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swr goes up on antenna
I have a home buit version of the Carolina Windom. An off center fed
antenna about 120 feet long with a 4:1 voltage balun and from the feedpoint it goes to an inline ferrite bead choke 20 feet from the feed point, then 80 feet of rg-8 to the shack. The balun is suspose to be rated for 3 kw. It does have 2 cores in it. By tests, I know if I run ssb at over 800 watts the balun will heat up and change the swr. I have noticed lately that running just 100 watts ssb on 80 meters the swr seems to be going up to about 2:1 and the rig cuts the power back as expected as I talk from a starting point of 1:1. That has hapened for the last two mornings. I don't recall it doing that before. The antenna has been up for several years. It is just over 1:1 when normal on the frequency I most often operate on. Today in the afternoon when 80 meters was dead I transmitted a carrier for about 5 minuits and let off to ID, then another carrier for about 5 minuits and the swr did not change. Any ideas why the swr went up for the last two mornings, but did not seem to go up this afternoon ? --- This email has been checked for viruses by Avast antivirus software. http://www.avast.com |
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SWR is a Citizens Band Radio terminology that describes events on the feed line. VSWR - Voltage Standing Wave Ratio refers to the voltage present. If you increase the voltage, what happens to the SWR? IT GOES UP! For some reason you don't even know how to use speel check, yet you must think that you are some kind of antenna expert. The permeability of the core of the balun is determined by it's quality. It appears to me as if you are cheap and you bought the cheapest balun you could find, trying to save money. If you do some research on the Carolina Windom antenna, you will read that if it's design frequency is 80 / 75m, that it will not work on 160 or 40m... Even if it is designed for 80m, 80m is such a large band, you only get about 455 kc's of resonance, and everywhere else it is not resonant. Had you bought a Guanella-balun and had this problem I would say that the Balun was bad. Because I do not know the origin of the balun you used, I can only make the same assumption. |
I'm not quite sure, but I don't think that it is legal, according to the Part 97 to transmit a carrier without saying anything.
One other thing - as Columbo would say, the SWR meter does not indicate resonance, it only tells you what the feed line tells it. By changing the length of the coax you can make the antenna appear as being resonant or non resonant, even though the antenna may or may not be a perfect 50 ohm load. Instead of taking down the antenna and looking at the components, you chose to visit this forum and ask us to look into our crystal ball for a solution to your problem. There is no way for sure for us to know how the PL connectors were installed, the condition of the coax you used for a feed line, the condition of the coax you used to build the antenna and the soldered connectors in between each and every junction. ONE guess would be that you have a intermittent loose connection and today it chose to work and tomorrow when it rains, snows, wind blows etc, it might not make contact and present a high swr as you put it. Find someone with an antenna analyzer to diagnose your problem for you. |
swr goes up on antenna
On 12/18/2014 2:12 PM, Channel Jumper wrote:
I'm not quite sure, but I don't think that it is legal, according to the Part 97 to transmit a carrier without saying anything. FCC demands that you must issue your call sign within the period required after beginning your transmission as he did. One other thing - as Columbo would say, the SWR meter does not indicate resonance, it only tells you what the feed line tells it. By changing the length of the coax you can make the antenna appear as being resonant or non resonant, even though the antenna may or may not be a perfect 50 ohm load. Your first sentence could be construed as accurate, depending on accepted terminology. Your second sentence is inaccurate and is in conflict with your first sentence. Instead of taking down the antenna and looking at the components, you chose to visit this forum and ask us to look into our crystal ball for a solution to your problem. I'm thinking you do not have such a crystal ball or you would have used it show your superiority over everyone else here. There is no way for sure for us to know how the PL connectors were installed, the condition of the coax you used for a feed line, the condition of the coax you used to build the antenna and the soldered connectors in between each and every junction. Who is "us"? You have someone sitting beside you? So, you want his problem handed to you(and the person next to you) on a platter? You don't really know how to help him, in other words. ONE guess would be that you have a intermittent loose connection and today it chose to work and tomorrow when it rains, snows, wind