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wilson 1000 coax legnth
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Thanks Sir, I did not think that was correct. This person said it was
needed because it was mag. mount. Kind of through it was a bunch of bull. Thanks, G.C. sideband wrote: Coax length is unimportant. However much you need to get from the antenna to your radio is plenty. If you're matching SWR with coax length, there's something wrong with your antenna. -SSB Guntier C. wrote: I received and old wilson 1000 mag. mount and want to put it on my quad (4 magnet) mount. I hear that I need a 18 ft. piece of coax for a counterpoise. Is that true? Any info appreciated. Thanks, G.C. |
"Guntier C." wrote in message ... Thanks Sir, I did not think that was correct. This person said it was needed because it was mag. mount. Kind of through it was a bunch of bull. Thanks, G.C. That person did not know the difference between RF ground and electrical ground. A solid electrical connection makes a decent RF ground in vehicles most of the time but it does not work backwards. Even though the base is insulated there is still an RF ground. Fuggedaboudit and enjoy your antenna!!! Chad |
On Wed, 03 Nov 2004 01:59:15 -0800, "Guntier C."
wrote: I received and old wilson 1000 mag. mount and want to put it on my quad (4 magnet) mount. I hear that I need a 18 ft. piece of coax for a counterpoise. Is that true? Any info appreciated. Thanks, G.C. I'll get slammed for posting the correct answer but what else is new. Manufacturers of Magnetic mount antennas recommend keeping the original length of coax.( Wilson, K40, RS, Antenna Specialist, ect.). Why do they do that? The answer is simple. They realize that there is no such thing as a perfect or exact scenario for an antenna installation. They know that in a typical installation of their antenna the coax length will most likely come into play and can be beneficial. It can be beneficial in that typically the radio will see a better load with the manufacturers recommended coax length. It's not that these antennas can't work with any length of coax. It's just that in most cases you'll get better results with the recommended length. |
Chad Wahls wrote:
"Guntier C." wrote in message ... Thanks Sir, I did not think that was correct. This person said it was needed because it was mag. mount. Kind of through it was a bunch of bull. Thanks, G.C. That person did not know the difference between RF ground and electrical ground. A solid electrical connection makes a decent RF ground in vehicles most of the time but it does not work backwards. Even though the base is insulated there is still an RF ground. Fuggedaboudit and enjoy your antenna!!! Chad Yep, sounds about right Chad.... Thanks, G.C. |
wrote in message ... On Wed, 03 Nov 2004 01:59:15 -0800, "Guntier C." wrote: I received and old wilson 1000 mag. mount and want to put it on my quad (4 magnet) mount. I hear that I need a 18 ft. piece of coax for a counterpoise. Is that true? Any info appreciated. Thanks, G.C. I'll get slammed for posting the correct answer but what else is new. Manufacturers of Magnetic mount antennas recommend keeping the original length of coax.( Wilson, K40, RS, Antenna Specialist, ect.). Why do they do that? The answer is simple. They realize that there is no such thing as a perfect or exact scenario for an antenna installation. They know that in a typical installation of their antenna the coax length will most likely come into play and can be beneficial. It can be beneficial in that typically the radio will see a better load with the manufacturers recommended coax length. It's not that these antennas can't work with any length of coax. It's just that in most cases you'll get better results with the recommended length. Agreed with tnon. As soon as someone mentions about using a certain length of coax with the CB antenna someone always asssumes it is to adjust SWR. It will not do this. It will change the impedance the radio sees which is not the same as changing the SWR. For example if the impedance of the antenna is 25 ohms 18ft of coax will make the antenna appear as 100 ohms at the radio. SWR will be 2:1 reguardless of the length of the cable. AS most radios are antennas are installed plug and go without benifit of tuning and most all CB antenas represent an impedance lower than 50 ohms antenna manufactures typically reccomend the 18ft coax for best results under these conditions. Most radios work better if they are mismatched above 50 than below 50. This really becomes noticable if SWR is around 3:1. Not uncommon on a typical plug and go installation. Otherwise length does not matter much on a well tuned and properly installed radio and antenna. |
Jimmie:
