| Page 1 of 2 | 1 | 2 |
|
|
SWR Tells Me??
OK, I have a new 2 meter j-pole antenna installed in my 3rd floor
attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? Thanks for any insight, jimbo |
SWR Tells Me??
jimbo wrote:
OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? That's what I would conclude from the data given. Wind some wire around the tip top and extend the antenna by a few inches. Then repeat your SWR measurements. -- 73, Cecil http://www.qsl.net/w5dxp |
SWR Tells Me??
"Cecil Moore" wrote in message om... jimbo wrote: OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? That's what I would conclude from the data given. Wind some wire around the tip top and extend the antenna by a few inches. Then repeat your SWR measurements. -- 73, Cecil One might also try adjusting the feed point location and see what effect that has. Harold KD5SAK |
SWR Tells Me??
kd5sak wrote:
"Cecil Moore" wrote in message Wind some wire around the tip top and extend the antenna by a few inches. Then repeat your SWR measurements. One might also try adjusting the feed point location and see what effect that has. That would be good to do after adjusting the length. -- 73, Cecil http://www.qsl.net/w5dxp |
SWR Tells Me??
On Mon, 20 Mar 2006 08:12:51 -0700, jimbo wrote:
OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? Have you a common mode choke on the feedline? J-poles are nortorious for feedline common mode problems. Danny, K6MHE |
SWR Tells Me??
Jimbo,
The most likely source of variation is inaccuracy of your SWR meter. It doesn't measure SWR on the feedline anyway. The questions you should ask is does it work? And is the transmitter roughly loaded with the right load impedance? ---- Reg |
SWR Tells Me??
In your set up the VSWR tells you very little!!
That 50 feet of LMR240 is modifying the actual VSWR as seen at your meter. It is not telling you the VSWR at the feed point. First, the antenna should have a balanced feed. With coax you need a 1/2 wavelength [coax corrected for velocity factor] matching stub to keep the line balanced. This will minimized coupling antenna current on the outside of the coax cable. Second, you should measure the antenna feed point impedance at the feedpoint, or 1/2 wavelength from the feed point [coax corrected for velocity factor]. Third, adjust the feedpoint location to get 50 + j0 ohms on your preferred frequency using an antenna analyzer [one or more members of your radio club should have one]. I'm located in southern NH, USA and if you are reasonably close I can assist in tuning your antenna. DD jimbo wrote: OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? Thanks for any insight, jimbo |
SWR Tells Me??
On Mon, 20 Mar 2006 15:37:26 -0500, Dave wrote:
In your set up the VSWR tells you very little!! That 50 feet of LMR240 is modifying the actual VSWR as seen at your meter. It is not telling you the VSWR at the feed point. The matched line loss of 50' LMR240 at 146MHz is 1.5dB. Jimbo tells us the source end VSWR at 146.2 is 2.3. One can make a reasonable estimate that the VSWR at the antenna end of the line is 3.5 (not a very good match for an antenna that should use an adjustable matching system). Total line loss is around 2.3dB, or about 0.9dB worse than matched line loss. Now look at the tx output power, has it decreased because of the bad load. Add that reduction (in dB) to the 0.9% above to get the overall degradation of transmit perfomance. A question: Is the proximity of other structures or feedline isolation a cause of the high VSWR, and should you resolve that before tweaking the matching? Owen -- |
SWR Tells Me??
On Mon, 20 Mar 2006 15:37:26 -0500, Dave wrote:
In your set up the VSWR tells you very little!! That 50 feet of LMR240 is modifying the actual VSWR as seen at your meter. It is not telling you the VSWR at the feed point. First, the antenna should have a balanced feed. With coax you need a 1/2 wavelength [coax corrected for velocity factor] matching stub to keep the line balanced. This will minimized coupling antenna current on the outside of the coax cable. Second, you should measure the antenna feed point impedance at the feedpoint, or 1/2 wavelength from the feed point [coax corrected for velocity factor]. I'm confused, wouldn't it be more important to have the right impedance and swr at the end of the feedline, where the transceiver is expecting 50 ohms? bob k5qwg Third, adjust the feedpoint location to get 50 + j0 ohms on your preferred frequency using an antenna analyzer [one or more members of your radio club should have one]. I'm located in southern NH, USA and if you are reasonably close I can assist in tuning your antenna. DD jimbo wrote: OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? Thanks for any insight, jimbo |
SWR Tells Me??
