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Help with commercial VHF mobile antenna
It's a long story, here's the short version. Our volunteer rescue squad dispatch operates in the 152 - 154 MHz range -- transmit on 154.XXX, receive 152.XXX. Our main antenna barely survived contact with a tree limb and needs to be replaced. Our local Motorola sales rep has his head stuck firmly up his ass and keeps trying to sell us some basic 1/4-wave verticals. The current antenna is a vertical whip with a loading coil wound along the length of the antenna. The dimensions a -- Overall height: 14.25 inches -- 4 inches from the base the antenna is wound into a coil, about 3/8 inch diameter, 5 turns -- the coil is 1.75 inches long -- above the coil is 8.5 inches of antenna -- NMO base I suspect this antenna is an old model 5/8-wave VHF antenna, shortened by winding a coil in the antenna. If it is a 5.8-wave, it should be giving us a few dB gain. The 1/4-wave whip he wants me to install would give unity or less gain. In our rural area, we need all the antenna help we can get. I an thinking about installing a full-length 5/8-wave whip, but, we go into a lot of driveways with low tree limbs and I doubt a full-length antenna would survive very long. I have Googled every term I can think of to find this antenna, Motorola sales rep tells me he thinks its a "cellular antenna" . . ..which it clearly is not. My MFJ antenna analyzer shows a resonance at 154 MHz. Anyone help me identify this antenna? - - - - - Fat, Dumb, and Ugly is no way to go through life. But, if you're a Republican, you have no choice. |
Help with commercial VHF mobile antenna
"Kickin' Ass and Takin' Names" wrote in message ... It's a long story, here's the short version. Our volunteer rescue squad dispatch operates in the 152 - 154 MHz range -- transmit on 154.XXX, receive 152.XXX. Our main antenna barely survived contact with a tree limb and needs to be replaced. Our local Motorola sales rep has his head stuck firmly up his ass and keeps trying to sell us some basic 1/4-wave verticals. The current antenna is a vertical whip with a loading coil wound along the length of the antenna. The dimensions a -- Overall height: 14.25 inches -- 4 inches from the base the antenna is wound into a coil, about 3/8 inch diameter, 5 turns -- the coil is 1.75 inches long -- above the coil is 8.5 inches of antenna -- NMO base I suspect this antenna is an old model 5/8-wave VHF antenna, shortened by winding a coil in the antenna. If it is a 5.8-wave, it should be giving us a few dB gain. The 1/4-wave whip he wants me to install would give unity or less gain. In our rural area, we need all the antenna help we can get. I an thinking about installing a full-length 5/8-wave whip, but, we go into a lot of driveways with low tree limbs and I doubt a full-length antenna would survive very long. I have Googled every term I can think of to find this antenna, Motorola sales rep tells me he thinks its a "cellular antenna" . . .which it clearly is not. My MFJ antenna analyzer shows a resonance at 154 MHz. Anyone help me identify this antenna? I don't think the Motorola man has his head anywhere, but maybe your head is. To be a 5/8 wave or gain antenna at 154 mhz the antenna will be about 35 to 45 inches long. If the antena you have is only 14.25 inches long, it is less than 1/4 and will have even less gain than a 1/4 wave whip. A 1/4 whip will be about 18 inches at 154 mhz. You do not shorten a 5/8 antenna by winding coils. The coils are either matching or phasing coils. |
Help with commercial VHF mobile antenna
I hate to break it to you, but what you have is a
less-than-quarter-wave vertical with a loading coil to bring the terminal impedance up to 50 ohms. Only the straight parts of the antenna radiate and that thing doesn't have very much. Your Moto guy was right in recommending a quarter-wave whip; it would out-perform this loaded version you have now. To counteract hitting trees and other stuff, you could substitute thin, strong 0.03"dia stainless spring stock for the radiator. It has a lot of "give" and could recover from close encounters. I use that for my 1/4-WL whip that daily bangs into my low-hanging garage door. Jim, K7JEB On Monday, June 24, 2013 5:57:05 AM UTC-7, Kickin' Ass and Takin' Names wrote: The current antenna is a vertical whip with a loading coil wound along the length of the antenna. The dimensions a -- Overall height: 14.25 inches -- 4 inches from the base the antenna is wound into a coil, about 3/8 inch diameter, 5 turns -- the coil is 1.75 inches long -- above the coil is 8.5 inches of antenna -- NMO base I suspect this antenna is an old model 5/8-wave VHF antenna, shortened by winding a coil in the antenna. |
Help with commercial VHF mobile antenna
El 24-06-13 14:57, Kickin' Ass and Takin' Names escribió:
It's a long story, here's the short version. Our volunteer rescue squad dispatch operates in the 152 - 154 MHz range -- transmit on 154.XXX, receive 152.XXX. Our main antenna barely survived contact with a tree limb and needs to be replaced. Our local Motorola sales rep has his head stuck firmly up his ass and keeps trying to sell us some basic 1/4-wave verticals. The current antenna is a vertical whip with a loading coil wound along the length of the antenna. The dimensions a -- Overall height: 14.25 inches -- 4 inches from the base the antenna is wound into a coil, about 3/8 inch diameter, 5 turns -- the coil is 1.75 inches long -- above the coil is 8.5 inches of antenna -- NMO base I suspect this antenna is an old model 5/8-wave VHF antenna, shortened by winding a coil in the antenna. If it is a 5.8-wave, it should be giving us a few dB gain. The 1/4-wave whip he wants me to install would give unity or less gain. In our rural area, we need all the antenna help we can get. I an thinking about installing a full-length 5/8-wave whip, but, we go into a lot of driveways with low tree limbs and I doubt a full-length antenna would survive very long. I have Googled every term I can think of to find this antenna, Motorola sales rep tells me he thinks its a "cellular antenna" . . .which it clearly is not. My MFJ antenna analyzer shows a resonance at 154 MHz. Anyone help me identify this antenna? - - - - - Fat, Dumb, and Ugly is no way to go through life. But, if you're a Republican, you have no choice. I agree with others. The full size quarter wave with correct size radials will perform better then the current 14.25' stick, no matter how you wind it. Make sure you have some VSWR indication to tune it to your frequency range, or just to check the complete installation. -- Wim PA3DJS www.tetech.nl Please remove abc first in case of PM |
Help with commercial VHF mobile antenna
On Mon, 24 Jun 2013 08:57:05 -0400, Kickin' Ass and Takin' Names
wrote: Our volunteer rescue squad dispatch operates in the 152 - 154 MHz range -- transmit on 154.XXX, receive 152.XXX. Antennas are normally tuned to the transmit frequency. Our main antenna barely survived contact with a tree limb and needs to be replaced. The tree limb will replace itself by growing back. All it takes is time. -- Overall height: 14.25 inches -- 4 inches from the base the antenna is wound into a coil, about 3/8 inch diameter, 5 turns -- the coil is 1.75 inches long -- above the coil is 8.5 inches of antenna -- NMO base Like this perhaps? http://www.11junk.com/jeffl/antennas/Misc/slides/Motorola-850mhz-NMO.html That's an 850 Mhz 5/8 wave antenna, commonly sold by Motorola for trunking systems. There's another model, with the same dimensions, but with thicker wire. It does work on VHF frequencies as a center loaded monopole, but is not designed for the purpose. I have about a dozen if you need one or two. Also, please check your frequency of operation just to be sure you're on 152 and not 850 MHz. I suspect this antenna is an old model 5/8-wave VHF antenna, shortened by winding a coil in the antenna. Nope. A 5/8 wave antenna is ummm... 5/8 wavelength at 152 MHz long or about 1 meter long. A 1/4 wave antenna will be ummm... 1/4 wavelength long or about 50 cm long. If it is a 5.8-wave, it should be giving us a few dB gain. The 1/4-wave whip he wants me to install would give unity or less gain. In our rural area, we need all the antenna help we can get. Some reading on the topic of 1/4 wave versus 5/8 wave mobile antennas: http://www.k0bg.com/images/pdf/mobile_vhf_ant.pdf Gain is a good thing, but not always beneficial. The mounting position has a big effect on the pattern. Any monopole, mounted on a flat ground plane, will have a tendency to "uptilt" the pattern, providing more RF to the sky than to the horizon. Too much gain and when the vehicle is at an angle, such as going uphill or downhill, there will be some pattern tilt. Fortunately, these type of problems are not much of a consideration, although they are important an UHF and up. Also, there are some mobile antennas, with open loading coils in the middle. Tree branches like to become entangled with the coil and try to rip the antenna out of the roof mount. I have one of those do exactly that to a previous vehicle. If you're going to be garaging the vehicle, there are 5/8 wave antennas that will tilt over with a hinge just above the base loading coil. I an thinking about installing a full-length 5/8-wave whip, but, we go into a lot of driveways with low tree limbs and I doubt a full-length antenna would survive very long. If there's a spring near the base, it will survive, as long as the tree limb doesn't hit the spring and get stuck. I prefer a magnet mount antenna, which simply falls over. The rectangular bases fall over better than the round bases. Do try to remember to put the magnet mount back when leaving the garage. Anyone help me identify this antenna? Sure. A photo would be helpful. If it's not the one in my photo, start he https://www.google.com/search?tbm=isch&q=vhf+mobile+antenna and see if any of the photos match your existing antenna. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Help with commercial VHF mobile antenna