blows etc, it might not make contact and present a high swr as you put it. Perhaps, as you put it. Can you suggest some tests that might be helpful for him? How would you suggest making tests or measurements that you think could be problems? Find someone with an antenna analyzer to diagnose your problem for you. Oh yeah! And learn nothing in the process. You would be standing around for weeks with an analyzer in your hands waiting for the first sign of intermittent connections. Then what? Your measurements have already told you have a problem. Forget that Channel Jumper non-ham idiot. You are much more capable than him. Try some more things and report back. Shake your feeder (you know what I mean) and have someone watch your meter. Look at each end of both the feeder and antenna to make sure you have no corrupted insulators or limbs touching. With a digital ohmmeter, you can measure resistance across the insulators (power off, of course). The idea is: can I find a way to measure something using what tools I have to give me a lead to the problem? Keep looking. The group would love to help, I'm sure. There are lots of gurus here. Good Luck! |
swr goes up on antenna
In article ,
Channel Jumper wrote: I'm not quite sure, but I don't think that it is legal, according to the Part 97 to transmit a carrier without saying anything. That is an over-generalization. Part 97 does allow you to make brief one-way transmissions, for the purpose of making adjustments to your station. Evaluating SWR or adjusting a tuner would fall into this category. The test transmission itself can be pretty much anything, as long as it does not violate any other terms of Part 97 (i.e. no foul language, no music, no interference with other stations on the frequency, and transmitted in a frequency allowed by your license privileges). A "blank" CW carrier need not violate any of these. And, in fact, that's just what many modern radios transmit whenever you hit the "tune" button to adjust the auto-tuner. You're still required to identify yourself somehow (verbally or CW) every ten minutes and at the end of the transmission. So, you do have to "say something", but you don't have to do it as part of the test carrier transmission itself... just do it when you're done, or after ten minutes (whichever comes sooner). |
swr goes up on antenna
On Wednesday, December 17, 2014 6:34:22 PM UTC-6, Ralph Mowery wrote:
I have a home buit version of the Carolina Windom. An off center fed antenna about 120 feet long with a 4:1 voltage balun and from the feedpoint it goes to an inline ferrite bead choke 20 feet from the feed point, then 80 feet of rg-8 to the shack. The balun is suspose to be rated for 3 kw. It does have 2 cores in it. By tests, I know if I run ssb at over 800 watts the balun will heat up and change the swr. I have noticed lately that running just 100 watts ssb on 80 meters the swr seems to be going up to about 2:1 and the rig cuts the power back as expected as I talk from a starting point of 1:1. That has hapened for the last two mornings. I don't recall it doing that before. The antenna has been up for several years. It is just over 1:1 when normal on the frequency I most often operate on. Today in the afternoon when 80 meters was dead I transmitted a carrier for about 5 minuits and let off to ID, then another carrier for about 5 minuits and the swr did not change. Any ideas why the swr went up for the last two mornings, but did not seem to go up this afternoon ? Hard to say really, but sounds sort of moisture related.. Maybe moisture freezing, and then melting, or wet moisture that later dries out. Just a guess though.. If that balun warms up, it's adding a substantial amount of loss. Not really related to your problem, but I hate to see perfectly useful RF turn to heat. :| Also kind of verifies my theory of the cause of the loss I saw when using one of those antennas at a field day several years ago. I had compared it to a regular coax fed dipole, and it was way down from the dipole. I always blamed the voltage balun it used, and your experience sort of verifies that assumption. |
swr goes up on antenna
wrote in message ... Hard to say really, but sounds sort of moisture related.. Maybe moisture freezing, and then melting, or wet moisture that later dries out. Just a guess though.. If that balun warms up, it's adding a substantial amount of loss. Not really related to your problem, but I hate to see perfectly useful RF turn to heat. :| Also kind of verifies my theory of the cause of the loss I saw when using one of those antennas at a field day several years ago. I had compared it to a regular coax fed dipole, and it was way down from the dipole. I always blamed the voltage balun it used, and your experience sort of verifies that assumption. While some of the RF is converted to heat, I also have a 80 meter dipole at the same height and at right angles to the OCF antenna. Switching back and forth between them, the OCF is usually beter. In a few cases the plane 80 mete dipole without a blun will work beter on 80 meters. Just a few months ago I hung an 18 and 24 MHz dipole about 6 inches bleow the 80 meter dipole and fed off the same coax. Still the OCF is usually beter. The ends of both antennas are about 50 to 60 feet off the ground and not suported in the middle or at the feed point. --- This email has been checked for viruses by Avast antivirus software. http://www.avast.com |