On Mon, 20 Mar 2006 21:31:02 GMT, Bob Miller
wrote: Second, you should measure the antenna feed point impedance at the feedpoint, or 1/2 wavelength from the feed point [coax corrected for velocity factor]. Why the half wavelength? The VSWR in this situation varies smoothly from the value at the feed point to the value at the source end of the line as dictated by the (known) line loss. In this scenario, knowing matched line loss and the VSWR at a point, it is possible to estimate the VSWR anywhere else on the line (as I have done in another post). I'm confused, wouldn't it be more important to have the right impedance and swr at the end of the feedline, where the transceiver is expecting 50 ohms? There are two objectives: 1. delivering the transmitter its rated load impedance (so that it safely delivers its rated power); 2. minimising line loss. 1. is achieved by a load approximately 50 ohms at the souce end of the line. 2. is achieved by a load approximately 50 ohms at the load end of the line (which will also satisfy 1.). Owen -- |
SWR Tells Me??
Cecil Moore wrote:
jimbo wrote: OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? That's what I would conclude from the data given. Wind some wire around the tip top and extend the antenna by a few inches. Then repeat your SWR measurements. I am not sure what you mean by "wrap some wire around the tip top." I could splice some wire on the long leg of the ladder line? |
SWR Tells Me??
kd5sak wrote:
"Cecil Moore" wrote in message om... jimbo wrote: OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? That's what I would conclude from the data given. Wind some wire around the tip top and extend the antenna by a few inches. Then repeat your SWR measurements. -- 73, Cecil One might also try adjusting the feed point location and see what effect that has. Harold KD5SAK I have done that on a j-pole that I made, but that antenna was in my shack, where it was accessible. This antenna is in a third floor attic where I have to wriggle my feet through 14 inches of insulation to find a rafter to stand on. Not conducive to much tweaking. Thanks, jimbo |
SWR Tells Me??
Dan Richardson k6mheatadelphia wrote:
On Mon, 20 Mar 2006 08:12:51 -0700, jimbo wrote: OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? Have you a common mode choke on the feedline? J-poles are nortorious for feedline common mode problems. Danny, K6MHE Maybe a 4-5 loop coil of coax close to the feed point? Thanks, jimbo |
SWR Tells Me??
Dave wrote:
In your set up the VSWR tells you very little!! That 50 feet of LMR240 is modifying the actual VSWR as seen at your meter. It is not telling you the VSWR at the feed point. First, the antenna should have a balanced feed. With coax you need a 1/2 wavelength [coax corrected for velocity factor] matching stub to keep the line balanced. This will minimized coupling antenna current on the outside of the coax cable. Second, you should measure the antenna feed point impedance at the feedpoint, or 1/2 wavelength from the feed point [coax corrected for velocity factor]. Third, adjust the feedpoint location to get 50 + j0 ohms on your preferred frequency using an antenna analyzer [one or more members of your radio club should have one]. I'm located in southern NH, USA and if you are reasonably close I can assist in tuning your antenna. DD jimbo wrote: OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? Thanks for any insight, jimbo Well, Utah is a little too far away for your offer of help. But, why shouldn't I measure SWR at the transmitter? Doesn't a SWR of 1.0 at the transmitter indicate 50 + j0 ohms which is what the transmitter wants? Thanks, jimbo |
SWR Tells Me??
jimbo wrote:
I am not sure what you mean by "wrap some wire around the tip top." I could splice some wire on the long leg of the ladder line? The antenna appears to be too short. Lengthen the antenna, normally accomplished at the tip top. -- 73, Cecil http://www.qsl.net/w5dxp |
SWR Tells Me??