On Mon, 24 Jun 2013 09:15:46 -0700, Jeff Liebermann
wrote: -- Overall height: 14.25 inches -- 4 inches from the base the antenna is wound into a coil, about 3/8 inch diameter, 5 turns -- the coil is 1.75 inches long -- above the coil is 8.5 inches of antenna -- NMO base Like this perhaps? http://www.11junk.com/jeffl/antennas/Misc/slides/Motorola-850mhz-NMO.html That's an 850 Mhz 5/8 wave antenna, commonly sold by Motorola for trunking systems. The antenna in the photo was made by Maxrad (PcTel). I have others that appear identical from Antennex (Laird). I couldn't find a similar antenna in either online catalog. The one's supplied by Motorola use a heavier gauge wire which I guess(tm) were Antenna Specialists (PcTel) like this: http://www.ebay.com/itm/321096706872 This is the closest equivalent that I could find: http://www.antenna.com/artifacts/2012511BASELOADEDNOGROUNDPLANEANTENNAS.pdf I would advise against using this antenna for 152 Mhz and suggest you get a real 1/4 wave or 5/8 wave antenna. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Help with commercial VHF mobile antenna
El 24-06-13 18:14, Wimpie escribió:
El 24-06-13 14:57, Kickin' Ass and Takin' Names escribió: It's a long story, here's the short version. Our volunteer rescue squad dispatch operates in the 152 - 154 MHz range -- transmit on 154.XXX, receive 152.XXX. Our main antenna barely survived contact with a tree limb and needs to be replaced. Our local Motorola sales rep has his head stuck firmly up his ass and keeps trying to sell us some basic 1/4-wave verticals. The current antenna is a vertical whip with a loading coil wound along the length of the antenna. The dimensions a -- Overall height: 14.25 inches -- 4 inches from the base the antenna is wound into a coil, about 3/8 inch diameter, 5 turns -- the coil is 1.75 inches long -- above the coil is 8.5 inches of antenna -- NMO base I suspect this antenna is an old model 5/8-wave VHF antenna, shortened by winding a coil in the antenna. If it is a 5.8-wave, it should be giving us a few dB gain. The 1/4-wave whip he wants me to install would give unity or less gain. In our rural area, we need all the antenna help we can get. I an thinking about installing a full-length 5/8-wave whip, but, we go into a lot of driveways with low tree limbs and I doubt a full-length antenna would survive very long. I have Googled every term I can think of to find this antenna, Motorola sales rep tells me he thinks its a "cellular antenna" . . .which it clearly is not. My MFJ antenna analyzer shows a resonance at 154 MHz. Anyone help me identify this antenna? - - - - - Fat, Dumb, and Ugly is no way to go through life. But, if you're a Republican, you have no choice. I agree with others. The full size quarter wave with correct size radials will perform better then the current 14.25' stick, no matter how you wind it. Make sure you have some VSWR indication to tune it to your frequency range, or just to check the complete installation. I overlooked the mobile operation from a car. Of course, when the antenna is mounted on a metal surface, you don't need radials.. -- Wim PA3DJS www.tetech.nl Please remove abc first in case of PM |
Help with commercial VHF mobile antenna
On Mon, 24 Jun 2013 18:46:46 +0200, Wimpie
wrote: I overlooked the mobile operation from a car. Of course, when the antenna is mounted on a metal surface, you don't need radials.. If it's a metal car roof, you don't need radials. Unfortunately, I've had to deal with verhicles that have a fiberglass roof. Aluminum duct tape ground plane (on the inside) to the rescue. http://www.homedepot.com/p/Nashua-Tape-322-1-57-64-in-x-50-yds-Aluminum-Foil-Tape-3220020500/100030120#.Uch4_Ng9pjZ The fiberglass roof problem is also common in marine VHF (156-163Mhz) installations. Those tend to use 1/2 wave antennas, which do not require a ground plane. The automobile version of the 1/2 wave: http://www.theantennafarm.com/catalog/laird-tech-bb1322w-4470.html?zenid=6bc9236b727ed1e483c9037fb2ac52db -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Help with commercial VHF mobile antenna
In article ,
Jeff Liebermann wrote: I overlooked the mobile operation from a car. Of course, when the antenna is mounted on a metal surface, you don't need radials.. If it's a metal car roof, you don't need radials. Unfortunately, I've had to deal with verhicles that have a fiberglass roof. Aluminum duct tape ground plane (on the inside) to the rescue. That will help but not entirely resolve the situation. What I have heard, is that the theoretical gain advantage of a 5/8-wavelength monopole over a 1/4-wave monopole, is dependent on the antenna being operated over a fairly large groundplane (one which reaches out several wavelengths from the feedpoint). A simple set of ground-radial "tapes" won't be big or extensive enough... and, actually, neither will be the typical vehicle roof (at VHF wavelengths at least). According to these sources, in the absence of a good groundplane, the 5/8-wave monopole tends to "squint" - its highest-gain lobes are not towards the horizon but aim upwards somewhat. Gain towards the horizon may be *less* than a quarter-wave monopole on the same vehicle mount. So, the theoretical gain advantage of a 5/8-wave vehicle antenna may not work out in practice. Testing would be required to see if there's actually an advantage, or whether a "high gain" antenna of this sort is actually a loss in practice because the gain is aimed in the wrong directions. And, I agree that for many vehicle mounting situations, a "ground independent" antenna such as an end-fed half-wave may be the best bet. I believe you can get these in a shortened form (with distributed or lumped inductive loading in the center of the radiator) to keep the height within reason... but going for a full-length end-fed radiator would give you somewhat better gain and efficiency, if it's safe to install on the vehicle. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Help with commercial VHF mobile antenna
Kickin' Ass and Takin' Names posted for all of
us... And I know how to SNIP It's a long story, here's the short version. Our volunteer rescue squad dispatch operates in the 152 - 154 MHz range -- transmit on 154.XXX, receive 152.XXX. Our main antenna barely survived contact with a tree limb and needs to be replaced. Our local Motorola sales rep has his head stuck firmly up his ass and keeps trying to sell us some basic 1/4-wave verticals. The current antenna is a vertical whip with a loading coil wound along the length of the antenna. The dimensions a -- Overall height: 14.25 inches -- 4 inches from the base the antenna is wound into a coil, about 3/8 inch diameter, 5 turns -- the coil is 1.75 inches long -- above the coil is 8.5 inches of antenna -- NMO base I suspect this antenna is an old model 5/8-wave VHF antenna, shortened by winding a coil in the antenna. If it is a 5.8-wave, it should be giving us a few dB gain. The 1/4-wave whip he wants me to install would give unity or less gain. In our rural area, we need all the antenna help we can get. I an thinking about installing a full-length 5/8-wave whip, but, we go into a lot of driveways with low tree limbs and I doubt a full-length antenna would survive very long. I have Googled every term I can think of to find this antenna, Motorola sales rep tells me he thinks its a "cellular antenna" . . .which it clearly is not. My MFJ antenna analyzer shows a resonance at 154 MHz. Anyone help me identify this antenna? look at Larsen antennas I believe they made many antennas for Moneyrola. -- Tekkie |
It would have helped - had the OP posted the model number for the antenna and a description of what he wanted to do with it.