swr goes up on antenna
On Thu, 18 Dec 2014 20:12:53 +0000, Channel Jumper
wrote: I'm not quite sure, but I don't think that it is legal, according to the Part 97 to transmit a carrier without saying anything. Wrong. Part 97.305(a)(b) (a) Except as specified elsewhere in this part, an amateur station may transmit a CW emission on any frequency authorized to the control operator. (b) A station may transmit a test emission on any frequency authorized to the control operator for brief periods for experimental purposes,... http://www.hallikainen.org/FCC/FccRules/2013/97/305/index.php Actually, you're half right. Much of what I hear on the air is seriously lacking in content and only a little better than not saying anything. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
swr goes up on antenna
On Thursday, December 18, 2014 5:05:35 PM UTC-6, Ralph Mowery wrote:
While some of the RF is converted to heat, I also have a 80 meter dipole at the same height and at right angles to the OCF antenna. Switching back and forth between them, the OCF is usually beter. In a few cases the plane 80 mete dipole without a blun will work beter on 80 meters. Just a few months ago I hung an 18 and 24 MHz dipole about 6 inches bleow the 80 meter dipole and fed off the same coax. Still the OCF is usually beter. The ends of both antennas are about 50 to 60 feet off the ground and not suported in the middle or at the feed point. I was thinking in terms of 80m. I suppose it's possible the windom could be better on some of the other bands. IE: a 80 dipole is not going to be too good on 40m unless you use a low loss method of feeding the high Z antenna. |
swr goes up on antenna
On Thursday, December 18, 2014 2:13:00 PM UTC-6, Channel Jumper wrote:
One other thing - as Columbo would say, the SWR meter does not indicate resonance, it only tells you what the feed line tells it. By changing the length of the coax you can make the antenna appear as being resonant or non resonant, even though the antenna may or may not be a perfect 50 ohm load. If changing the length of the coax makes large changes in the SWR, that just shows you have poor decoupling from the antenna to the feed line. Need a balun or choke, or a better balun or choke than what is being used. What you state is largely a CB radio wives tale, due to most of them not properly decoupling the antenna from the line. With proper decoupling, the length of the coax will have little bearing on the SWR seen at the rig. It will be the same as what is seen at the antenna input, minus any decrease in SWR due to coax loss. IE: very high coax loss can make anything look good at the rig. |
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On 12/19/2014 2:33 AM, Jeff wrote:
By changing the length of the coax you can make the antenna appear as being resonant or non resonant, even though the antenna may or may not be a perfect 50 ohm load. Incorrect, changing the length of the feeder will not change the SWR beyond any extra loss in the cable. It will change the phase of the mismatch that is all; (rotate around a constant VSWR circle on a Smith Chart). Jeff Incorrect. The basic VSWR meter measures the voltage, not the power. And if the SWR is other than 1:1, this voltage will change depending on the distance to the mismatch. Additionally, a shorted coax 1/2 wavelength long shows a short (0 ohms). But a shorted 1/4 wavelength coax shows an open (infinite impedance). Somewhere in between (I'm not going to bother to figure out exactly where because it's not that important) it will show an effective 50 ohm impedance. Coax length is unimportant when you have a 1:1 SWR, but if you don't, the coax will act as a matching network. And length will affect the overall system. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
swr goes up on antenna
On 12/19/2014 10:14 AM, Jerry Stuckle wrote:
On 12/19/2014 2:33 AM, Jeff wrote: By changing the length of the coax you can make the antenna appear as being resonant or non resonant, even though the antenna may or may not be a perfect 50 ohm load. Incorrect, changing the length of the feeder will not change the SWR beyond any extra loss in the cable. It will change the phase of the mismatch that is all; (rotate around a constant VSWR circle on a Smith Chart). Jeff Incorrect. The basic VSWR meter measures the voltage, not the power. And if the SWR is other than 1:1, this voltage will change depending on the distance to the mismatch. Additionally, a shorted coax 1/2 wavelength long shows a short (0 ohms). But a shorted 1/4 wavelength coax shows an open (infinite impedance). Somewhere in between (I'm not going to bother to figure out exactly where because it's not that important) it will show an effective 50 ohm impedance. Please bother. It is important to me to understand these things. And are you saying that there is a length of transmission line that will satisfy both the shorted and open conditions. What do you mean by "effective"? Are the lengths different in the two cases? Please help and thanks. Coax length is unimportant when you have a 1:1 SWR, but if you don't, the coax will act as a matching network. And length will affect the overall system. |