SWR is a direct measurement of impedance mismatch. If you change the coax length, thereby changing the angle the coax is conducting, and the SWR changes, then there is something wrong with the antenna itself that needs to be fixed. Physics dictates this. There's no way around it. Coax Length DOES NOT MATTER for a single antenna setup. Period. -SSB Jimmie wrote: wrote in message ... On Wed, 03 Nov 2004 01:59:15 -0800, "Guntier C." wrote: I received and old wilson 1000 mag. mount and want to put it on my quad (4 magnet) mount. I hear that I need a 18 ft. piece of coax for a counterpoise. Is that true? Any info appreciated. Thanks, G.C. I'll get slammed for posting the correct answer but what else is new. Manufacturers of Magnetic mount antennas recommend keeping the original length of coax.( Wilson, K40, RS, Antenna Specialist, ect.). Why do they do that? The answer is simple. They realize that there is no such thing as a perfect or exact scenario for an antenna installation. They know that in a typical installation of their antenna the coax length will most likely come into play and can be beneficial. It can be beneficial in that typically the radio will see a better load with the manufacturers recommended coax length. It's not that these antennas can't work with any length of coax. It's just that in most cases you'll get better results with the recommended length. Agreed with tnon. As soon as someone mentions about using a certain length of coax with the CB antenna someone always asssumes it is to adjust SWR. It will not do this. It will change the impedance the radio sees which is not the same as changing the SWR. For example if the impedance of the antenna is 25 ohms 18ft of coax will make the antenna appear as 100 ohms at the radio. SWR will be 2:1 reguardless of the length of the cable. AS most radios are antennas are installed plug and go without benifit of tuning and most all CB antenas represent an impedance lower than 50 ohms antenna manufactures typically reccomend the 18ft coax for best results under these conditions. Most radios work better if they are mismatched above 50 than below 50. This really becomes noticable if SWR is around 3:1. Not uncommon on a typical plug and go installation. Otherwise length does not matter much on a well tuned and properly installed radio and antenna. |
On Thu, 04 Nov 2004 19:33:08 GMT, sideband wrote:
Jimmie: SWR is a direct measurement of impedance mismatch. If you change the coax length, thereby changing the angle the coax is conducting, and the SWR changes, then there is something wrong with the antenna itself that needs to be fixed. Physics dictates this. There's no way around it. Coax Length DOES NOT MATTER for a single antenna setup. Period. -SSB I'd only agree if the antenna perfectly decoupled and was a perfect load for the radio. In other words if everything operated perfectly in a perfect world. As far as mag mounts and using a automobiles as a counterpoise ? ? ? That scenario is not even close to a perfect world. I often wonder why there is so much criticism when one suggests using the manufacturers recommended length. This criticism seems to be followed by the statement that "any length will do". They believe that 10 feet, 11 feet, 13.5 feet,ect. ect. are fine. Just don't pick 18 feet !!!!!! How hypocritical is that? Isn't 18' also a length contained within the statement "any length will do"? |
tnom:
Explain this, then... CB radio in my Peterbilt, with a mirror mounted Hustler HQ-27 on the driver's side.. 6 feet of coax shows a 1:1 on 19, 1.1:1 on 1, and 1.1:1 on 40... 12 feet of coax shows the same readings... 18 feet of coax shows.. guess what? the SAME RESULTS. IT IS possible to get near "perfect world" results with a little work, and attention to detail. By the way, I've gotten similar results on two meters with a magnet mount 5/8 2M Lakeview whip on my old Ford LTLA9000. Had to shorten the coax because I was sick of the bundle of it in the cab... no change in SWR over a four MHz bandwidth... There goes the "magmount theory" as well.. If changing the length of the coax changes the SWR, then there is something wrong with the antenna. -SSB wrote: I'd only agree if the antenna perfectly decoupled and was a perfect load for the radio. In other words if everything operated perfectly in a perfect world. As far as mag mounts and using a automobiles as a counterpoise ? ? ? That scenario is not even close to a perfect world. I often wonder why there is so much criticism when one suggests using the manufacturers recommended length. This criticism seems to be followed by the statement that "any length will do". They believe that 10 feet, 11 feet, 13.5 feet,ect. ect. are fine. Just don't pick 18 feet !!!!!! How hypocritical is that? Isn't 18' also a length contained within the statement "any length will do"? |
No. Not true. I run 6 feet on my 1000 mag. 1.1 swr's on channel 35.