A mobile vertical antenna has no gain - gain is only achieved when you have gain in one direction and rejection in one or more directions. The only measurable gain would be gain as compared to a dipole or gain over isotropic. 1/4 wave antenna's can sometimes produce a better signal locally, because the radiation pattern is spread out over a larger area. It will give no distance gain - just local reception.] Because it is all one radio - the reception length does not matter, just that it is resonant at X mhz - transmit. Public service here all uses Larsen antenna's - especially the PA State Police, and they have very deep pockets. Shakespeare also makes a decent mobile antenna. You need a antenna analyzer or a Dip Meter to set to resonance. The only thing the SWR meter can do is tell you what is happening in the feed line. |
Help with commercial VHF mobile antenna
On Wed, 26 Jun 2013 13:16:39 +0100, Channel Jumper
wrote: A mobile vertical antenna has no gain - gain is only achieved when you have gain in one direction and rejection in one or more directions. Wrong. Gain on a mobile antenna can be achieved without directionality. Just reduce the vertical radiation angle, which puts more RF towards the horizon, and less RF towards the sky and the ground. The only measurable gain would be gain as compared to a dipole or gain over isotropic. Yep. That's the way gain is normally measured. dBd or dBi. 1/4 wave antenna's can sometimes produce a better signal locally, because the radiation pattern is spread out over a larger area. I've seen that. However, it's usually the result of misusing a "gain" type antenna, such as a dual band 5/8 ham antenna being used on marine or commercial frequencies, or a 5/8 commercial antenna, being used on ham frequencies. Lots of ways to do it wrong. Where a 1/4 wave antenna really shines is when one needs to cover a wide range of VHF frequencies, from aircraft to marine. It will give no distance gain - just local reception.] Ummm... the range depends more on the terrain than on the antenna. Because it is all one radio - the reception length does not matter, just that it is resonant at X mhz - transmit. VSWR is highly over-rated. The only real reason to keep VSWR low is that high VSWR will cause the transmitter to protect itself and partly shut down. Try this experiment. Take a piece of sheet metal (or aluminum foil covered cardboard) to act as a ground plane. Insert and SO-239 connector in the middle. Add a length of moderately stiff electrical wire to the SO-239 that is longer than 1/4 wave at the weather frequency (162.xxxx). Find a receiver that will measure the actual receive signal strength. An all mode or AM (not FM) receiver will work nicely. Extra credit for using a service monitor. Make a measurement and start cutting the length of the antenna in roughly 1/2" intervals. Measure the receive signal strength. What I've found when I've done this, is that the antenna gain, which is what the receive signal level indicates, doesn't change very much until you get down to about 1/8th wavelength. I modeled this test using 4NEC2 and found the same thing. Now, if you believe that the tx and rx performance of an antenna are identical, this would suggest that you could make the antenna almost any length, and still have adequate gain and function if you could fix the VSWR. Public service here all uses Larsen antenna's - especially the PA State Police, and they have very deep pockets. Obviously, the more expensive the antenna, the better it works. Shakespeare also makes a decent mobile antenna. They mostly make marine and military antennas. Their commercial antennas are overpriced versions of the antennas that they sell to the military. They're very well built, rugged, but not cheap. http://shakespeare-military.com You need a antenna analyzer or a Dip Meter to set to resonance. Have you ever tried to resonate a 1/4 wave antenna with either of those? You'll find that it's affected by the position and location of just about everything within about a 20 ft radius. I run a sweep generator, directional coupler, detector, and scope combination to test antennas, but no way would I ever use that to tune the antenna. Just getting near the antenna ruins the display. Incidentally, for complex antennas, such as a dual band J-pole, minimum VSWR isn't always at resonance. The only thing the SWR meter can do is tell you what is happening in the feed line. Wrong. A VSWR meter reading is affected by the xmitter output impedance, feed line impedance to the VSWR meter, characteristic impedance of the coax cables(s), feed line impedance after the VSWR meter, and of course, the antenna impedance. That's actually a problem because a VSWR meter is affected by literally everything. Drivel: I run mostly 75 ohm systems (because the coax is cheap and easy and has less loss). I had to build my own 75 ohm directional coupler in order to get accurate VSWR measurements. (Yes, Bird makes a 75 ohm wattmeter 4307, but I don't want to spend the money). -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
And so Jeff speaks.
Jeff - the radiation pattern of a 1/4 wave antenna pancakes compared to a longer antenna, but you cannot use weather radio as a barometer to measure how efficient a antenna works. Weather radio is broadcast in such a way that a signal is available to most people within a 40 mile radius circle from each transmitter - at least in Western Pennsylvania. The signals here - about 350 watts at the transmitter - is enough that a simple table top radio or a cheap bubble pack GMRS radio will usually receive it - even with a 4 inch antenna. However - when working any type of DX - you best better bring your A game or go home. You are not going to net much with a simple 1/4 wave antenna on VHF frequencies when the bands are not wide open. This man wants to replace his mobile antenna with something better, but does not want to take the advice of his radio technician - because he thinks the guy is ripping him off. The bottom line is - most people involved in communications doesn't just start selling radios without any type of formal education. Even if the only education the person received was from the Military, it is usually based on sound practices and principals. I don't usually deal with anyone that carries their money in a snapper purse. If someone wants to tell me how to do something that I have been doing for 40 years - I just walk away. I laugh at these so called roadside CB radio shops that sells all this garbage to these CB radio guys, including their peek n tunes - which does nothing except reduce the life of the radio. |
Help with commercial VHF mobile antenna
Good Catch Jeff, and thanks for straightening thins out -- it's amazing
some of the things we think we know about antennas and antenna measurement until someone corrects us! Furthermore, it's difficult for someone who does not fully understand the concepts to receive misinformation! Irv VE6BP "Jeff Liebermann" wrote in message ... On Wed, 26 Jun 2013 13:16:39 +0100, Channel Jumper wrote: A mobile vertical antenna has no gain - gain is only achieved when you have gain in one direction and rejection in one or more directions. Wrong. Gain on a mobile antenna can be achieved without directionality. Just reduce the vertical radiation angle, which puts more RF towards the horizon, and less RF towards the sky and the ground. The only measurable gain would be gain as compared to a dipole or gain over isotropic. Yep. That's the way gain is normally measured. dBd or dBi. 1/4 wave antenna's can sometimes produce a better signal locally, because the radiation pattern is spread out over a larger area. I've seen that. However, it's usually the result of misusing a "gain" type antenna, such as a dual band 5/8 ham antenna being used on marine or commercial frequencies, or a 5/8 commercial antenna, being used on ham frequencies. Lots of ways to do it wrong. Where a 1/4 wave antenna really shines is when one needs to cover a wide range of VHF frequencies, from aircraft to marine. It will give no distance gain - just local reception.] Ummm... the range depends more on the terrain than on the antenna. Because it is all one radio - the reception length does not matter, just that it is resonant at X mhz - transmit. VSWR is highly over-rated. The only real reason to keep VSWR low is that high VSWR will cause the transmitter to protect itself and partly shut down. Try this experiment. Take a piece of sheet metal (or aluminum foil covered cardboard) to act as a ground plane. Insert and SO-239 connector in the middle. Add a length of moderately stiff electrical wire to the SO-239 that is longer than 1/4 wave at the weather frequency (162.xxxx). Find a receiver that will measure the actual receive signal strength. An all mode or AM (not FM) receiver will work nicely. Extra credit for using a service monitor. Make a measurement and start cutting the length of the antenna in roughly 1/2" intervals. Measure the receive signal strength. What I've found when I've done this, is that the antenna gain, which is what the receive signal level indicates, doesn't change very much until you get down to about 1/8th wavelength. I modeled this test using 4NEC2 and found the same thing. Now, if you believe that the tx and rx performance of an antenna are identical, this would suggest that you could make the antenna almost any length, and still have adequate gain and function if you could fix the VSWR. Public service here all uses Larsen antenna's - especially the PA State Police, and they have very deep pockets. Obviously, the more expensive the antenna, the better it works. Shakespeare also makes a decent mobile antenna. They mostly make marine and military antennas. Their commercial antennas are overpriced versions of the antennas that they sell to the military. They're very well built, rugged, but not cheap. http://shakespeare-military.com You need a antenna analyzer or a Dip Meter to set to resonance. Have you ever tried to resonate a 1/4 wave antenna with either of those? You'll find that it's affected by the position and location of just about everything within about a 20 ft radius. I run a sweep generator, directional coupler, detector, and scope combination to test antennas, but no way would I ever use that to tune the antenna. Just getting near the antenna ruins the display. Incidentally, for complex antennas, such as a dual band J-pole, minimum VSWR isn't always at resonance. The only thing the SWR meter can do is tell you what is happening in the feed line. Wrong. A VSWR meter reading is affected by the xmitter output impedance, feed line impedance to the VSWR meter, characteristic impedance of the coax cables(s), feed line impedance after the VSWR meter, and of course, the antenna impedance. That's actually a problem because a VSWR meter is affected by literally everything. Drivel: I run mostly 75 ohm systems (because the coax is cheap and easy and has less loss). I had to build my own 75 ohm directional coupler in order to get accurate VSWR measurements. (Yes, Bird makes a 75 ohm wattmeter 4307, but I don't want to spend the money). -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Help with commercial VHF mobile antenna