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If you transmit a dead carrier and you don't say anything you are in fact violating the Part 97! All you people did was reaffirm what I had already said. The sad fact is that we give out licenses like lolly pops at a barber shop with no real instruction involved and once a person gets a license they automatically think that they don't need to know the rules or anything that they learned to pass the exam to get the license. Had we had real Elmer's when we gave out those licenses, we wouldn't have 90% of the garbage we hear today on the HF radio! As far as amplifiers goes, yes I can see someone using a amplifier on 160 meters in the summertime, but the rest of the time, all they are doing is broadcasting - not really serving any purpose. If the OP bought a commercial OCFD in the first place - he wouldn't have these problems. The people that made comments about their OCFD out performing a center cut dipole for X frequency doesn't understand how a OCFD works. If you have a 80m OCFD and you use it on 10 meters, it acts like a 8 wavelength long 10m dipole. You actually get a realization of GAIN from the OCFD when you use it on 40 or 20 or 10m... Unfortunately in my book to realize gain you must give up something in one direction to improve your signal in another. So the term GAIN really isn't relevant here. Instead of using the term GAIN I should say improvement, because improvement would be a more relative term. |
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The reason I use the term CB radio is because most people gets their start on the Chicken Band and you don't have to have a lot of intelligence to use something that only takes place on less then 1 MC of bandwidth. You can practically cut a piece of wire to a resonant length and throw it up in a tree and not even use a piece of coax and talk on all 40 channels on a CB radio without a tuner. When you become a ham and you want to talk everywhere, you must learn a little about feed lines and antenna's and what works and what does not and what a antenna tuner really does and what it cannot do. The most hilarious people to me are the ones that uses non resonant antenna's, an amplifier and a antenna tuner and just because their signal is loud and strong they think that they can overcome being resonant. They end up talking to other LIDS with the same situation and the only people they can hear are those that are the loudest! I have owned a G5RV that I bought at a hamfest 4 years ago that was only out of the bag once, and even then only for one week. The first time I tried to operate on 40m with it 20' off the ground, which was as high as I could get it on poles placed on my lot - everyone on 40m told me to get the heck off the G5RV, which was incentive enough to find something better and quit fooling around with junk! As far as me not being a ham - you have your opinions and I have mine! 10-4 Rodger Dodger, I'm gone! |
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"Jerry Stuckle" wrote in message ... On 12/19/2014 2:33 AM, Jeff wrote: By changing the length of the coax you can make the antenna appear as being resonant or non resonant, even though the antenna may or may not be a perfect 50 ohm load. Incorrect, changing the length of the feeder will not change the SWR beyond any extra loss in the cable. It will change the phase of the mismatch that is all; (rotate around a constant VSWR circle on a Smith Chart). Jeff # Incorrect. The basic VSWR meter measures the voltage, not the power. # And if the SWR is other than 1:1, this voltage will change depending on # the distance to the mismatch. But isn't it still Vmax over Vmin, regardless of where that happens on the feedline? # Additionally, a shorted coax 1/2 wavelength long shows a short (0 ohms). # But a shorted 1/4 wavelength coax shows an open (infinite impedance). # Somewhere in between (I'm not going to bother to figure out exactly # where because it's not that important) it will show an effective 50 ohm # impedance. # Coax length is unimportant when you have a 1:1 SWR, but if you don't, # the coax will act as a matching network. And length will affect the # overall system. I always understood the VSWR to be constant with the feedline length moving the parameters around the Smith chart constant VSWR circle. Thus it is possible by changing length to provide an antenna tuner with R and X values that the tuner can handle better. |
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On Fri, 19 Dec 2014 16:38:58 +0000, Channel Jumper
