Posted: 3/16/2011 10:24:04 AM EDT
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Ok, I fully admit I crammed for the tech a year ago, bought a bunch of gear and am not an engineer. My understanding of SWR is limited to how much of the signal is getting out vs being reflected back.
I have an Arrow dual band j-pole on the roof and an Arrow 4 element take down yagi. Both are fed with 100ft of LMR 400 to the basement, with the excess coiled within 10 feet of the radio. I had never checked SWR on these as they seemed adequate. I recently picked up a cheap meter and hooked it up to the 857 and have an SWR of over 2 on each antenna. The yagi is adjustable I see, and I will do that. My question is where to I start with troubleshooting this? Can I troubleshoot this with my cheap meter or is a $300 analyzer required? |
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Length of coax should be irrelevant. The exact length in wavelengths might change the SWR a little due to transformer effects but if the load is well matched there would be little change.
First I would check and make sure that you know how to operate your meter, to make sure you are getting accurate readings. Some meters you have to adjust a calibration on the forward power, then switch to reverse to get a reading - if you don't adjust the forward calibration correctly on these kind, the SWR you get will be bogus. If it's a cross needle type or the like, just ensure that you're reading it correctly. Either way, maybe get a "second opinion" from another instrument if available. High SWR could be a result of an antenna problem, bad connector installation, coax cable damage, antenna nearby to other objects, or other potential issues. Best way to troubleshoot is to eliminate as many variables as possible, and to test each part of the system independently as best as possible. Test your meter with a dummy load connected right to the meter, to test meter function. Test the antenna at the antenna using a short test connection, preferably a test jumper that's a 1/2 wavelength at the test frequency. If this is high, start adjusting the tuning on the antenna to bring it down. Test the full run of the coax with a dummy load at the end instead of your antenna, to ensure that your coax and connectors are sound. |
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Quoted:
Length of coax should be irrelevant. The exact length in wavelengths might change the SWR a little due to transformer effects but if the load is well matched there would be little change. First I would check and make sure that you know how to operate your meter, to make sure you are getting accurate readings. Some meters you have to adjust a calibration on the forward power, then switch to reverse to get a reading - if you don't adjust the forward calibration correctly on these kind, the SWR you get will be bogus. If it's a cross needle type or the like, just ensure that you're reading it correctly. Either way, maybe get a "second opinion" from another instrument if available. High SWR could be a result of an antenna problem, bad connector installation, coax cable damage, antenna nearby to other objects, or other potential issues. Best way to troubleshoot is to eliminate as many variables as possible, and to test each part of the system independently as best as possible. Test your meter with a dummy load connected right to the meter, to test meter function. Test the antenna at the antenna using a short test connection, preferably a test jumper that's a 1/2 wavelength at the test frequency. If this is high, start adjusting the tuning on the antenna to bring it down. Test the full run of the coax with a dummy load at the end instead of your antenna, to ensure that your coax and connectors are sound. This!!!! de W1EL |
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Antenna analyzer. Ask an Elmer in your locale if they have one you can borrow.
I just finished putting together a home-brew Jpole similar to the one that Arrow sells. I measured twice, cut once, measured again, and couldn't wait, so I connected a radio to it and put it up on the roof. Signal reports were good to my local repeater at low power 5w (about 15 miles away over flat ground) so I didn't think another thing of it. Well, after a conversation with a sage local elder in the radio club, he told me to meet him and that he had something for me. I show up and he hands me an antenna analyzer and says "I heard you put up a new antenna. Try this out and let me know what you find out." And then he hands me a 20-ft and 40-ft section of LMR-400 with PL-259s attached and says "Hey, I had these laying around the house. Try 'em out and let me know what you think." And then, he hands me a bag full of Anderson Powerpoles and a crimper and says "I heard on the net you needed these. Use what you need to." So needless to say, I was shocked and speechless. Back to the point, I ran home and hooked up the antenna analyzer. Wouldn't you know it, but my carefully built JPole was over 2:1 SWR across the VHF band and 3:1 SWR on UHF. D'oh! I finally figured out that while I had paid close attention to the vertical length of the elements, I hadn't paid very close attention to the horizontal space between them. After I fixed that, I was sub-2:1 across VHF and UHF, with the sweet spots of less than 1.4:1 SWR at 146.00 and 446.00 MHz respectively. Eureka! Mind you, an antenna analyzer won't tell you what is wrong with your antenna, but it'll give you an instantaneous readout of any changes or tuning you might make. I really ought to hate the guy that loaned me all this stuff because it just means I've got to spend about $400 on an antenna analyzer and another $100 on coax cable. :) |