"Kickin' Ass and Takin' Names" wrote in message ... It's a long story, here's the short version. Our volunteer rescue squad dispatch operates in the 152 - 154 MHz range -- transmit on 154.XXX, receive 152.XXX. SNIP I have Googled every term I can think of to find this antenna, Motorola sales rep tells me he thinks its a "cellular antenna" . . .which it clearly is not. My MFJ antenna analyzer shows a resonance at 154 MHz. Anyone help me identify this antenna? - - - - - Fat, Dumb, and Ugly is no way to go through life. But, if you're a Republican, you have no choice. When you're asking people for help, an insulting sig cuts down the number of people who care about your problems. |
Help with commercial VHF mobile antenna
El lunes, 24 de junio de 2013 20:04:01 UTC+2, Dave Platt escribió:
In article , Jeff Liebermann wrote: I overlooked the mobile operation from a car. Of course, when the antenna is mounted on a metal surface, you don't need radials.. If it's a metal car roof, you don't need radials. Unfortunately, I've had to deal with verhicles that have a fiberglass roof. Aluminum duct tape ground plane (on the inside) to the rescue. That will help but not entirely resolve the situation. What I have heard, is that the theoretical gain advantage of a 5/8-wavelength monopole over a 1/4-wave monopole, is dependent on the antenna being operated over a fairly large groundplane (one which reaches out several wavelengths from the feedpoint). A simple set of ground-radial "tapes" won't be big or extensive enough... and, actually, neither will be the typical vehicle roof (at VHF wavelengths at least). According to these sources, in the absence of a good groundplane, the 5/8-wave monopole tends to "squint" - its highest-gain lobes are not towards the horizon but aim upwards somewhat. Gain towards the horizon may be *less* than a quarter-wave monopole on the same vehicle mount. So, the theoretical gain advantage of a 5/8-wave vehicle antenna may not work out in practice. Testing would be required to see if there's actually an advantage, or whether a "high gain" antenna of this sort is actually a loss in practice because the gain is aimed in the wrong directions. And, I agree that for many vehicle mounting situations, a "ground independent" antenna such as an end-fed half-wave may be the best bet. I believe you can get these in a shortened form (with distributed or lumped inductive loading in the center of the radiator) to keep the height within reason... but going for a full-length end-fed radiator would give you somewhat better gain and efficiency, if it's safe to install on the vehicle. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! To Jeff and Dave, I agree on the end-fed half-wave. I like them but you need to take care of matching and good capacitors (high voltage breakdown). The myth of the gain advantage of the 5/8lambda is from the AM broadcast antenna patterns where we have a large ground plane (mother earth, I am sure you both know). I fully agree; the half-wave, and even the quarter-wave will win with real world ground planes/radials on HF/VHF/UHF terrestrial links. As long as people think "longer = better", the myth will continue and peoople keep buying 5/8 lambda verticals with radials (the pigeons like them!). To avoid long discussion with others: I know stacking with good phasing does help to increase gain. Wim PA3DJS please instruct your racing pigeon to skip abc. |
Help with commercial VHF mobile antenna
On 6/26/2013 10:17 AM, Jeff Liebermann wrote:
On Wed, 26 Jun 2013 13:16:39 +0100, Channel Jumper wrote: A mobile vertical antenna has no gain - gain is only achieved when you have gain in one direction and rejection in one or more directions. Wrong. Gain on a mobile antenna can be achieved without directionality. Just reduce the vertical radiation angle, which puts more RF towards the horizon, and less RF towards the sky and the ground. The only measurable gain would be gain as compared to a dipole or gain over isotropic. Yep. That's the way gain is normally measured. dBd or dBi. 1/4 wave antenna's can sometimes produce a better signal locally, because the radiation pattern is spread out over a larger area. I've seen that. However, it's usually the result of misusing a "gain" type antenna, such as a dual band 5/8 ham antenna being used on marine or commercial frequencies, or a 5/8 commercial antenna, being used on ham frequencies. Lots of ways to do it wrong. Where a 1/4 wave antenna really shines is when one needs to cover a wide range of VHF frequencies, from aircraft to marine. It will give no distance gain - just local reception.] Ummm... the range depends more on the terrain than on the antenna. Because it is all one radio - the reception length does not matter, just that it is resonant at X mhz - transmit. VSWR is highly over-rated. The only real reason to keep VSWR low is that high VSWR will cause the transmitter to protect itself and partly shut down. Try this experiment. Take a piece of sheet metal (or aluminum foil covered cardboard) to act as a ground plane. Insert and SO-239 connector in the middle. Add a length of moderately stiff electrical wire to the SO-239 that is longer than 1/4 wave at the weather frequency (162.xxxx). Find a receiver that will measure the actual receive signal strength. An all mode or AM (not FM) receiver will work nicely. Extra credit for using a service monitor. Make a measurement and start cutting the length of the antenna in roughly 1/2" intervals. Measure the receive signal strength. What I've found when I've done this, is that the antenna gain, which is what the receive signal level indicates, doesn't change very much until you get down to about 1/8th wavelength. I modeled this test using 4NEC2 and found the same thing. Now, if you believe that the tx and rx performance of an antenna are identical, this would suggest that you could make the antenna almost any length, and still have adequate gain and function if you could fix the VSWR. I do not doubt your information here. However, it seems to conflict with my experiences working 75 meters. I work 75 each day using a 75 meter horizontal loop. I hear the same characters on each day. Often a newbie pops up with a poor signal. He is in the same area as "the gang" and yet his signal stinks. Almost invariably we ask him about his G5RV. "Gee guys how did you know I was using a G5RV?" Poor signals shows up every time. He is using a dipole that is way too short to resonate on 75 meters. I think they are 82 feet long. It seems to me if VSWR made little difference, then his 82 foot long dipole on 75 meters should work just fine. Not trying for a fight, just want an opinion about why we are hearing this effect. Of course they are using tuners to make a match to their transceivers. My own loop is carefully cut for 3.9 mhz. I need a tuner because it is feed with 600 ohm open wire line and has a nasty VSWR because of mismatch between the lead-in and antenna. The online calculator for loss using my antenna system comes out to be 1/2 db. I can live with that. However, if I put up a loop that was 1/2 the size I need, and then matched it with a tuner, it would hardly work at all. I know. I tried loading mine on 160 meters. I could make a match with the tuner. But it was a bust. Public service here all uses Larsen antenna's - especially the PA State Police, and they have very deep pockets. Obviously, the more expensive the antenna, the better it works. Shakespeare also makes a decent mobile antenna. They mostly make marine and military antennas. Their commercial antennas are overpriced versions of the antennas that they sell to the military. They're very well built, rugged, but not cheap. http://shakespeare-military.com You need a antenna analyzer or a Dip Meter to set to resonance. Have you ever tried to resonate a 1/4 wave antenna with either of those? You'll find that it's affected by the position and location of just about everything within about a 20 ft radius. I run a sweep generator, directional coupler, detector, and scope combination to test antennas, but no way would I ever use that to tune the antenna. Just getting near the antenna ruins the display. Incidentally, for complex antennas, such as a dual band J-pole, minimum VSWR isn't always at resonance. The only thing the SWR meter can do is tell you what is happening in the feed line. Wrong. A VSWR meter reading is affected by the xmitter output impedance, feed line impedance to the VSWR meter, characteristic impedance of the coax cables(s), feed line impedance after the VSWR meter, and of course, the antenna impedance. That's actually a problem because a VSWR meter is affected by literally everything. Drivel: I run mostly 75 ohm systems (because the coax is cheap and easy and has less loss). I had to build my own 75 ohm directional coupler in order to get accurate VSWR measurements. (Yes, Bird makes a 75 ohm wattmeter 4307, but I don't want to spend the money). |
Help with commercial VHF mobile antenna