Great antenna btw. I don't know if you can beat one for the ease/money/performance. Don "Guntier C." wrote in message ... I received and old wilson 1000 mag. mount and want to put it on my quad (4 magnet) mount. I hear that I need a 18 ft. piece of coax for a counterpoise. Is that true? Any info appreciated. Thanks, G.C. |
On Thu, 04 Nov 2004 21:59:02 GMT, sideband wrote:
tnom: Explain this, then... CB radio in my Peterbilt, with a mirror mounted Hustler HQ-27 on the driver's side.. 6 feet of coax shows a 1:1 on 19, 1.1:1 on 1, and 1.1:1 on 40... 12 feet of coax shows the same readings... 18 feet of coax shows.. guess what? the SAME RESULTS. IT IS possible to get near "perfect world" results with a little work, and attention to detail. By the way, I've gotten similar results on two meters with a magnet mount 5/8 2M Lakeview whip on my old Ford LTLA9000. Had to shorten the coax because I was sick of the bundle of it in the cab... no change in SWR over a four MHz bandwidth... There goes the "magmount theory" as well.. If changing the length of the coax changes the SWR, then there is something wrong with the antenna. -SSB SSB; I know this was addressed to Tnom, hope you don't mind my 2-cents worth. Very few mag mounts have an adequate "RF" ground, at least on 11 meters or lower. There isn't enough capacitive coupling to the vehicle body. In some cases the length of the coax can matter, not from a matching point of view, but as a counterpoise for the antenna. I have seen mag mount installations where you could see the SWR change just by moving the coax around. |
"Lancer" wrote in message ews.com... On Thu, 04 Nov 2004 21:59:02 GMT, sideband wrote: tnom: Explain this, then... CB radio in my Peterbilt, with a mirror mounted Hustler HQ-27 on the driver's side.. 6 feet of coax shows a 1:1 on 19, 1.1:1 on 1, and 1.1:1 on 40... 12 feet of coax shows the same readings... 18 feet of coax shows.. guess what? the SAME RESULTS. IT IS possible to get near "perfect world" results with a little work, and attention to detail. By the way, I've gotten similar results on two meters with a magnet mount 5/8 2M Lakeview whip on my old Ford LTLA9000. Had to shorten the coax because I was sick of the bundle of it in the cab... no change in SWR over a four MHz bandwidth... There goes the "magmount theory" as well.. If changing the length of the coax changes the SWR, then there is something wrong with the antenna. -SSB SSB; I know this was addressed to Tnom, hope you don't mind my 2-cents worth. Very few mag mounts have an adequate "RF" ground, at least on 11 meters or lower. There isn't enough capacitive coupling to the vehicle body. In some cases the length of the coax can matter, not from a matching point of view, but as a counterpoise for the antenna. I have seen mag mount installations where you could see the SWR change just by moving the coax around. Please explain how a poor "RF ground" can be corrected by increasing coax length?? Don |
On Thu, 04 Nov 2004 21:59:02 GMT, sideband wrote
in : tnom: Explain this, then... CB radio in my Peterbilt, with a mirror mounted Hustler HQ-27 on the driver's side.. Nice antenna! Installed one recently and plan on getting one for myself. 6 feet of coax shows a 1:1 on 19, 1.1:1 on 1, and 1.1:1 on 40... 12 feet of coax shows the same readings... 18 feet of coax shows.. guess what? the SAME RESULTS. Your antenna mount has a good RF ground. Coax length is going to make little if any difference. IT IS possible to get near "perfect world" results with a little work, and attention to detail. By the way, I've gotten similar results on two meters with a magnet mount 5/8 2M Lakeview whip on my old Ford LTLA9000. Had to shorten the coax because I was sick of the bundle of it in the cab... no change in SWR over a four MHz bandwidth... Now -that- I find hard to believe. If you get relatively flat bandwidth over 4MHz then your antenna is basically nothing more than a dummy load, and coax length doesn't matter there either. There goes the "magmount theory" as well.. You didn't read the results of my tests a few months ago, did you? If changing the length of the coax changes the SWR, then there is something wrong with the antenna. Absolutely correct, and the problem is an inadequate RF ground at the mount, which is typical of mag-mounts (unless they are so heavily loaded with resistance that they act like dummy loads). ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
On Thu, 4 Nov 2004 20:07:21 -0500, "M-Tech"
wrote in : snip Please explain how a poor "RF ground" can be corrected by increasing coax length?? A given length of coax runs from the radio to the antenna mount. If the shield is not adequately grounded (RF ground) then RF will crawl onto the outside of the shield. When that happens your coax behaves like an antenna, having resonant and non-resonant lengths. When that happens, different lengths of the coax will put the antenna mount at different phase angles, therefore changing the load impedance and SWR. There are only two solutions: Provide a good RF ground to the antenna mount (preferred), or change the length of the coax to 'tune' the input impedance of the antenna. That's all assuming your radio is fixed to a good RF ground. ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
On Thu, 04 Nov 2004 21:59:02 GMT, sideband wrote:
tnom: Explain this, then... CB radio in my Peterbilt, with a mirror mounted Hustler HQ-27 on the driver's side.. 6 feet of coax shows a 1:1 on 19, 1.1:1 on 1, and 1.1:1 on 40... 12 feet of coax shows the same readings... 18 feet of coax shows.. guess what? the SAME RESULTS. Good for you. I bet that if you used 18' you'd get good results also. IT IS possible to get near "perfect world" results with a little work, and attention to detail. Near is relative. Anything short of perfect can benefit from using the manufacturers recommended length. By the way, I've gotten similar results on two meters with a magnet mount 5/8 2M Lakeview whip on my old Ford LTLA9000. Had to shorten the coax because I was sick of the bundle of it in the cab... no change in SWR over a four MHz bandwidth... ???rec.radio.cb.........11 meters........27mhz ??? There goes the "magmount theory" as well.. You haven't proven anything other than you are inflexible in your belief that other antenna installation can't benefit from recommended coax lengths. Your installations are obviously perfect because you are perfect and being so you have done everything perfectly with your perfect counterpoise in your perfect world. If changing the length of the coax changes the SWR, then there is something wrong with the antenna. -SSB No. When it comes to mag mounts on vehicles it most likely means that you left the fantasy world behind and realize that you live in the real world where things aren't so perfect. |
Very few mag mounts have an adequate "RF" ground, at least on 11 meters or lower. There isn't enough capacitive coupling to the vehicle body. In some cases the length of the coax can matter, not from a matching point of view, but as a counterpoise for the antenna. I have seen mag mount installations where you could see the SWR change just by moving the coax around. Please explain how a poor "RF ground" can be corrected by increasing coax length?? Don He never said that a poor RF ground could be corrected by coax length. He said in effect that the side effects of a poor RF ground could be reduced by a specific length of coax. |
Dunno about you, Lancer, but the last thing I'd want is RF inside the
vehicle.. which is exactly what's happening if you're using the coax braid as the counterpoise/image plane/groundplane...even partially. -SSB Lancer wrote: SSB; I know this was addressed to Tnom, hope you don't mind my 2-cents worth. Very few mag mounts have an adequate "RF" ground, at least on 11 meters or lower. There isn't enough capacitive coupling to the vehicle body. In some cases the length of the coax can matter, not from a matching point of view, but as a counterpoise for the antenna. I have seen mag mount installations where you could see the SWR change just by moving the coax around. |
I said no change in swr... I should have been more specific.. I got
the same readings at 144.000, 146.000 and 148.000 on the original length as I did on the shortened length. Sorry about the confusion. -SSB Frank Gilliland wrote: On Thu, 04 Nov 2004 21:59:02 GMT, sideband wrote in : tnom: Explain this, then... CB radio in my Peterbilt, with a mirror mounted Hustler HQ-27 on the driver's side.. Nice antenna! Installed one recently and plan on getting one for myself. 6 feet of coax shows a 1:1 on 19, 1.1:1 on 1, and 1.1:1 on 40... 12 feet of coax shows the same readings... 18 feet of coax shows.. guess what? the SAME RESULTS. Your antenna mount has a good RF ground. Coax length is going to make little if any difference. IT IS possible to get near "perfect world" results with a little work, and attention to detail. By the way, I've gotten similar results on two meters with a magnet mount 5/8 2M Lakeview whip on my old Ford LTLA9000. Had to shorten the coax because I was sick of the bundle of it in the cab... no change in SWR over a four MHz bandwidth... Now -that- I find hard to believe. If you get relatively flat bandwidth over 4MHz then your antenna is basically nothing more than a dummy load, and coax length doesn't matter there either. There goes the "magmount theory" as well.. You didn't read the results of my tests a few months ago, did you? If changing the length of the coax changes the SWR, then there is something wrong with the antenna. Absolutely correct, and the problem is an inadequate RF ground at the mount, which is typical of mag-mounts (unless they are so heavily loaded with resistance that they act like dummy loads). ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
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Again, with a properly installed single antenna, coax length does not matter. -SSB This thread was about a ...................... 1. mag mount 2. on a vehilcle 3. with a recommended coax length Does this thread represent a "properly installed single antenna"? |
wrote:
Again, with a properly installed single antenna, coax length does not matter. -SSB This thread was about a ...................... 1. mag mount 2. on a vehilcle 3. with a recommended coax length Does this thread represent a "properly installed single antenna"? It usually represents a disagreement no matter how it's worded. :) -- http://NewsReader.Com 30GB/Month |
On 05 Nov 2004 13:08:42 GMT, Steveo
wrote: wrote: Again, with a properly installed single antenna, coax length does not matter. -SSB This thread was about a ...................... 1. mag mount 2. on a vehilcle 3. with a recommended coax length Does this thread represent a "properly installed single antenna"? It usually represents a disagreement no matter how it's worded. :) Yes. Coax length creates static. It's an electrical fact. |
Hello All:
I have been reading most of the thread here and its quit good reading. Great inputs from Frank, Sideband, Jimmie Lancer, Tnom, and others. One main consideration on the magnetic mount antennas is that the coax is being used as a counter poise and a lead for capacitance to ground. So I can see where a manufacture will specify a 3/4 wavelength length of coax, 18 feet. Anytime you use the coax as a counter poise or other end of the antenna coax lengths will effect the SWR, as the coax is no longer a simply a transmission line, but now also a antenna. I would love to have the time to do more testing. You will not see this type of design in Military, Commercial, or Amateur Antennas. I got a old Wilson 5000 magnetmatic mount antenna that I rebuilt and use on my car. I replaced the Krapy RG58C coax with "18" Ft of RG142B Teflon type coax that is bullet proof. It works great. I get it surplus at times. But I wanted to add 3 or 4 each, 9 foot ground plane radial wires, being run over the body of the car, from the base of the antenna, connected to the shield of the coax. And do a few tests. Food for thought. One thing I may do as a quick add on improvement is add in a Toroid Core in the coax about 9 feet down the coax from the antenna. Wrapping the coax around a 2.4 inch OD Amidon Toroid Core, will choke the outer coax shield currents from going farther down the coax, but allowing a 1/4 wave of coax to be a counter poise. Worth a try I think. It will also reduce some interference noise heard by the radio, that are coming in from the ungrounded shield of the coax. Neat-Oh! Suggested Toroid Core Amidon FT-240-61 Amidon is at: http://www.amidoncorp.com/aai_productselection.htm Jay in the Mojave Kreedentials: Rock n Roll Fan Owner 1967 Ford F250, 4x4, with worlds loudest PA System Took Electronics in 8th grade, way more better than that ackademic crap that schools now teach kids now a days ancient European history, liberal arts, and in incoming paper clips, in place of industrial arts. Paid Member of Mojave Desert Radio Club, 9 dollars now. Radio enthusiast type of guy |
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"sideband" wrote in message om... tnom: Explain this, then... CB radio in my Peterbilt, with a mirror mounted Hustler HQ-27 on the driver's side.. 6 feet of coax shows a 1:1 on 19, 1.1:1 on 1, and 1.1:1 on 40... 12 feet of coax shows the same readings... 18 feet of coax shows.. guess what? the SAME RESULTS. Nobody said it would be any different ,go back and read my post. I was very clear on why manufactureers say to use 18 ft and why you do not have to. UNLESS SOMETHING IS FUBAR CHANGING COAX LENGTH DOES NOT CHANGE SWR. JUST THE IMPEDANCE THE RADIO SEES.SWR IS DETERMINED SOLEY BY THE IMPEDANCE OF THE ANTENNA AND THE IMPEDANCE OF THE TRANSMISSION LINE. YOUR STATEMENT PROVES NOTHING. IF something is FUBAR making it read right by changing coax length will not fix it. You just sound like another ham that likes to hang around the CB group so they can sound important. Nobody even made the claims you are refuting. |
"Lancer" wrote in message ews.com... On Thu, 04 Nov 2004 21:59:02 GMT, sideband wrote: tnom: Explain this, then... CB radio in my Peterbilt, with a mirror mounted Hustler HQ-27 on the driver's side.. 6 feet of coax shows a 1:1 on 19, 1.1:1 on 1, and 1.1:1 on 40... 12 feet of coax shows the same readings... 18 feet of coax shows.. guess what? the SAME RESULTS. IT IS possible to get near "perfect world" results with a little work, and attention to detail. By the way, I've gotten similar results on two meters with a magnet mount 5/8 2M Lakeview whip on my old Ford LTLA9000. Had to shorten the coax because I was sick of the bundle of it in the cab... no change in SWR over a four MHz bandwidth... There goes the "magmount theory" as well.. If changing the length of the coax changes the SWR, then there is something wrong with the antenna. -SSB SSB; I know this was addressed to Tnom, hope you don't mind my 2-cents worth. Very few mag mounts have an adequate "RF" ground, at least on 11 meters or lower. There isn't enough capacitive coupling to the vehicle body. In some cases the length of the coax can matter, not from a matching point of view, but as a counterpoise for the antenna. I have seen mag mount installations where you could see the SWR change just by moving the coax around. This is true but is totally beside the point of why manufacturers reccomend 18ft of coax. If you have the problem you mentioned fix the ground dont play with the coax length. A good test for a CB antenna is to temporarily splice in 6 ft of coax . If your SWR changes your ground is probably bad.Poorly made SWR meters will cause this too. |
"Jimmie" wrote in message om... "Lancer" wrote in message ews.com... On Thu, 04 Nov 2004 21:59:02 GMT, sideband wrote: tnom: Explain this, then... CB radio in my Peterbilt, with a mirror mounted Hustler HQ-27 on the driver's side.. 6 feet of coax shows a 1:1 on 19, 1.1:1 on 1, and 1.1:1 on 40... 12 feet of coax shows the same readings... 18 feet of coax shows.. guess what? the SAME RESULTS. IT IS possible to get near "perfect world" results with a little work, and attention to detail. By the way, I've gotten similar results on two meters with a magnet mount 5/8 2M Lakeview whip on my old Ford LTLA9000. Had to shorten the coax because I was sick of the bundle of it in the cab... no change in SWR over a four MHz bandwidth... There goes the "magmount theory" as well.. If changing the length of the coax changes the SWR, then there is something wrong with the antenna. -SSB SSB; I know this was addressed to Tnom, hope you don't mind my 2-cents worth. Very few mag mounts have an adequate "RF" ground, at least on 11 meters or lower. There isn't enough capacitive coupling to the vehicle body. In some cases the length of the coax can matter, not from a matching point of view, but as a counterpoise for the antenna. I have seen mag mount installations where you could see the SWR change just by moving the coax around. This is true but is totally beside