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Antenna analyzer. Once you use one, and find out how easy and useful they are, then you have to go buy one. We have mentioned that this radio stuff is as bad as ARs, haven't we? Friggin' A, right. I guess I'm lucky –– I've got this one on loan for as little or as long as I need it, and I know at least three other folks in my club that have them as well. |
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Quoted:
Antenna analyzer. Ask an Elmer in your locale if they have one you can borrow. I just finished putting together a home-brew Jpole similar to the one that Arrow sells. I measured twice, cut once, measured again, and couldn't wait, so I connected a radio to it and put it up on the roof. Signal reports were good to my local repeater at low power 5w (about 15 miles away over flat ground) so I didn't think another thing of it. Well, after a conversation with a sage local elder in the radio club, he told me to meet him and that he had something for me. I show up and he hands me an antenna analyzer and says "I heard you put up a new antenna. Try this out and let me know what you find out." And then he hands me a 20-ft and 40-ft section of LMR-400 with PL-259s attached and says "Hey, I had these laying around the house. Try 'em out and let me know what you think." And then, he hands me a bag full of Anderson Powerpoles and a crimper and says "I heard on the net you needed these. Use what you need to." So needless to say, I was shocked and speechless. Back to the point, I ran home and hooked up the antenna analyzer. Wouldn't you know it, but my carefully built JPole was over 2:1 SWR across the VHF band and 3:1 SWR on UHF. D'oh! I finally figured out that while I had paid close attention to the vertical length of the elements, I hadn't paid very close attention to the horizontal space between them. After I fixed that, I was sub-2:1 across VHF and UHF, with the sweet spots of less than 1.4:1 SWR at 146.00 and 446.00 MHz respectively. Eureka! Mind you, an antenna analyzer won't tell you what is wrong with your antenna, but it'll give you an instantaneous readout of any changes or tuning you might make. I really ought to hate the guy that loaned me all this stuff because it just means I've got to spend about $400 on an antenna analyzer and another $100 on coax cable. :) i built the same antenna. under 1.7:1 across both bands. the only down side was i couldnt find a single piece of Al long enough so i cut and threaded 2 pieces and use a coupling nut to connect. so now i have a portable J pole. LOL. also build an "arrow" 2m yagi that i stole the design from them. 8 aAl arrow shafts, couple ft of 1" square Al tubing and a home made gamma match. i have to redo the gama but when prop was up last summer i talked 180 miles into a PA repeater with full quieting. |
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Update, ordered a dummy load and a MFJ-259b. Checked the rooftop j-pole and the attic yagi both are acceptable, shitty swr meter before was possible. I am seeing high coax loss's but I need to wait awhile before I get the climbing gear on and put a dummy load on the end of my coax. (snow on the roof this am)
I tried making a MURS band roll up j-pole out of 450 ohm ladder line for a friend to help me learn how to use the new meter. Used these plans Plans and an online calculator for 151.88. Measured everything in mm as close as I could and I could not get any swr below 2 in the entire range. Of course I started hacking away at the long element it got better but no where close. Tried the advanced resonance mode and got X to zero at around 138 if I remember correctly and SWR was still high. So the ant can be resonant and you can still have a high swr due to other factors like cable etc? I will further test the cable and connections. I am using 30 feet of 8x, that I crimped and soldered. I did not air choke this, I will try that also. |
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... shitty swr meter before was possible. I did not air choke this, I will try that also. "Cheap" SWR meters (crapshack, etc) are usually "pretty good" for HF but often read high at UHF and above, as it seems you have discovered. Your 100' of LMR400 is going to have some loss, can't help it. But at the same time it'll serve to lower the apparent SWR at the radio and reduce the Q of the system, give you a little more bandwidth. IMO no big deal on FM unless your length is an odd quarter wave (times velocity factor) on one of the operating bands. I'm not sure about the air choke. It isn't usually done at VF/UHF and I'd expect it to introduce a fair amount of extra loss. On the J-pole you could put some decoupling radials at the bottom of the antenna (as is done with the Ringo Ranger, A99, etc.) and maybe get back some of the performance lost in the feedline. On the Yagi there's probably not much to do but tune carefully and observe proper feedline routing. |