On Mon, 20 Mar 2006 15:01:53 -0700, jimbo wrote:
Dan Richardson k6mheatadelphia wrote: Have you a common mode choke on the feedline? J-poles are nortorious for feedline common mode problems. Danny, K6MHE Maybe a 4-5 loop coil of coax close to the feed point? Thanks, jimbo Yes, that would help. Danny |
SWR Tells Me??
Bob Miller wrote:
SNIPPED I'm confused, wouldn't it be more important to have the right impedance and swr at the end of the feedline, where the transceiver is expecting 50 ohms? bob k5qwg SNIPPED If you have 50 ohms at the antenna, then you will have 50 ohms regardless of the length of coax. A 3:1 VSWR, assuming other responses to this post are correct, means that your antenna connection could be anything from 16 ohms to 150 ohms and all sorts of combinations of resistance and reactance within that range. An antenna analyzer will let you determine antenna resonance, antenna feedpoint impedance [hopefully 50 ohms], coax line loss, and allow proper adjustment at the connection points for your coax. Once again, find someone in your area with an antenna analyzer. I repeat my offer if you are within 60 +/- miles of Concord NH. |
SWR Tells Me??
Owen Duffy wrote:
On Mon, 20 Mar 2006 21:31:02 GMT, Bob Miller wrote: Second, you should measure the antenna feed point impedance at the feedpoint, or 1/2 wavelength from the feed point [coax corrected for velocity factor]. Why the half wavelength? A 1/2 wavelength transmission line repeats the load impedance at the antenna with minimum loss, assuming you cannot get an analyzer at the antenna itself. IMO, measuring directly at the antenna and solving the matching/load problem there is the easiest and best method. You are free to disagree. |
SWR Tells Me??
jimbo wrote:
SNIPPED Well, Utah is a little too far away for your offer of help. But, why shouldn't I measure SWR at the transmitter? Doesn't a SWR of 1.0 at the transmitter indicate 50 + j0 ohms which is what the transmitter wants? Thanks, jimbo It IS what the transmitter wants! But you may be introducing un-necessary power losses in your coax from any true VSWR 1:1. It is possible to have a VSWR 25:1 at the antenna and still have close to 1:1 at the transmitter! REALLY !! Your transmitter is happy. But you are wasting power heating the coax, not radiating it at the antenna! This is caused by impedance transformation along the coax. VSWR ~ 1:1 at the antenna provides best power transfer from coax to antenna. BTW, I used to live in Kaysville and Layton [ZIP=85015] in the olden days [30+ years ago] |
SWR Tells Me??
On Mon, 20 Mar 2006 17:54:19 -0500, Dave wrote:
Owen Duffy wrote: On Mon, 20 Mar 2006 21:31:02 GMT, Bob Miller wrote: Second, you should measure the antenna feed point impedance at the feedpoint, or 1/2 wavelength from the feed point [coax corrected for velocity factor]. Why the half wavelength? A 1/2 wavelength transmission line repeats the load impedance at the antenna with minimum loss, assuming you cannot get an analyzer at the antenna itself. IMO, measuring directly at the antenna and solving the matching/load problem there is the easiest and best method. You are free to disagree. Ok, but again you can measure the impedance anywhere and knowing line parameters, calculate Z at any other point. Owen -- |
SWR Tells Me??
|
SWR Tells Me??
I have no personal experience with your conditions.
However, the scenario is familiar. You would like to "tidy up" an installation that is "sort of" working. You can minimize the effort involved by going to http://free.prohosting.com/~w0rcy/Jpole/jpole.html That will allow you to plan the fewest trips to the attic that will enable you achieve your goals with your resources. OTH, if you are achieving acceptable results with what you have now I would advise leaving it alone. If you feel 2-4db more is worth the effort, lay out your plan! BTW, I totally agree with Cecil. The fundamental problem appears that the antenna is a bit short! Any change other than that is a patch! On Mon, 20 Mar 2006 08:12:51 -0700, jimbo wrote: OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? Thanks for any insight, jimbo John Ferrell W8CCW |
SWR Tells Me??