the point of why manufacturers reccomend 18ft of coax. If you have the problem you mentioned fix the ground dont play with the coax length. A good test for a CB antenna is to temporarily splice in 6 ft of coax . If your SWR changes your ground is probably bad.Poorly made SWR meters will cause this too. The 18ft rule is also there for "just in case" . As you change the lenght of the coax the impedance seen by the radio will also change. Assuming the antenna is at resonance you will see a pure resistance at the radio for only 2 lengths, an odd 1/4wl(16ft) and an even 1/4wl. At all other length unless the SWR is a perfect 1:1 match the impedances seen at the radio end will be complex. This is not always a bad thing. However there are some complex impedances that the radio may not like. It may dislike it so much the finals fry, the modulation becomes mushy or rx is nearly dead or it may be no problem at all for any length on your radio. There are a lot of different radios out there so the manufacturer usually recommends something safe they figure will work with all radios. 18 ft is a length that will normally allow enough cable to reach just about anywhere on the vehicle and provide a radio friendly impedance at the other end of the cable. Does it really need to be 18ft, probably not.. If you find the rare case where it does you can chase your tail till hell freezes over. |
On Fri, 05 Nov 2004 08:13:49 -0800, Jay in the Mojave
wrote in : Hello All: I have been reading most of the thread here and its quit good reading. Great inputs from Frank, Sideband, Jimmie Lancer, Tnom, and others. One main consideration on the magnetic mount antennas is that the coax is being used as a counter poise and a lead for capacitance to ground. So I can see where a manufacture will specify a 3/4 wavelength length of coax, 18 feet. The purpose of using 18 feet is not to effect a counterpoise but to mirror the RF potential of the shield at both ends of the coax, assuming that at least one end is properly grounded. IOW, if the radio has a good RF ground but the antenna mount is poorly grounded (like a mag-mount), using a 1/2 wave (as measured on the -outside- of the shield) will prevent the antenna end from floating. Or vice-versa. Anytime you use the coax as a counter poise or other end of the antenna coax lengths will effect the SWR, as the coax is no longer a simply a transmission line, but now also a antenna. I would love to have the time to do more testing. You will not see this type of design in Military, Commercial, or Amateur Antennas. I got a old Wilson 5000 magnetmatic mount antenna that I rebuilt and use on my car. I replaced the Krapy RG58C coax with "18" Ft of RG142B Teflon type coax that is bullet proof. It works great. I get it surplus at times. But I wanted to add 3 or 4 each, 9 foot ground plane radial wires, being run over the body of the car, from the base of the antenna, connected to the shield of the coax. And do a few tests. Food for thought. Here's something to try: Set your GDO for 27MHz and run it over your vehicle. Some places will dip more than others indicating better RF grounds, and therefore better places to drill. One thing I may do as a quick add on improvement is add in a Toroid Core in the coax about 9 feet down the coax from the antenna. Wrapping the coax around a 2.4 inch OD Amidon Toroid Core, will choke the outer coax shield currents from going farther down the coax, but allowing a 1/4 wave of coax to be a counter poise. Worth a try I think. It will also reduce some interference noise heard by the radio, that are coming in from the ungrounded shield of the coax. Neat-Oh! Suggested Toroid Core Amidon FT-240-61 Amidon is at: http://www.amidoncorp.com/aai_productselection.htm A sleeve with a high perm (u=850) should also work at 27MHz. As chokes I've seen them used at the radio end, but maybe using one at the antenna end will improve the performance of a mag-mount....? I might just try that myself someday. ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
On Fri, 05 Nov 2004 19:09:51 GMT, "Jimmie"
wrote in : snip .... Assuming the antenna is at resonance you will see a pure resistance at the radio for only 2 lengths, an odd 1/4wl(16ft) and an even 1/4wl. At all other length unless the SWR is a perfect 1:1 match the impedances seen at the radio end will be complex. The input impedance to the antenna (and coax) is complex regardless of resonance simply because the feedpoint is slightly out of phase with ground (the antenna wouldn't work if it wasn't). Even if the antenna -was- purely resistive at resonance the impedance would be on the order of 36.5 ohms (ideal), which does -not- present a 1:1 match to a 50 ohm coax. If you are getting 1:1 SWR then either the antenna has some sort of matching mechanism or it's a dummy load. But you are right, using 18 feet is a quick-&-dirty fix for a poor installation. ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Jay:
Thanks for your input.. A couple of things, though.. If the coax is being used as a counterpoise, it's not part of a capacitive grounding system. The capacitive ground comes from the metallic base/magnet mount to the vehicle body, not from the coax. -SSB Jay in the Mojave wrote: Hello All: I have been reading most of the thread here and its quit good reading. Great inputs from Frank, Sideband, Jimmie Lancer, Tnom, and others. One main consideration on the magnetic mount antennas is that the coax is being used as a counter poise and a lead for capacitance to ground. So I can see where a manufacture will specify a 3/4 wavelength length of coax, 18 feet. Anytime you use the coax as a counter poise or other end of the antenna coax lengths will effect the SWR, as the coax is no longer a simply a transmission line, but now also a antenna. I would love to have the time to do more testing. You will not see this type of design in Military, Commercial, or Amateur Antennas. I got a old Wilson 5000 magnetmatic mount antenna that I rebuilt and use on my car. I replaced the Krapy RG58C coax with "18" Ft of RG142B Teflon type coax that is bullet proof. It works great. I get it surplus at times. But I wanted to add 3 or 4 each, 9 foot ground plane radial wires, being run over the body of the car, from the base of the antenna, connected to the shield of the coax. And do a few tests. Food for thought. One thing I may do as a quick add on improvement is add in a Toroid Core in the coax about 9 feet down the coax from the antenna. Wrapping the coax around a 2.4 inch OD Amidon Toroid Core, will choke the outer coax shield currents from going farther down the coax, but allowing a 1/4 wave of coax to be a counter poise. Worth a try I think. It will also reduce some interference noise heard by the radio, that are coming in from the ungrounded shield of the coax. Neat-Oh! Suggested Toroid Core Amidon FT-240-61 Amidon is at: http://www.amidoncorp.com/aai_productselection.htm Jay in the Mojave Kreedentials: Rock n Roll Fan Owner 1967 Ford F250, 4x4, with worlds loudest PA System Took Electronics in 8th grade, way more better than that ackademic crap that schools now teach kids now a days ancient European history, liberal arts, and in incoming paper clips, in place of industrial arts. Paid Member of Mojave Desert Radio Club, 9 dollars now. Radio enthusiast type of guy |
Take your angst and put it somewhere else.. your comments don't even
justify a reply.. Just remember this, I am a HAM, but I'll always be a CBer, as well. If I see an opportunity to help, I'll take it. Methinks you could stand to take some of your own advice and go back and read the posts made. Jimmie wrote: drivel clipped |
On Fri, 05 Nov 2004 22:35:20 GMT, sideband wrote:
Take your angst and put it somewhere else.. your comments don't even justify a reply.. Just remember this, I am a HAM, but I'll always be a CBer, as well. If I see an opportunity to help, I'll take it. Methinks you could stand to take some of your own advice and go back and read the posts made. Jimmie wrote: drivel clipped Wait ! Everybody cool your jets !!!! We finally get a legit rec.radio.cb thread complete with information and passionate beliefs. It doesn't get any better than that. What more can one ask for? More Dougay trash? |
"Frank Gilliland" wrote in message ... On Thu, 4 Nov 2004 20:07:21 -0500, "M-Tech" wrote in : snip Please explain how a poor "RF ground" can be corrected by increasing coax length?? A given length of coax runs from the radio to the antenna mount. If the shield is not adequately grounded (RF ground) then RF will crawl onto the outside of the shield. When that happens your coax behaves like an antenna, having resonant and non-resonant lengths. When that happens, different lengths of the coax will put the antenna mount at different phase angles, therefore changing the load impedance and SWR. There are only two solutions: Provide a good RF ground to the antenna mount (preferred), or change the length of the coax to 'tune' the input impedance of the antenna. That's all assuming your radio is fixed to a good RF ground. Yeah, check-check. I'm with ya. I was just trying to ask(albeit, sarcastically) who the hell would loop coax through their vehicle instead of looking for a ground problem!!! I'm also assuming that if the above case was true, you'd NEVER get enough of a ground to get a decent(say 2:1)swr unless you have a few hundred feet in the trunk!! I just always was told that if you have a swr problem, it won't be fixed by changing coax lengths. Also, why do people have negative comments about mag mounts? Mine works fantastic. Maybe the paint on my Passat is thin:-) Don 313 |
isn't a 1:1 swr reading a fallicy (sp?). not saying that your equipment
is relaying false readings but a 1:1 match in theory should be impossible. i have the 1000 also and agree that it works well. on the other hand the old 102" whip worked the falkland islands on 12 watts. M-Tech wrote: No. Not true. I run 6 feet on my 1000 mag. 1.1 swr's on channel 35. Great antenna btw. I don't know if you can beat one for the ease/money/performance. Don "Guntier C." wrote in message ... I received and old wilson 1000 mag. mount and want to put it on my quad (4 magnet) mount. I hear that I need a 18 ft. piece of coax for a counterpoise. Is that true? Any info appreciated. Thanks, G.C. |
wrote in message ... Very few mag mounts have an adequate "RF" ground, at least on 11 meters or lower. There isn't enough capacitive coupling to the vehicle body. In some cases the length of the coax can matter, not from a matching point of view, but as a counterpoise for the antenna. I have seen mag mount installations where you could see the SWR change just by moving the coax around. Please explain how a poor "RF ground" can be corrected by increasing coax length?? Don He never said that a poor RF ground could be corrected by coax length. He said in effect that the side effects of a poor RF ground could be reduced by a specific length of coax. He sure did. He said mag mounts have a poor ground. He said the length of the coax can help by being "part of the ground plane"(counterpoise). I'm simply saying you'd need a TON of coax to do that...and get a decent swr(point of post). Hence, length of coax(within reason, obviously)on a wilson 1000 mag mount means diddly squat. I'm I right, or wrong:-) Don 313 |
Amen.