"seediq" wrote in message ... I do not doubt your information here. However, it seems to conflict with my experiences working 75 meters. I work 75 each day using a 75 meter horizontal loop. I hear the same characters on each day. Often a newbie pops up with a poor signal. He is in the same area as "the gang" and yet his signal stinks. Almost invariably we ask him about his G5RV. "Gee guys how did you know I was using a G5RV?" Poor signals shows up every time. He is using a dipole that is way too short to resonate on 75 meters. I think they are 82 feet long. It seems to me if VSWR made little difference, then his 82 foot long dipole on 75 meters should work just fine. Not trying for a fight, just want an opinion about why we are hearing this effect. Of course they are using tuners to make a match to their transceivers. My own loop is carefully cut for 3.9 mhz. I need a tuner because it is feed with 600 ohm open wire line and has a nasty VSWR because of mismatch between the lead-in and antenna. The online calculator for loss using my antenna system comes out to be 1/2 db. I can live with that. However, if I put up a loop that was 1/2 the size I need, and then matched it with a tuner, it would hardly work at all. I know. I tried loading mine on 160 meters. I could make a match with the tuner. But it was a bust. You can not compair what goes on at 75 meters with 6 meters and above. Less than 99.9 % of the hams can not put up equal antennas. For the mobile on VHF it would have to be around 200 feet high and the truck would have ot be 200 to 400 feet wide and long.. I don't like the g5rv either, but they seem to work ok. Your loop works fine for talking to the same people each day. Try it at other distances. Going say 3000 miles away, a short vertical may be beter. I don't do much on 75, but do some on 20 meters. It all depends on the propogation. One day some stations with beams were hardly workable and a state or two away I worked a mobile and another with a temporary vertical that was running 5 watts, they were both s9 or beter. I have played with vhf repeaters for about 40 years. It may depend on the area you are in as to the best kind of all around vhf antenna. One day a fellow ham and I rounded up several antennas of all kinds. From 1/4 wave to one about 6 feet long for 2 meters. There did not seem to be a clear winner. Even a 40 meter antenna mounted to the bumper that was about 10 feet long worked as well receiving one repeater while the car was parked in the same spot. The area around here averages about 700 feet above sea level. Some of the repeaters are from about the same height to around 5000 feet above sea level. About the only overall differance we have found is the 5 or 6 foot long antennas do not seem to work very well while in motion and the 5/8 antennas need to be stiff enough that they do not lay back at highway speeds. In areas that are flat it may be a whole differant story and the antennas that keep the signal near the earth such as a 5/8 may work a lot beter overall. We did notice a big differance when going from a 4 bay dipole to a colinear about the same overall length. The dipole and colinear were both Phelps Dodge, not the cheap ham antennas. While the rated gain differance was about 1 db infavor of the dipole aray, the noted coverage was much less when using the colinear. Years later, we switched back to the dipole aray and the coverage came back. If used in a differant area, there could be another differance in coverage when compaired. |
Help with commercial VHF mobile antenna
On 6/27/2013 1:07 PM, Wimpie wrote:
El lunes, 24 de junio de 2013 20:04:01 UTC+2, Dave Platt escribió: In article , Jeff Liebermann wrote: I overlooked the mobile operation from a car. Of course, when the antenna is mounted on a metal surface, you don't need radials.. If it's a metal car roof, you don't need radials. Unfortunately, I've had to deal with verhicles that have a fiberglass roof. Aluminum duct tape ground plane (on the inside) to the rescue. That will help but not entirely resolve the situation. What I have heard, is that the theoretical gain advantage of a 5/8-wavelength monopole over a 1/4-wave monopole, is dependent on the antenna being operated over a fairly large groundplane (one which reaches out several wavelengths from the feedpoint). A simple set of ground-radial "tapes" won't be big or extensive enough... and, actually, neither will be the typical vehicle roof (at VHF wavelengths at least). According to these sources, in the absence of a good groundplane, the 5/8-wave monopole tends to "squint" - its highest-gain lobes are not towards the horizon but aim upwards somewhat. Gain towards the horizon may be *less* than a quarter-wave monopole on the same vehicle mount. So, the theoretical gain advantage of a 5/8-wave vehicle antenna may not work out in practice. Testing would be required to see if there's actually an advantage, or whether a "high gain" antenna of this sort is actually a loss in practice because the gain is aimed in the wrong directions. And, I agree that for many vehicle mounting situations, a "ground independent" antenna such as an end-fed half-wave may be the best bet. I believe you can get these in a shortened form (with distributed or lumped inductive loading in the center of the radiator) to keep the height within reason... but going for a full-length end-fed radiator would give you somewhat better gain and efficiency, if it's safe to install on the vehicle. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! To Jeff and Dave, I agree on the end-fed half-wave. I like them but you need to take care of matching and good capacitors (high voltage breakdown). The myth of the gain advantage of the 5/8lambda is from the AM broadcast antenna patterns where we have a large ground plane (mother earth, I am sure you both know). I fully agree; the half-wave, and even the quarter-wave will win with real world ground planes/radials on HF/VHF/UHF terrestrial links. As long as people think "longer = better", the myth will continue and peoople keep buying 5/8 lambda verticals with radials (the pigeons like them!). To avoid long discussion with others: I know stacking with good phasing does help to increase gain. Wim PA3DJS please instruct your racing pigeon to skip abc. How refreshing! Thanks, Wim, for attempting to dispel myths. John KD5YI |
Help with commercial VHF mobile antenna
On Thu, 27 Jun 2013 13:34:20 -0500, seediq
wrote: I do not doubt your information here. However, it seems to conflict with my experiences working 75 meters. Ummm... is there a weather channel on 75 meters? My "cut the antenna length" test was intended for VHF, which is a very different beast from 75 meters. The big difference between VHF and HF is that HF turns the nearby earth, ground, water table, hills, buildings, fences, and neighbors into part of the antenna system. For VHF, once I get out of the near field region, most of that stuff can be ignored (unless it's also in the Fresnel Zone). I can elaborate more on this if you like, but I'm not an expert or seriously experience with land based HF antennas, just marine HF, which is yet a different beast. I'm stuck at home today with a foot problem. So, I get to sit at the computer instead of the workbench. I'll throw together a web page showing that cutting the antenna short does NOT reduce it's gain and efficiency very much (but does mangle the pattern and VSWR). Stay tuned. I work 75 each day using a 75 meter horizontal loop. I hear the same characters on each day. Often a newbie pops up with a poor signal. He is in the same area as "the gang" and yet his signal stinks. Almost invariably we ask him about his G5RV. "Gee guys how did you know I was using a G5RV?" Poor signals shows up every time. He is using a dipole that is way too short to resonate on 75 meters. I think they are 82 feet long. It seems to me if VSWR made little difference, then his 82 foot long dipole on 75 meters should work just fine. Not trying for a fight, just want an opinion about why we are hearing this effect. Can I pass on this right now? I have some definite opinions on HF antennas and the G5RV, which unfortunately I cannot substantiate with either experience or calculations. Rather than post erroneous information, I'll keep my foot in my mouth where it belongs. However, I can't resist giving you a clue as to what's different between an excessively short dipole and a real antenna. A hint is that unless the VSWR is outrageously high or the antenna was made from barbed wire, nearly 100.0% of the RF that is applied to it gets radiated in some direction. The some direction is the key. With a decent antenna, it's going in the right direction. With a not so wonderful antenna, it's going in useless directions, such as into the ground. I'll stop there before I get myself into trouble. Of course they are using tuners to make a match to their transceivers. I should also point out that one can always make things worse with an antenna tuner. Try the loss on 160m and 75m with this Java applet: http://fermi.la.asu.edu/w9cf/tuner/tuner.html Tweak the values of Q for the caps and inductors for a more realistic calculation. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Help with commercial VHF mobile antenna
On Thursday, June 27, 2013 1:34:20 PM UTC-5, seediq wrote:
I do not doubt your information here. However, it seems to conflict with my experiences working 75 meters. I work 75 each day using a 75 meter horizontal loop. I hear the same characters on each day. Often a newbie pops up with a poor signal. He is in the same area as "the gang" and yet his signal stinks. Almost invariably we ask him about his G5RV. "Gee guys how did you know I was using a G5RV?" Poor signals shows up every time. He is using a dipole that is way too short to resonate on 75 meters. I think they are 82 feet long. It seems to me if VSWR made little difference, then his 82 foot long dipole on 75 meters should work just fine. Not trying for a fight, just want an opinion about why we are hearing this effect. Of course they are using tuners to make a match to their transceivers. A high SWR on it's own is not always bad news. It depends on the feed line used, freq, etc. The main reason the G5RV's don't do so hot is the goofy method of feeding most seem to use. IE: a length of twin lead to a choke, to coax. And then some add insult to injury and run a tuner at the shack. A good amount of power is turned to heat. But if you feed the same antenna with ladder line the whole route, the losses are not so bad, even with a high SWR. If you tune the line and use no tuner, fairly low losses. If you use a tuner, not quite as good, but not too bad if you use the least amount of inductance needed to get a usable match. All antennas will radiate nearly all power applied to them. The trick is getting it from the rig to the antenna without turning some into heat. This is where the usual G5RV is failing. Some of the power is not making it to the antenna due to obtuse lossy methods of feeding. Same issue with some of the windoms, etc that are sold. |
Help with commercial VHF mobile antenna
wrote in message ... A high SWR on it's own is not always bad news. It depends on the feed line used, freq, etc. The main reason the G5RV's don't do so hot is the goofy method of feeding most seem to use. IE: a length of twin lead to a choke, to coax. And then some add insult to injury and run a tuner at the shack. A good amount of power is turned to heat. But if you feed the same antenna with ladder line the whole route, the losses are not so bad, even with a high SWR. If you tune the line and use no tuner, fairly low losses. If you use a tuner, not quite as good, but not too bad if you use the least amount of inductance needed to get a usable match. All antennas will radiate nearly all power applied to them. The trick is getting it from the rig to the antenna without turning some into heat. This is where the usual G5RV is failing. Some of the power is not making it to the antenna due to obtuse lossy methods of feeding. Same issue with some of the windoms, etc that are sold. I have not used the g5rv except for some the club uses at field day. From what I understand about them, they were designed to work on 20 meters. It was just luck that they will have a reasonable low swr on other ham bands. If the swr goes up over 3 or 4 to 1 I can see lots of power being lost in the coax part. I use a home made version of the off center fed. I can compair it to an 80 meter dipole and a triband beam. It usually matches the dipole or is sometimes beter depending on the direction of the other stations on 80 meters. On 20 and 10 meters the beam is usually much beter, but if a station hapens to be in certain places there is not too much differance . The ocf does not work very well on 15, but it is not suspose to. All antennas are fed with Davis Bury flex rg 8 type which does not have too much loss. |