Owen Duffy wrote:
On Mon, 20 Mar 2006 15:37:26 -0500, Dave wrote: In your set up the VSWR tells you very little!! That 50 feet of LMR240 is modifying the actual VSWR as seen at your meter. It is not telling you the VSWR at the feed point. The matched line loss of 50' LMR240 at 146MHz is 1.5dB. Jimbo tells us the source end VSWR at 146.2 is 2.3. One can make a reasonable estimate that the VSWR at the antenna end of the line is 3.5 (not a very good match for an antenna that should use an adjustable matching system). Total line loss is around 2.3dB, or about 0.9dB worse than matched line loss. Now look at the tx output power, has it decreased because of the bad load. Add that reduction (in dB) to the 0.9% above to get the overall degradation of transmit perfomance. A question: Is the proximity of other structures or feedline isolation a cause of the high VSWR, and should you resolve that before tweaking the matching? Owen -- The numbers are confusing. 50 feet of LMR240 1.5 dB, (I understand that.) Total line loss 2.3 dB. (Where did you get that?) .9 dB worse than matched line loss. (Where did you get that number?) There is wooden structure near since the antenna is located in an attic. But I don't think there are any large metal structures close to the antenna. The coax is in PVC pipe. It may go close to a metal AC duct on it's way to the basement.. Thanks, jimbo |
SWR Tells Me??
Owen Duffy wrote:
SNIPPED Ok, but again you can measure the impedance anywhere and knowing line parameters, calculate Z at any other point. Owen -- Owen, I don't disagree with you. But, IMO, it is beyond Bob Miller's expertise/knowledge, at this point in his ham experience, to perform those calculations. I didn't perform those calculations until my 3rd year in engineering school. KISS, and fix it at the source. |
SWR Tells Me??
jimbo wrote:
SNIPPED Adding some length to the long leg would be relatively easy, BUT moving the feed point would be a real challenge! The coax shield and center conductor are soldered to the ladder line wires. When I tuned a j-pole in my work shop, tiny movements made a change amd maintaining the position while trying to get it soldered was nearly impossible. (At least for my clumsy fingers.) Is there a better way to connect the feed point? Thanks, jimbo Jimbo, you just added another variable to the discussion. Please describe the details of the coax to ladder line and ladder line to antenna for us. |
SWR Tells Me??
On Mon, 20 Mar 2006 16:51:44 -0700, jimbo wrote:
Owen Duffy wrote: On Mon, 20 Mar 2006 15:37:26 -0500, Dave wrote: In your set up the VSWR tells you very little!! That 50 feet of LMR240 is modifying the actual VSWR as seen at your meter. It is not telling you the VSWR at the feed point. The matched line loss of 50' LMR240 at 146MHz is 1.5dB. Jimbo tells us the source end VSWR at 146.2 is 2.3. One can make a reasonable estimate that the VSWR at the antenna end of the line is 3.5 (not a very good match for an antenna that should use an adjustable matching system). Total line loss is around 2.3dB, or about 0.9dB worse than matched line loss. Now look at the tx output power, has it decreased because of the bad load. Add that reduction (in dB) to the 0.9% above to get the overall degradation of transmit perfomance. A question: Is the proximity of other structures or feedline isolation a cause of the high VSWR, and should you resolve that before tweaking the matching? Owen -- One step at a time... The numbers are confusing. 50 feet of LMR240 1.5 dB, (I understand that.) Ok. Total line loss 2.3 dB. (Where did you get that?) It can be calculated knowing the line loss and source end VSWR. The additional 0.8dB is increased line loss due to operation with VSWR1. Try http://www.vk1od.net/tl/tllce.php for the calcs. .9 dB worse than matched line loss. (Where did you get that number?) Sorry, my rounding error, call it 0.8dB. It is by subtraction of the matched line loss from the total line loss. There is wooden structure near since the antenna is located in an attic. But I don't think there are any large metal structures close to the antenna. The coax is in PVC pipe. It may go close to a metal AC duct on it's way to the basement.. Is the roof sarked (does it have a metal foil lining under the tiles or shingles), presumably it isnt a metal roof. Thanks, jimbo -- |
SWR Tells Me??