wrote: If you transmit a dead carrier and you don't say anything you are in fact violating the Part 97! As long as you identify yourself at the end of the transmission (and every 10 minutes), and not interfering with anyone, methinks it's legal. There's no lower limit on the speed of a CW transmission. Let's do the math for the MINIMUM Morse code speed: http://www.w8ji.com/cw_bandwidth_described.htm 1 / (0.002 WPM * 0.83 baud/wpm) = 602 seconds (10 mins) So, if I were experimenting with ultra narrow band long range Morse code or data transmission, that requires that I send very slowly, my MINIMUM speed would be 0.002 words per minute before one needs to identify. Disclaimer: I am not an attorney. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
swr goes up on antenna
"Channel Jumper" wrote in message ... Sorry Jeff, but you just shot yourself in the foot! If you transmit a dead carrier and you don't say anything you are in fact violating the Part 97! All you people did was reaffirm what I had already said. The sad fact is that we give out licenses like lolly pops at a barber shop with no real instruction involved and once a person gets a license they automatically think that they don't need to know the rules or anything that they learned to pass the exam to get the license. Had we had real Elmer's when we gave out those licenses, we wouldn't have 90% of the garbage we hear today on the HF radio! As far as amplifiers goes, yes I can see someone using a amplifier on 160 meters in the summertime, but the rest of the time, all they are doing is broadcasting - not really serving any purpose. If the OP bought a commercial OCFD in the first place - he wouldn't have these problems. The people that made comments about their OCFD out performing a center cut dipole for X frequency doesn't understand how a OCFD works. If you have a 80m OCFD and you use it on 10 meters, it acts like a 8 wavelength long 10m dipole. You actually get a realization of GAIN from the OCFD when you use it on 40 or 20 or 10m... Unfortunately in my book to realize gain you must give up something in one direction to improve your signal in another. So the term GAIN really isn't relevant here. Instead of using the term GAIN I should say improvement, because improvement would be a more relative term. Yes, I got my ham license out of the same Cracker Jack box that I got my First Class Phone license out of around 1972. The OCF I have is home built, but the blaun was bought. It does not take too much to add about 40 feet of wire to one side and 80 feet to the other side. The testing I did was on 80 meters around 2 or 3 in the afteroon. The band was almost dead, Not a signal within 50 khz of either side of where I was testing. I did ID and ask if the frequency was in useseveral times during the test. Ham radio is partly for testing and experminenting. That is what I was doing for about the 10 minuits total. You can look in the books and theory and everything else, but unless you put up several antennas and compair them in the same area like I have , it is all just a guess. The ground, trees,and atmosphere are almost impossiable to modle on a computer for every location. The OCF I was compairing was to an 80 meter dipole on 80 meters only, not other bands. Without being able to rotate an anenna, you have to take what you get as far as where the main lobe of the signal goes. Most of the ones that complain about amplifiers not being needed are the ones that don't have one,but wish they did. I seldom run one,but at times it does make a big differance in making good contacts and not. Same as with a beam. People with poor antennas say they can work all they can hear,but they do not realise they are not hearing much either. I don't knock people with poor antennas. At one time I did not have a very good antenna system either. Lived on a small lot without any trees and the best I could do was a dipole up about 20 feet. Still had fun with what I had. The name is real and the call is KU4PT, unlike some that post on here witout a real name or showing a call. --- This email has been checked for viruses by Avast antivirus software. http://www.avast.com |
swr goes up on antenna
On 12/19/2014 12:10 PM, Wayne wrote:
"Jerry Stuckle" wrote in message ... On 12/19/2014 2:33 AM, Jeff wrote: By changing the length of the coax you can make the antenna appear as being resonant or non resonant, even though the antenna may or may not be a perfect 50 ohm load. Incorrect, changing the length of the feeder will not change the SWR beyond any extra loss in the cable. It will change the phase of the mismatch that is all; (rotate around a constant VSWR circle on a Smith Chart). Jeff # Incorrect. The basic VSWR meter measures the voltage, not the power. # And if the SWR is other than 1:1, this voltage will change depending on # the distance to the mismatch. But isn't it still Vmax over Vmin, regardless of where that happens on the feedline? Yes, but that changes along the coax, depending on the length from