Help with commercial VHF mobile antenna
On Thu, 27 Jun 2013 16:11:50 -0700, Jeff Liebermann
wrote: I'm stuck at home today with a foot problem. So, I get to sit at the computer instead of the workbench. I'll throw together a web page showing that cutting the antenna short does NOT reduce it's gain and efficiency very much (but does mangle the pattern and VSWR). Stay tuned. 5 hours (minus dinner) later and I'm dead tired. What started as a simple little demonstration turned into a time burning nightmare. Here's where I stopped: http://www.11junk.com/jeffl/antennas/Monopole/index.html The various sub-directories are NEC2 models for various length monopole antennas over a perfect ground plane. That a rough approximation of what one would expect to see on the roof of a car with a large metal roof at VHF/UHF frequencies. It's not quite correct, but close enough for this exercise. The directories are named after the length of the monopole antenna. For example: monopole_0_625 is a 0.625 or 5/8th wavelength antenna. The underscores were used because Windoze XP detests more than one period in a filename. The NEC deck is really simple. CM Monopole antenna over perfect ground. CM by Jeff Liebermann AE6KS 06/25/2013 CE SY LENGTH = 0.625 'Length in wavelengths GW 1 21 0 0 0 0 0 LENGTH 0.001 GE 1 GN 1 EK EX 0 1 1 0 1 0 FR 0 0 0 0 299.8 0 EN The only value that changes for each antenna is the label: LENGTH = X.XXX The 0.001 is 0.001 wavelengths for a wavelength = 1 meter, which is a 2mm diameter monopole antenna. The 299.8 MHz frequency is a convenient trick to make 1 wavelength equal to 1 meter, making all the dimension appear in wavelengths. That allow this antenna to be easily scaled to any frequency. If you feel ambitious, download and install 4NEC2 from: http://www.qsl.net/4nec2/ and try it. If you're really into big models, I suggest you also get the multi-core/processor NEC2 engine from: http://users.otenet.gr/~jmsp/ which really speeds things up. So much for the background stuff... Start with the 1/4 wave antenna at: http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_250/index.html http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_250/slides/monopole_0_250.html Note that the gain is 5.19dBi. At this point, I usually get an outrage from everyone who knows that a dipole is 2.15dBi and that this monopole can't possibly have more. Well, we have a perfectly reflective ground under this antenna, that reflects 100.0% of everything that hits it, effectively doubling the gain. 2.15dBi + 3.01dB doubling = 5.16dBi You'll see the extra 3dB gain throughout the various pages. The common misconception is that shorter antennas have less gain. Yes, they do, but it's not really proportional to the length. For example, the 1/4 wave monopole may have 5.19dBi gain, http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_250/slides/monopole_0_250.html but the 1/8th wave monopole still has 4.86dbi gain http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html and the 1/20th wave monopole still has 4.79dBi gain. Going the other direction with longer monopole antennas, the full wave monopole at: http://www.11junk.com/jeffl/antennas/Monopole/monopole_1_000/slides/monopole_1_000.html has 7.06dBi gain or less than 2dB more than a 1/4 monopole. One might expect that having 4 times as much wire as the 1/4 wave monopole would produce a 6dB gain increase, but that's not how it works. I did some tweaking and arranged to produce the antenna impedance in polar form. For example, the 1/4 wave antenna at: http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_250/slides/monopole_0_250.html has an impedance of 48.7 ohms with a phase angle of 30.2 degrees. Close enough to 50 ohms. However, as we get into even multiples of 1/4 wavelength, the impedances become very high. For example, the infamous 1/2 wave monopole shows 934 ohms: http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_500/slides/monopole_0_500.html which is not going to be easy to match. On the short end of the scale, the 1/8th wave antenna at: http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html shows 254 ohms, which will work with a 2:1 turns ratio transformer. If you look at the antennas that are odd multiples of 1/4 wavelength, you'll notice that their impedances are tolerably close to 50 ohms. For example, the 1.25 wavelength antenna is 72.9 ohms, which will probably work without any matching xformer. http://www.11junk.com/jeffl/antennas/Monopole/monopole_1_250/slides/monopole_1_250.html If you look at the patterns at: http://www.11junk.com/jeffl/antennas/Monopole/index.html you'll see some interesting things. The pattern for the 1/2 wave monopole and shorter are all very similar. The gain is also fairly constant. I can't say the same for the impedance, which varies radically and the takeoff angle, which keeps creeping upward as the antenna gets longer. As the antenna gets really long, such as this 5 wavelength monopole monster: http://www.11junk.com/jeffl/antennas/Monopole/monopole_5_000/slides/pattern.html the major lobes are almost straight up, which might be useful for talking to satellites but not terrestrial repeaters. Note that the gain has increased to 10.7dBi or 5.5dB more than the 1/4 wave monopole. Lots more can be extracted from the simulations. I'll clean up the mess, contrive a web page, make it pretty, but not tonite. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Help with commercial VHF mobile antenna
On 6/27/2013 11:49 PM, Jeff Liebermann wrote:
On the short end of the scale, the 1/8th wave antenna at: http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html shows 254 ohms, which will work with a 2:1 turns ratio transformer. I don't think a transformer is a significant help. Without the transformer the SWR is about 158:1. With the transformer, the SWR is still up to about 61:1. That will probably kick in the SWR protection of the transmitter. John - KD5YI |
Help with commercial VHF mobile antenna
On Sun, 30 Jun 2013 07:24:34 -0500, John S
wrote: On 6/27/2013 11:49 PM, Jeff Liebermann wrote: On the short end of the scale, the 1/8th wave antenna at: http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html shows 254 ohms, which will work with a 2:1 turns ratio transformer. I don't think a transformer is a significant help. Without the transformer the SWR is about 158:1. With the transformer, the SWR is still up to about 61:1. That will probably kick in the SWR protection of the transmitter. John - KD5YI Nope. A 2:1 turns ratio tranformer will provide a 4:1 impedance ratio, not a 2:1 impedance ratio. The required transformer ratio would be: (254 / 50)^0.5 = sqrt(5) = 2.3 A 2:1 turns ratio xformer should be close enough. Another way is to take the 2:1 turns ratio transformer, which has a 4:1 impedance ratio, and divide the antenna impedance by the impedance ratio: 254 / 4 = 63.5 ohms. Not exactly 50 ohms, but close enough. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Help with commercial VHF mobile antenna
On Sun, 30 Jun 2013 10:58:08 -0700, Jeff Liebermann
wrote: On Sun, 30 Jun 2013 07:24:34 -0500, John S wrote: On 6/27/2013 11:49 PM, Jeff Liebermann wrote: On the short end of the scale, the 1/8th wave antenna at: http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html shows 254 ohms, which will work with a 2:1 turns ratio transformer. I don't think a transformer is a significant help. Without the transformer the SWR is about 158:1. With the transformer, the SWR is still up to about 61:1. That will probably kick in the SWR protection of the transmitter. John - KD5YI Nope. A 2:1 turns ratio tranformer will provide a 4:1 impedance ratio, not a 2:1 impedance ratio. The required transformer ratio would be: (254 / 50)^0.5 = sqrt(5) = 2.3 A 2:1 turns ratio xformer should be close enough. Another way is to take the 2:1 turns ratio transformer, which has a 4:1 impedance ratio, and divide the antenna impedance by the impedance ratio: 254 / 4 = 63.5 ohms. Not exactly 50 ohms, but close enough. Oops. My mistake. I couldn't recall if a 2:1 transformer referred to the turns ratio or the impedance ratio. I've seen it done both ways in other industries and transformer applications. I usually qualify the label with either turns or impedance ratio but forgot this time. However, skimming the available literature with Google, I find that the common usage for RF xformers is the impedance ratio. Therefore, your comments are correct and I should have specified a 4:1 transformer. Sorry(tm). -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Help with commercial VHF mobile antenna
On 6/30/2013 1:05 PM, Jeff Liebermann wrote:
On Sun, 30 Jun 2013 10:58:08 -0700, Jeff Liebermann wrote: On Sun, 30 Jun 2013 07:24:34 -0500, John S wrote: On 6/27/2013 11:49 PM, Jeff Liebermann wrote: On the short end of the scale, the 1/8th wave antenna at: http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html shows 254 ohms, which will work with a 2:1 turns ratio transformer. I don't think a transformer is a significant help. Without the transformer the SWR is about 158:1. With the transformer, the SWR is still up to about 61:1. That will probably kick in the SWR protection of the transmitter. John - KD5YI Nope. A 2:1 turns ratio tranformer will provide a 4:1 impedance ratio, not a 2:1 impedance ratio. The required transformer ratio would be: (254 / 50)^0.5 = sqrt(5) = 2.3 A 2:1 turns ratio xformer should be close enough. Another way is to take the 2:1 turns ratio transformer, which has a 4:1 impedance ratio, and divide the antenna impedance by the impedance ratio: 254 / 4 = 63.5 ohms. Not exactly 50 ohms, but close enough. Oops. My mistake. I couldn't recall if a 2:1 transformer referred to the turns ratio or the impedance ratio. I've seen it done both ways in other industries and transformer applications. I usually qualify the label with either turns or impedance ratio but forgot this time. However, skimming the available literature with Google, I find that the common usage for RF xformers is the impedance ratio. Therefore, your comments are correct and I should have specified a 4:1 transformer. Sorry(tm). No problem and no reason to apologize. For the sake of those who read this forum, I will provide my analysis upon request. John - KD5YI |
The ladder line portion of the G5RV is the matching network.