On Mon, 20 Mar 2006 19:00:16 -0500, Dave wrote:
Owen Duffy wrote: SNIPPED Ok, but again you can measure the impedance anywhere and knowing line parameters, calculate Z at any other point. Owen -- Owen, I don't disagree with you. But, IMO, it is beyond Bob Miller's expertise/knowledge, at this point in his ham experience, to perform those calculations. I didn't perform those calculations until my 3rd year in engineering school. KISS, and fix it at the source. Dave, there is no doubt that the closer you are to the antenna, the less error introduced by tolerance on line parameters. On the otherhand, the instrumentation / user can be disruptive or real inconvenient. As for the calcs, one can be lazy and use http://www.vk1od.net/tl/tllce.php (if one's cable type is supported). My antenna analyser can't do the calcs internally, I don't know if some can, but it would be a pretty handy feature. Owen -- |
SWR Tells Me??
Dave wrote:
jimbo wrote: SNIPPED Adding some length to the long leg would be relatively easy, BUT moving the feed point would be a real challenge! The coax shield and center conductor are soldered to the ladder line wires. When I tuned a j-pole in my work shop, tiny movements made a change amd maintaining the position while trying to get it soldered was nearly impossible. (At least for my clumsy fingers.) Is there a better way to connect the feed point? Thanks, jimbo Jimbo, you just added another variable to the discussion. Please describe the details of the coax to ladder line and ladder line to antenna for us. Sorry for the confusion. The ladder line is the j-pole antenna. Cut to the appropriate length, etc. The coax feed line is attached to the "J" near the bottom, maybe 2 inches up. Thanks, jimbo |
SWR Tells Me??
John Ferrell wrote:
I have no personal experience with your conditions. However, the scenario is familiar. You would like to "tidy up" an installation that is "sort of" working. You can minimize the effort involved by going to http://free.prohosting.com/~w0rcy/Jpole/jpole.html That will allow you to plan the fewest trips to the attic that will enable you achieve your goals with your resources. OTH, if you are achieving acceptable results with what you have now I would advise leaving it alone. If you feel 2-4db more is worth the effort, lay out your plan! BTW, I totally agree with Cecil. The fundamental problem appears that the antenna is a bit short! Any change other than that is a patch! On Mon, 20 Mar 2006 08:12:51 -0700, jimbo wrote: OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? Thanks for any insight, jimbo John Ferrell W8CCW Well, the thing operates OK as far as I can tell. But it doesn't take much to hit a 2 meter repeater. I guess I am more concerned about damage to the transmitter than I am about getting full power radiated. Thanks, jimbo |
SWR Tells Me??