the mismatch. # Additionally, a shorted coax 1/2 wavelength long shows a short (0 ohms). # But a shorted 1/4 wavelength coax shows an open (infinite impedance). # Somewhere in between (I'm not going to bother to figure out exactly # where because it's not that important) it will show an effective 50 ohm # impedance. # Coax length is unimportant when you have a 1:1 SWR, but if you don't, # the coax will act as a matching network. And length will affect the # overall system. I always understood the VSWR to be constant with the feedline length moving the parameters around the Smith chart constant VSWR circle. Thus it is possible by changing length to provide an antenna tuner with R and X values that the tuner can handle better. No, VSWR is not constant along the length of the feedline. A transmission line connected to a load of a different impedance will act as an impedance transformer. The actual impedance at the source will be different than that at the load, depending on the load impedance, the transmission line impedance and the length of the transmission line. This is why changing the length of the coax allows the tuner to work better. Also since impedance is a function for the R and X values, when these change, the impedance changes. Yes, changing the length of the coax does move the parameters around the Smith chart in a circle, but it is not a constant VSWR circle. Plot it out and you will see the impedance changes, depending on the length of the coax. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
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On 12/19/2014 10:14 AM, Jerry Stuckle wrote:
On 12/19/2014 2:33 AM, Jeff wrote: By changing the length of the coax you can make the antenna appear as being resonant or non resonant, even though the antenna may or may not be a perfect 50 ohm load. Incorrect, changing the length of the feeder will not change the SWR beyond any extra loss in the cable. It will change the phase of the mismatch that is all; (rotate around a constant VSWR circle on a Smith Chart). Jeff Incorrect. The basic VSWR meter measures the voltage, not the power. And if the SWR is other than 1:1, this voltage will change depending on the distance to the mismatch. Additionally, a shorted coax 1/2 wavelength long shows a short (0 ohms). But a shorted 1/4 wavelength coax shows an open (infinite impedance). Somewhere in between (I'm not going to bother to figure out exactly where because it's not that important) it will show an effective 50 ohm impedance. With a short, an open, or with what kind of load? Coax length is unimportant when you have a 1:1 SWR, but if you don't, the coax will act as a matching network. And length will affect the overall system. |
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On 12/19/2014 8:08 PM, Jerry Stuckle wrote:
On 12/19/2014 12:10 PM, Wayne wrote: "Jerry Stuckle" wrote in message ... On 12/19/2014 2:33 AM, Jeff wrote: By changing the length of the coax you can make the antenna appear as being resonant or non resonant, even though the antenna may or may not be a perfect 50 ohm load. Incorrect, changing the length of the feeder will not change the SWR beyond any extra loss in the cable. It will change the phase of the mismatch that is all; (rotate around a constant VSWR circle on a Smith Chart). Jeff # Incorrect. The basic VSWR meter measures the voltage, not the power. # And if the SWR is other than 1:1, this voltage will change depending on # the distance to the mismatch. But isn't it still Vmax over Vmin, regardless of where that happens on the feedline? Yes, but that changes along the coax, depending on the length from the mismatch. # Additionally, a shorted coax 1/2 wavelength long shows a short (0 ohms). # But a shorted 1/4 wavelength coax shows an open (infinite impedance). # Somewhere in between (I'm not going to bother to figure out exactly # where because it's not that important) it will show an effective 50 ohm # impedance. # Coax length is unimportant when you have a 1:1 SWR, but if you don't, # the coax will act as a matching network. And length will affect the # overall system. I always understood the VSWR to be constant with the feedline length moving the parameters around the Smith chart constant VSWR circle. Thus it is possible by changing length to provide an antenna tuner with R and X values that the tuner can handle better. Let me take this one piece at a time. No, VSWR is not constant along the length of the feedline. Yes - it is for lossless lines, and as Jeff wrote, for lines with attenuation, the SWR spirals inward. Consider either a very lossy or a very long line. No matter what is attached at the load end, the source will see the line characteristic impedance called "surge impedance" of the line. The variation that you see when you change the location of your instrument is due to common mode current on the outside of the shield of your coax. It will give you a false indication. A transmission line connected to a load of a different impedance will act as an impedance transformer. Yes, it