You cannot use a tuner with a tuner. If the matching network is the ladder line and you connect a tuner to it - yes you can trick the transceiver into believing that is is seeing a 50 ohm matched load - but all you are going to create is heat. On the other side of the coin, I hear all the time - I can work everything that I can hear - with my G5RV - the problem is - what can you hear? Unless you have a real 80 meter dipole and you compare them side by side - within one hour of each other, at the same height and in the same neighborhood - you cannot compare the two. In the end - you will realize that the efficiency is so low - you are not hearing much - just the strongest of signals - when the band is open, and not much of anything when the bands are no cooperating. The thing that tricks people into thinking that they are doing something is the fact that they see 100 watts into the meter and they think that they are modulating all 100 watts - when in fact a single side splatter signal is only fully modulated part of the time - most of the time - we aren't really using more then maybe 15 or 20 watts out of 100. Only the digital modes and CW - which is the original digital modes - dots and dah's - is 100% fully modulated. That is the reason why we turn down the power when we work digital modes. Most transceivers do not have a 100% duty cycle - hence if you operate at 100 watts for very long - your transceiver will not take it! |
Help with commercial VHF mobile antenna
On Thu, 4 Jul 2013 00:33:18 +0100, Channel Jumper
wrote: You cannot use a tuner with a tuner. Nope. I've done that for fun. I just happen to have two identical MFJ tuners available and thought it might be amusing to put them back to back and measure the losses at the 50 ohm output. One tuner was set to be capacitive, while the other was matched to have the conjugate inductive reactance. It worked nicely until I tried 80 meters, where I heard some internal arcing. Measured losses were fairly high on 40 and 75 meters. If the matching network is the ladder line and you connect a tuner to it - yes you can trick the transceiver into believing that is is seeing a 50 ohm matched load - but all you are going to create is heat. Baloney. The losses come from the limited Q and high resistive losses of the inductors used in the antenna tuner. That's why really good antenna tuners use big fat silver plated coils. Try it yourself with this Java app: http://www.rsq-info.net/PSK-modelling.html You'll start to see substantial losses on 80 meters with the default values. The example uses Q=100 for the inductor, which might be a bit optimistic for 80 meters. (I haven't done a tuner in 30 years so I forget the typical Q values). If you plug in real values extracted from your favorite MFJ antenna tuner, you'll see losses at higher frequencies. On the other side of the coin, I hear all the time - I can work everything that I can hear - with my G5RV - the problem is - what can you hear? Unless you have a real 80 meter dipole and you compare them side by side - within one hour of each other, at the same height and in the same neighborhood - you cannot compare the two. In the end - you will realize that the efficiency is so low - you are not hearing much - just the strongest of signals - when the band is open, and not much of anything when the bands are no cooperating. Sigh. In the 1970's, I did some work with diversity reception on HF. In order for diversity to work, the reception between the two antennas needed to be different presumably via a different skywave path. The tests were on WWV at 2.5, 5.0, 10.0, and 15.0Mhz with a simple dipole and balun tuned to 5.0Mhz. We started with the antennas on opposite sides of the parking lot. The signal levels tracked each other. I ran 1000ft of RG-58c/u down the roadway and the signal still tracked. I ran another 1000ft down the roadway in the opposite direction, and the signals still tracked. I moved one of the receivers about 10,000 ft away and ran twisted pair audio back to the factory. Finally, with 11,000ft of separation, I was able to see frequency selective fading at HF frequencies suitable for diversity reception. (Incidentally, this was adjacent to SJO airport, which added a political layer to such testing). The real problem with comparing antennas closely located is that they interact with each other. Ideally, I would want to see 2-3 wavelengths separation between antennas to prevent interaction. Well, at 80 meters, that's 500 to 750 ft separation, which is difficult to achieve. For added amusement and confusion, there's the commonly ignored problem of takeoff angle. The usual drawings in the books show a signal bouncing between the ground and the ionosphere several time with the angle of incidence equal to the angle of reflection. We'll it doesn't quite work like that. There was an article in QST last year demonstrating that the signal comes from directly overhead. While DX'er try to optimize the takeoff angle to match the equal angles of incidence and reflection, perhaps it would more interesting to try maximizing the gain straight up? I'll see if I can find the issue and article. How the G5RV fits into the picture is beyond my limited imagination. The thing that tricks people into thinking that they are doing something is the fact that they see 100 watts into the meter and they think that they are modulating all 100 watts - when in fact a single side splatter signal is only fully modulated part of the time - most of the time - we aren't really using more then maybe 15 or 20 watts out of 100. Well, you can set the % modulation to 100% and get 100% modulation. The problem is that it can easily splatter as you describe. 25% of CW power is the recommended maximum. Note that none of this diversion has anything to do with antennas. Only the digital modes and CW - which is the original digital modes - dots and dah's - is 100% fully modulated. Wrong. Percent modulation is the radio of the peak-to-peak voltage at the waveform peaks, divided into the peak-to-peak voltage in the modulation troughs, as shown on an oscilloscope. 100% is very common and easily obtained. Please look at the RF on a scope and see for yourself. http://electriciantraining.tpub.com/14193/css/14193_146.htm That is the reason why we turn down the power when we work digital modes. Nope. The reason we turn down the percent modulation is to reduce splatter, not because the transmitter is somehow inherently unable to produce 100% modulation. Most transceivers do not have a 100% duty cycle - hence if you operate at 100 watts for very long - your transceiver will not take it! Wrong again. The reason for the low percentage of modulation for most digital modes is to keep the occupied bandwidth fairly reasonable. As you approach 100% modulation, the signal starts to become wide and begins to splatter. Beyond 100%, it's really wide and ugly. Here's the math for PSK31: http://www.rsq-info.net/PSK-modelling.html Compare the occupied bandwidth and spurious junk at 25% modulation (Fig 3) with the others showing various anomalies. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Help with commercial VHF mobile antenna
On Wed, 03 Jul 2013 18:36:44 -0700, Jeff Liebermann wrote:
On Thu, 4 Jul 2013 00:33:18 +0100, Channel Jumper wrote: You cannot use a tuner with a tuner. Nope. I've done that for fun. I just happen to have two identical MFJ tuners available and thought it might be amusing to put them back to back and measure the losses at the 50 ohm output. One tuner was set to be capacitive, while the other was matched to have the conjugate inductive reactance. It worked nicely until I tried 80 meters, where I heard some internal arcing. Measured losses were fairly high on 40 and 75 meters. If the matching network is the ladder line and you connect a tuner to it - yes you can trick the transceiver into believing that is is seeing a 50 ohm matched load - but all you are going to create is heat. Baloney. The losses come from the limited Q and high resistive losses of the inductors used in the antenna tuner. That's why really good antenna tuners use big fat silver plated coils. Try it yourself with this Java app: http://www.rsq-info.net/PSK-modelling.html You'll start to see substantial losses on 80 meters with the default values. The example uses Q=100 for the inductor, which might be a bit optimistic for 80 meters. (I haven't done a tuner in 30 years so I forget the typical Q values). If you plug in real values extracted from your favorite MFJ antenna tuner, you'll see losses at higher frequencies. On the other side of the coin, I hear all the time - I can work everything that I can hear - with my G5RV - the problem is - what can you hear? Unless you have a real 80 meter dipole and you compare them side by side - within one hour of each other, at the same height and in the same neighborhood - you cannot compare the two. In the end - you will realize that the efficiency is so low - you are not hearing much - just the strongest of signals - when the band is open, and not much of anything when the bands are no cooperating. Sigh. In the 1970's, I did some work with diversity reception on HF. In order for diversity to work, the reception between the two antennas needed to be different presumably via a different skywave path. The tests were on WWV at 2.5, 5.0, 10.0, and 15.0Mhz with a simple dipole and balun tuned to 5.0Mhz. We started with the antennas on opposite sides of the parking lot. The signal levels tracked each other. I ran 1000ft of RG-58c/u down the roadway and the signal still tracked. I ran another 1000ft down the roadway in the opposite direction, and the signals still tracked. I moved one of the receivers about 10,000 ft away and ran twisted pair audio back to the factory. Finally, with 11,000ft of separation, I was able to see frequency selective fading at HF frequencies suitable for diversity reception. (Incidentally, this was adjacent to SJO airport, which added a political layer to such testing). The real problem with comparing antennas closely located is that they interact with each other. Ideally, I would want to see 2-3 wavelengths separation between antennas to prevent interaction. Well, at 80 meters, that's 500 to 750 ft separation, which is difficult to achieve. For added amusement and confusion, there's the commonly ignored problem of takeoff angle. The usual drawings in the books show a signal bouncing between the ground and the ionosphere several time with the angle of incidence equal to the angle of reflection. We'll it doesn't quite work like that. There was an article in QST last year demonstrating that the signal comes from directly overhead. While DX'er try to optimize the takeoff angle to match the equal angles of incidence and reflection, perhaps it would more interesting to try maximizing the gain straight up? I'll see if I can find the issue and article. How the G5RV fits into the picture is beyond my limited imagination. The thing that tricks people into thinking that they are doing something is the fact that they see 100 watts into the meter and they think that they are modulating all 100 watts - when in fact a single side splatter signal is only fully modulated part of the time - most of the time - we aren't really using more then maybe 15 or 20 watts out of 100. Well, you can set the % modulation to 100% and get 100% modulation. The problem is that it can easily splatter as you describe. 