But-out flag cleared//
"jimbo" wrote in message . .. Dave wrote: In your set up the VSWR tells you very little!! That 50 feet of LMR240 is modifying the actual VSWR as seen at your meter. It is not telling you the VSWR at the feed point.... Second, you should measure the antenna feed point impedance at the feedpoint, or 1/2 wavelength from the feed point [coax corrected for velocity factor]. ..... DD jimbo wrote: OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? Thanks for any insight, jimbo Seems to me there are several items being asked / answered here. Sort of going backwards through the parts of the thread I can see... 1 - I think it is a safe bet that if the VSWR at the transmitter (regardless of feed like loss BUT within normally expected values) is lowest at the high end of the band, then the antenna system is either too short or resonant too high in frequency - take your pick of wording. 2- As you measure VSWR further and further from the load (antenna) you will get a better and better value because of the loss in the transmission line. This is because as signal is lost to the loss (attenuation) of the line, there is less to reflect at the load and less makes it back to the measuring device - when compared to tehat sent from teh transmitter. Less power to measure in the backward direction = better VSWR. Side Note that the loss is twice the line loss in this case, because it travels it twice in a round trip. Another result of this is that a simple resistive (lossy) attenuator (or a hunk of t-line used as an attenuator) has a return loss that is equal to twice its attenuation rating. In other words, a 10dB pad has a 20 dB return loss and is a pretty good load, since only 1% of the power makes it back to be measured for the VSWR measurement. 3- Then there are comments about measuring an *impedance* *at* the load, or 1/2 wavelength away. This is because the *Impedance* repeats every half wavelength in a *LOSS-LESS* line (due to the much discussed physics of waves). For practical lines, with minimal loss, it's pretty close, but, as explained above, approaches 50 ohms (Zo) further and further from the load. Therefore, accuracy in the measured *impedance* degrades further and further from the load. But/out flag set// 73, Steve, K,9.D;C'I Hi Owen. |
SWR Tells Me??
OK, I misunderstood.
jimbo wrote: Dave wrote: jimbo wrote: SNIPPED Adding some length to the long leg would be relatively easy, BUT moving the feed point would be a real challenge! The coax shield and center conductor are soldered to the ladder line wires. When I tuned a j-pole in my work shop, tiny movements made a change amd maintaining the position while trying to get it soldered was nearly impossible. (At least for my clumsy fingers.) Is there a better way to connect the feed point? Thanks, jimbo Jimbo, you just added another variable to the discussion. Please describe the details of the coax to ladder line and ladder line to antenna for us. Sorry for the confusion. The ladder line is the j-pole antenna. Cut to the appropriate length, etc. The coax feed line is attached to the "J" near the bottom, maybe 2 inches up. Thanks, jimbo |
SWR Tells Me??
jimbo wrote:
OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? Thanks for any insight, jimbo Thanks for all of the advice. I have one more question. Can I tune a J-Pole antenna in my shop and then move it to the attic and expect the same performance? In other words, is an antenna an antenna or is an antenna and it's installation an antenna? It would seem to me that if the antenna is tuned and works in my shop it should work in the attic. Otherwise, how do "store bought" antennas work? Thanks, jimbo |
SWR Tells Me??
jimbo wrote:
Thanks for all of the advice. I have one more question. Can I tune a J-Pole antenna in my shop and then move it to the attic and expect the same performance? In other words, is an antenna an antenna or is an antenna and it's installation an antenna? You can tune a J-Pole antenna at one place in free space, move it to another place in free space, and expect the same results. The real world is not so forgiving. The conditions will not be identical but they may (or may not) be close enough to be acceptable to you. Welcome to the real world of ham radio. -- 73, Cecil http://www.qsl.net/w5dxp |
SWR Tells Me??
On Tue, 21 Mar 2006 06:51:16 -0700, jimbo wrote:
In other words, is an antenna an antenna or is an antenna and it's installation an antenna? Hi jimbo The second, the wire you call an antenna, its method of feed, and its environment are the radiator. With care, it may work straight out of the box, but your numbers already offer a different story; thus the question becomes "what constitutes care?" It would seem to me that if the antenna is tuned and works in my shop it should work in the attic. Otherwise, how do "store bought" antennas work? The market for antennas destined to be put in an attic is very small, hence you see very few for sale. 73's Richard Clark, KB7QHC |
SWR Tells Me??
"jimbo" wrote in message ... jimbo wrote: Thanks for all of the advice. I have one more question. Can I tune a J-Pole antenna in my shop and then move it to the attic and expect the same performance? In other words, is an antenna an antenna or is an antenna and it's installation an antenna? It would seem to me that if the antenna is tuned and works in my shop it should work in the attic. Otherwise, how do "store bought" antennas work? Thanks, jimbo I have built about twenty copper-pipe j-poles and I have found that with five feet or so between the antenna and surroundings, the tuning doesn't change enough to be annoying. The copper pipe j-pole can be dropped into a pipe and hoisted into the clear for VSWR evaluations if you really want to get gnats-assey about it. Speaking of that, I have found that j-poles usually don't hit 1:1, even when the low point is mid-band. However, after I get the best match, I insert a 100 pF capacitor between the coax center pin and the antenna and "Viola!" I stole that idea from the gamma match. |
SWR Tells Me??
|
SWR Tells Me??