will. The actual impedance at the source will be different than that at the load, depending on the load impedance, the transmission line impedance and the length of the transmission line. Of course. This is why changing the length of the coax allows the tuner to work better. Also since impedance is a function for the R and X values, when these change, the impedance changes. Impedance IS the R and X values which we commonly write as R+jX in what is called complex notation. When the R and X values change the complex will change, but not necessarily the absolute value which, unfortately is also called impedance. Consider that |Z| = sqrt(R^2 + X^2) and note that this is the MAGNITUDE of the vector and carries no information about phase. What two values can you put under the radical and have the same |Z|? So, do you mean the impedance R+jX or |Z|? They are not the same although one is derived from the other using Pythagorean's theorum. Please specify. Yes, changing the length of the coax does move the parameters around the Smith chart in a circle, but it is not a constant VSWR circle. Plot it out and you will see the impedance changes, depending on the length of the coax. I have done it many times and see very little difference using common transmission lines. As mentioned by Jeff, Wayne, and possibly others, it diminishes as a function of the transmission line length and loss (commonly called alpha). If you want to experiment with Smith charts rapidly and easily, then get the Smith V3.10 from: http://www.fritz.dellsperger.net/ If you learn to use it, it will open a world that you will become addicted to. |
swr goes up on antenna
"Jerry Stuckle" wrote in message ... On 12/19/2014 12:10 PM, Wayne wrote: "Jerry Stuckle" wrote in message ... On 12/19/2014 2:33 AM, Jeff wrote: By changing the length of the coax you can make the antenna appear as being resonant or non resonant, even though the antenna may or may not be a perfect 50 ohm load. Incorrect, changing the length of the feeder will not change the SWR beyond any extra loss in the cable. It will change the phase of the mismatch that is all; (rotate around a constant VSWR circle on a Smith Chart). Jeff # Incorrect. The basic VSWR meter measures the voltage, not the power. # And if the SWR is other than 1:1, this voltage will change depending on # the distance to the mismatch. But isn't it still Vmax over Vmin, regardless of where that happens on the feedline? # Yes, but that changes along the coax, depending on the length from the # mismatch. # Additionally, a shorted coax 1/2 wavelength long shows a short (0 ohms). # But a shorted 1/4 wavelength coax shows an open (infinite impedance). # Somewhere in between (I'm not going to bother to figure out exactly # where because it's not that important) it will show an effective 50 ohm # impedance. # Coax length is unimportant when you have a 1:1 SWR, but if you don't, # the coax will act as a matching network. And length will affect the # overall system. I always understood the VSWR to be constant with the feedline length moving the parameters around the Smith chart constant VSWR circle. Thus it is possible by changing length to provide an antenna tuner with R and X values that the tuner can handle better. # No, VSWR is not constant along the length of the feedline. # A transmission line connected to a load of a different impedance will # act as an impedance transformer. The actual impedance at the source # will be different than that at the load, depending on the load # impedance, the transmission line impedance and the length of the # transmission line. This is why changing the length of the coax allows # the tuner to work better. # Also since impedance is a function for the R and X values, when these # change, the impedance changes. # Yes, changing the length of the coax does move the parameters around the # Smith chart in a circle, but it is not a constant VSWR circle. Plot it # out and you will see the impedance changes, depending on the length of # the coax. Well, I was talking about "lossless" line. Otherwise, the constant VSWR circle will spiral inward to the center as the line length is increased. If you use a crappy lossy line of sufficient length, you can make an open circuit look like 50 ohms. |
swr goes up on antenna
On 12/20/2014 10:56 AM, Wayne wrote:
"Jerry Stuckle" wrote in message ... On 12/19/2014 12:10 PM, Wayne wrote: "Jerry Stuckle" wrote in message ... On 12/19/2014 2:33 AM, Jeff wrote: By changing the length of the coax you can make the antenna appear as being resonant or non resonant, even though the antenna may or may not be a perfect 50 ohm load. Incorrect, changing the length of the feeder will not change the SWR beyond any extra loss in the cable. It will change the phase of the mismatch that is all; (rotate around a constant VSWR circle on a Smith Chart). Jeff # Incorrect. The basic VSWR meter measures the voltage, not the power. # And if the SWR is other than 1:1, this voltage will change depending on # the distance to the mismatch. But isn't it still Vmax over Vmin, regardless of where that happens on the feedline? # Yes, but that changes along the coax, depending on the length from the # mismatch. # Additionally, a shorted coax 1/2 wavelength long shows a short (0 ohms). # But a shorted 1/4 wavelength coax shows an open (infinite impedance). # Somewhere in between (I'm not going to bother to figure out exactly # where because it's not that important) it will show an effective 50 ohm # impedance. # Coax length is unimportant when you have a 1:1 SWR, but if you don't, # the coax will act as a matching network. And length will affect the # overall system. I always understood the VSWR to be constant with the feedline length moving the parameters around the Smith chart constant VSWR circle. Thus it is possible by changing length to provide an antenna tuner with R and X values that the tuner can handle better. # No, VSWR is not constant along the length of the feedline. # A transmission line connected to a load of a different impedance will # act as an impedance transformer. The actual impedance at the source # will be different than that at the load, depending on the load # impedance, the transmission line impedance and the length of the # transmission line. This is why changing the length of the coax allows # the tuner to work better. # Also since impedance is a function for the R and X values, when these # change, the impedance changes. # Yes, changing the length of the coax does move the parameters around the # Smith chart in a circle, but it is not a constant VSWR circle. Plot it # out and you will see the impedance changes, depending on the length of # the coax. Well, I was talking about "lossless" line. Otherwise, the constant VSWR circle will spiral inward to the center as the line length is increased. If you use a crappy lossy line of sufficient length, you can make an open circuit look like 50 ohms. Same for a short circuit. The load condition never gets back to the source. |
swr goes up on antenna
"John S" wrote in message ... On 12/20/2014 10:56 AM, Wayne wrote: "Jerry Stuckle" wrote in message ... On 12/19/2014 12:10 PM, Wayne wrote: "Jerry Stuckle" wrote in message ... On 12/19/2014 2:33 AM, Jeff wrote: By changing the length of the coax you can make the antenna appear as being resonant or non resonant, even though the antenna may or may not be a perfect 50 ohm load. Incorrect, changing the length of the feeder will not change the SWR beyond any extra loss in the cable. It will change the phase of the mismatch that is all; (rotate around a constant VSWR circle on a Smith Chart). Jeff # Incorrect. The basic VSWR meter measures the voltage, not the power. # And if the SWR is other than 1:1, this voltage will change depending on # the distance to the mismatch. But isn't it still Vmax over Vmin, regardless of where that happens on the feedline? # Yes, but that changes along the coax, depending on the length from the # mismatch. # Additionally, a shorted coax 1/2 wavelength long shows a short (0 ohms). # But a shorted 1/4 wavelength coax shows an open (infinite impedance). # Somewhere in between (I'm not going to bother to figure out exactly # where because it's not that important) it will show an effective 50 ohm # impedance. # Coax length is unimportant when you have a 1:1 SWR, but if you don't, # the coax will act as a matching network. And length will affect the # overall system. I always understood the VSWR to be constant with the feedline length moving the parameters around the Smith chart constant VSWR circle. Thus it is possible by changing length to provide an antenna tuner with R and X values that the tuner can handle better. # No, VSWR is not constant along the length of the feedline. # A transmission line connected to a load of a different impedance will # act as an impedance transformer. The actual impedance at the source # will be different than that at the load, depending on the load # impedance, the transmission line impedance and the length of the # transmission line. This is why changing the length of the coax allows # the tuner to work better. # Also since impedance is a function for the R and X values, when these # change, the impedance changes. # Yes, changing the length of the coax does move the parameters around the # Smith chart in a circle, but it is not a constant VSWR circle. Plot it # out and you will see the impedance changes, depending on the length of # the coax. Well, I was talking about "lossless" line. Otherwise, the constant VSWR circle will spiral inward to the center as the line length is increased. If you use a crappy lossy line of sufficient length, you can make an open circuit look like 50 ohms. # Same for a short circuit. The load condition never gets back to the source. Many years ago, I was called in to help set up a HF/VHF comm station in a large building. The VHF transceiver wasn't working and the VSWR measured 1:1. So, I climbed up to the roof and found the cable was not connected to the antenna. |
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