25% of CW power is the recommended maximum. Note that none of this diversion has anything to do with antennas. Only the digital modes and CW - which is the original digital modes - dots and dah's - is 100% fully modulated. Wrong. Percent modulation is the radio of the peak-to-peak voltage at the waveform peaks, divided into the peak-to-peak voltage in the modulation troughs, as shown on an oscilloscope. 100% is very common and easily obtained. Please look at the RF on a scope and see for yourself. http://electriciantraining.tpub.com/14193/css/14193_146.htm That is the reason why we turn down the power when we work digital modes. Nope. The reason we turn down the percent modulation is to reduce splatter, not because the transmitter is somehow inherently unable to produce 100% modulation. Most transceivers do not have a 100% duty cycle - hence if you operate at 100 watts for very long - your transceiver will not take it! Wrong again. The reason for the low percentage of modulation for most digital modes is to keep the occupied bandwidth fairly reasonable. As you approach 100% modulation, the signal starts to become wide and begins to splatter. Beyond 100%, it's really wide and ugly. Here's the math for PSK31: http://www.rsq-info.net/PSK-modelling.html Compare the occupied bandwidth and spurious junk at 25% modulation (Fig 3) with the others showing various anomalies. Gee. I sure hope the OP got "Help with commercial VHF mobile antenna". |
Help with commercial VHF mobile antenna
On 4 Jul 2013 12:44:59 GMT, Allodoxaphobia
wrote: Gee. I sure hope the OP got "Help with commercial VHF mobile antenna". Well, I posted a photo of what I suspected was his "VHF" antenna, which turned out to be an 850MHz antenna. The dimensions fit. http://www.11junk.com/jeffl/antennas/Misc/slides/Motorola-850mhz-NMO.html Nobody seemed to care much about answering the question. Of course, the OP (Mr Kickin' Ass and Takin' Names) didn't bother to respond, so I'll assume he doesn't care and has turned the problem over to the Motorola shop, which will surely find the most expensive replacement antenna available. Drivel: I wrote one of many Usenet rules in about 1995. Some applicable quotes: No usenet discussion can survive without topic drift after about 5 replies. The really good postings, the ones that are illuminating, informative, and worth keeping, usually receive no replies or comments. The higher the authority, the bigger the mistakes. Usenet postings are not written for the benefits of the current reader. Rather, they are historical documents, written for the benefit of future readers, who will then cite the incorrect information within to perpetuate the mistakes. Those who don't bother to trim their quotes, also don't bother to read what's in the quotes. One line unsubstantiated replies are usually not worth reading. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Help with commercial VHF mobile antenna
"Jeff Liebermann" wrote in message ... On 4 Jul 2013 12:44:59 GMT, Allodoxaphobia wrote: Gee. I sure hope the OP got "Help with commercial VHF mobile antenna". Well, I posted a photo of what I suspected was his "VHF" antenna, which turned out to be an 850MHz antenna. The dimensions fit. http://www.11junk.com/jeffl/antennas/Misc/slides/Motorola-850mhz-NMO.html Nobody seemed to care much about answering the question. Of course, the OP (Mr Kickin' Ass and Takin' Names) didn't bother to respond, so I'll assume he doesn't care and has turned the problem over to the Motorola shop, which will surely find the most expensive replacement antenna available. He probably got put off when many on here were siding with the Motorola shop instead of him. |
Help with commercial VHF mobile antenna
On Thu, 4 Jul 2013 12:48:10 -0400, "Ralph Mowery"
wrote: He probably got put off when many on here were siding with the Motorola shop instead of him. It's possible. I have no way to tell. Reading between the lines in the original posting: "I an thinking about installing a full-length 5/8-wave whip, but, we go into a lot of driveways with low tree limbs and I doubt a full-length antenna would survive very long." it seems like the antenna is on a county owned service tall van or high truck. They probably have a service contract with the local Motorola MSS to maintain the county owned radios. The shop is required to use only genuine Motorola parts. If he wants to keep his contract, he's probably stuck with using whatever the shop wants. Incidentally, I forgot to mumble something about how to deal with tree branch grabbing springs and coils. The problem is that a bent over spring has large gaps into which tree branches fit nicely. When the antenna straightens up again, it locks the branch into it's stainless grip, and begins a tug of war. Sometimes, the grip is strong enough to rip the antenna out of the vehicle roof. Protecting coils are easy. Just use shrink tube over the coil. Springs are not so easy. I tried various experiments and eventually settled on flexible irrigation pipe or vinyl tubing. Find a size that slips over the spring loosely. Run a ty-wrap around only the top of the spring, not the bottom. When bent over, this sleeve will slide upward, so make it a bit longer than the spring. I tried a sheet of vinyl wrapped around the spring, but that tended to fall apart. Best to use tubing. Yes, it's ugly, but the uglier the antenna, the better it works. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Help with commercial VHF mobile antenna
On Wednesday, July 3, 2013 8:36:44 PM UTC-5, Jeff Liebermann wrote:
There was an article in QST last year demonstrating that the signal comes from directly overhead. While DX'er try to optimize the takeoff angle to match the equal angles of incidence and reflection, perhaps it would more interesting to try maximizing the gain straight up? Heck, that's what we have done for years on the lower bands. "NVIS" On 80m, with the usual distances used for general jibber jabber, most signals do arrive at fairly high angles. And this was always on our minds when choosing an antenna. But it's fairly handy that a dipole or horizontal loop at the most used heights does shoot the bulk of the power at high angles, with max often straight up. For rag chew type stuff close in, a dipole is almost always preferred over a vertical. In my case, I always had max gain at high angles, so the only thing left to improve was system efficiency. Which leads me to feed with coax with no tuner used for a very high system efficiency. Coax is slightly more lossy than ladder line, but at 4 mhz the loss using good coax is so low you would be hard pressed to tell the difference with the average length feed line vs say the Cecil method using a tuned ladder line with no tuner. |
Help with commercial VHF mobile antenna
On 6/26/2013 2:03 PM, Jeff Liebermann wrote:
On Wed, 26 Jun 2013 18:22:44 +0100, Channel Jumper wrote: And so Jeff speaks. snip The bottom line is - most people involved in communications doesn't just start selling radios without any type of formal education. Which apparently he hasn't had since he can't build a sentence properly. I ran a 2-way radio shop for many years in Stanton CA. The best salesman didn't know anything about radio. That was my job. I went with him to meetings and filled in the techy details. Later, other employers demonstrated the same principle. At one place, the only technically competent person in management was the VP of engineering. Both sales and marketing were clueless and relied on engineering to deal with the technical details. I'm not sure how much formal education any of these people had but they were all very effective at selling. Good way to do it. I am the sales engineer and the sales team knows when they are skirting the edge of their knowledge and brings me in. As far as I know this is how it works everywhere when there is tech involved. Even if the only education the person received was from the Military, it is usually based on sound practices and principals. Nope. They are taught just enough to get it done, and often done poorly. Unless things have changed. If someone wants to tell me how to do something that I have been doing for 40 years - I just walk away. I pity your customers, since you appear to think you know it all. I'm still learning and will until the day I die. tom K0TAR |
Help with commercial VHF mobile antenna
On 7/4/2013 11:14 AM, Jeff Liebermann wrote:
One line unsubstantiated replies are usually not worth reading. I can prove that's wrong. tom K0TAR |
Help with commercial VHF mobile antenna
"tom" wrote in message ... On 6/26/2013 2:03 PM, Jeff Liebermann wrote: On Wed, 26 Jun 2013 18:22:44 +0100, Channel Jumper wrote: And so Jeff speaks. snip The bottom line is - most people involved in communications doesn't just start selling radios without any type of formal education. Which apparently he hasn't had since he can't build a sentence properly. You mean the verb doesn't always agree with the nearest noun?!? Who'd'a thunk it? (That's one of my pettest of peeves, by the way.) "Sal" (KD6VKW) |
Help with commercial VHF mobile antenna
On Sun, 07 Jul 2013 19:26:04 -0500, tom wrote:
On 7/4/2013 11:14 AM, Jeff Liebermann wrote: One line unsubstantiated replies are usually not worth reading. I can prove that's wrong. +1 |
Help with commercial VHF mobile antenna
On 7/7/2013 9:52 PM, Sal wrote:
"tom" wrote in message ... On 6/26/2013 2:03 PM, Jeff Liebermann wrote: On Wed, 26 Jun 2013 18:22:44 +0100, Channel Jumper wrote: And so Jeff speaks. snip The bottom line is - most people involved in communications doesn't just start selling radios without any type of formal education. Which apparently he hasn't had since he can't build a sentence properly. You mean the verb doesn't always agree with the nearest noun?!? Who'd'a thunk it? (That's one of my pettest of peeves, by the way.) "Sal" (KD6VKW) I read scopes of work (and other corp. docs) as a significant part of my living. In other words, I'm a proofreader. So I understand being picky about building sentences and paragraphs and documents. It has to make sense as a whole, and if someone can't make even a single sentence work, they surely can't make the whole work. tom K0TAR |
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