On Wed, 22 Mar 2006 06:22:12 -0700, jimbo wrote:
wrote: On Tue, 21 Mar 2006 22:15:22 -0800, "Sal M. Onella" wrote: "jimbo" wrote in message ... jimbo wrote: Thanks for all of the advice. I have one more question. Can I tune a J-Pole antenna in my shop and then move it to the attic and expect the same performance? In other words, is an antenna an antenna or is an antenna and it's installation an antenna? It would seem to me that if the antenna is tuned and works in my shop it should work in the attic. Otherwise, how do "store bought" antennas work? Thanks, jimbo I have built about twenty copper-pipe j-poles and I have found that with five feet or so between the antenna and surroundings, the tuning doesn't change enough to be annoying. The copper pipe j-pole can be dropped into a pipe and hoisted into the clear for VSWR evaluations if you really want to get gnats-assey about it. I've found getting them 6ft (for 2m ) above the ground was enough that another 20ft didn't change it enough to measure. Speaking of that, I have found that j-poles usually don't hit 1:1, even when the low point is mid-band. However, after I get the best match, I insert a 100 pF capacitor between the coax center pin and the antenna and "Viola!" I stole that idea from the gamma match. I've had good swr match(1:1 or unmeasurably close) without the cap. Also the original poster has the ladder line based jpole in the attic. If it's like most I've seen the attic is within 5ft . Like I'd said elsewhere the twinlead and ladder line versions are fussier and tend to be different from one batch of wire to another. Allison Yes, the attic has the roof within five feet. No walls but lots of cross braces. I am going to tune a J-Pole in my shop and then take it to the attic and check the difference. jimbo I have a ladderline j-pole right outside my attic, in a small tree, and I lucked out on swr, just following the measurements I found on the internet diagram of the thing. One trick you might try is to stick a straight pin into the insulation, touching the longer wire element. Raise and lower the pin for resonance. bob k5qwg |
SWR Tells Me??
Must comment- that the answer is "MAYBE"! It depends on what is
UNDER that roof! If you have metal flashing (near vents, furnace pipeing), or insulation with a "VAPOR BARRIER" usually, aluminium foil backed, it may affect your ability to xmit a signal , and in the case of the latter, might make a great "FARIDAY SHIELD" (signal proof room)! And this would apply to comercial antennas as well! Tho, the wood in a roof, and the roofing material should be generally passive to even UHF, the other materials might be a problem. Dont give up hope, as have friend witha antenna (quad), on 6 meters, and 2 meters, in an attic dormer, if it had been insulated correctly, would have been relatively RF Proof! Might want to examine it before betting too complex! Jim -NN7K "jimbo" wrote in message ... jimbo wrote: Thanks for all of the advice. I have one more question. Can I tune a J-Pole antenna in my shop and then move it to the attic and expect the same performance? In other words, is an antenna an antenna or is an antenna and it's installation an antenna? It would seem to me that if the antenna is tuned and works in my shop it should work in the attic. Otherwise, how do "store bought" antennas work? Thanks, jimbo |
SWR Tells Me??
On Mon, 20 Mar 2006 08:12:51 -0700, jimbo wrote:
OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic. I have 50 feet of LMR240 coax running to the basement. I measure SWR at the following frequencies on simplex. 144.2 2.5 145.2 2.4 146.2 2.3 147.2 1.9 147.9 1.7 Can I conclude that the antenna is electrically short for the 2 meter band? Thanks for any insight, jimbo That or it is mis-matched -- 73 for now Buck N4PGW |
| All times are GMT +1. The time now is 03:02 PM. |
|
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com