Don 313 wrote in message ... On Fri, 05 Nov 2004 22:35:20 GMT, sideband wrote: Take your angst and put it somewhere else.. your comments don't even justify a reply.. Just remember this, I am a HAM, but I'll always be a CBer, as well. If I see an opportunity to help, I'll take it. Methinks you could stand to take some of your own advice and go back and read the posts made. Jimmie wrote: drivel clipped Wait ! Everybody cool your jets !!!! We finally get a legit rec.radio.cb thread complete with information and passionate beliefs. It doesn't get any better than that. What more can one ask for? More Dougay trash? |
Hell Jim, every measurement made using ANY equipment could be a fallacy if
taken out the correct(to disprove)decimal point:-) I'm just saying (for my own comparative measurements) on my Dosy meter I've tuned my 1000 for 1:1 on 35 and I KNOW it talks nice:-) I'm SURE it's not because of the 1:1 measurment, but because of the antenna....and I'm doing it without 18' of coax....just 6'. I also talk all over, from Alaska to Turkey from my mudduck mobile here in pa:-) Don 313 "jim" wrote in message et... isn't a 1:1 swr reading a fallicy (sp?). not saying that your equipment is relaying false readings but a 1:1 match in theory should be impossible. i have the 1000 also and agree that it works well. on the other hand the old 102" whip worked the falkland islands on 12 watts. M-Tech wrote: No. Not true. I run 6 feet on my 1000 mag. 1.1 swr's on channel 35. Great antenna btw. I don't know if you can beat one for the ease/money/performance. Don "Guntier C." wrote in message ... I received and old wilson 1000 mag. mount and want to put it on my quad (4 magnet) mount. I hear that I need a 18 ft. piece of coax for a counterpoise. Is that true? Any info appreciated. Thanks, G.C. |
Jim:
No, a 1:1 match isn't a fallacy... but it's extremely rare. It IS possible to have a 50 ohm output impedance on the radio, a coax cable that is actually 50 ohms impedance, and an antenna that presents a 50 ohm impedance at its feedpoint. Possible, but highly unlikely... For example, the 1:1 I'm seeing on the meter in my Pete is probably 1.03:1 or something... but the meter I'm using doesn't have that kind of resolution. 1.03 is close enough to 1 to say, ok.. good enough.. especially considering the fact that anything under 2:1 will be just fine. -SSB jim wrote: isn't a 1:1 swr reading a fallicy (sp?). not saying that your equipment is relaying false readings but a 1:1 match in theory should be impossible. i have the 1000 also and agree that it works well. on the other hand the old 102" whip worked the falkland islands on 12 watts. M-Tech wrote: No. Not true. I run 6 feet on my 1000 mag. 1.1 swr's on channel 35. Great antenna btw. I don't know if you can beat one for the ease/money/performance. Don "Guntier C." wrote in message ... I received and old wilson 1000 mag. mount and want to put it on my quad (4 magnet) mount. I hear that I need a 18 ft. piece of coax for a counterpoise. Is that true? Any info appreciated. Thanks, G.C. |
jim wrote:
isn't a 1:1 swr reading a fallicy (sp?). not saying that your equipment is relaying false readings but a 1:1 match in theory should be impossible. i have the 1000 also and agree that it works well. on the other hand the old 102" whip worked the falkland islands on 12 watts. M-Tech wrote: No. Not true. I run 6 feet on my 1000 mag. 1.1 swr's on channel 35. Great antenna btw. I don't know if you can beat one for the ease/money/performance. Don "Guntier C." wrote in message ... I received and old wilson 1000 mag. mount and want to put it on my quad (4 magnet) mount. I hear that I need a 18 ft. piece of coax for a counterpoise. Is that true? Any info appreciated. Thanks, G.C. cheers on the spell check guys :) 50 ohm impedance on all points may be fine but inherent loss on the connections makes it impossible to achieve perfect matching. closest thing i've seen is hardline connections on some of the navy test equipment (CASS) that i've worked with. that was up in the gigahertz range. |
On Fri, 05 Nov 2004 22:22:03 GMT, sideband wrote
in : Jay: Thanks for your input.. A couple of things, though.. If the coax is being used as a counterpoise, it's not part of a capacitive grounding system. The capacitive ground comes from the metallic base/magnet mount to the vehicle body, not from the coax. That was an issue when I did the mag-mount tests a few months ago. As it turns out, the coax can provide plenty of capacitive coupling to the vehicle, even more than the mag-mount under certain conditions. But neither of them provide much coupling, not nearly as much as would be needed to compare with a properly mounted base. ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
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