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| Widely variable SWR over time | |
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| Tweet Topic Started: Sunday, 23. October 2016, 20:25 (318 Views) | |
| Krupp Stahl | Sunday, 23. October 2016, 20:25 Post #1 |
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26 Charlie Tango 2248
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I wonder if you can help a beginner.... I'm having a bit of an issue an with widely variable SWR readings. Over the last few evenings (since I got my rig up and running), SWR readings have been quite reasonable but varying from around 0.5 to 2 over time. This evening, I found that reception was suddenly extremely poor and when I tested the SWR on channels 1 and 40, the readings were extremely high (>3). Now all of a sudden a few hours later and for no apparent reason, SWR is showing as: 2.5 on channel 1 2.0 on channel 20 1.4 on channel 40. My main concern is the apparent variation in SWR over time. I'm loathe to adjust the antenna if things are simply going to change an hour afterwards. I am running a Communicators NI-440 DX (from 1981) with a MAAS KM-500 magbase antenna sat on a table. I plan to upgrade the antenna soon, but in the meantime any advice on how to deal with the SWR would be much appreciated. I wonder if my SWR meter is a duffer. |
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| Mirmanuk | Sunday, 23. October 2016, 20:52 Post #2 |
26 Charlie Tango 2064
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Is the table you mention outside and is it metal? If not I would look at that as a cause first, if indoors anything moving around it may have an effect as the signal gets reflected back off of objects , walls etc. If not on a good metal base then ground plane issues could be a problem, if outside then dampness may be changing the ground plane effect improving or de-minishing it. You may have to add some ground planes, wire etc. Double check the coax and connectors, for looseness, damage, are the connectors dry, check the mag mount base, if its old the inner connections could be damaged. It is mainly a process of elimination, could be one thing or a combination of more. Once you get a good antenna up in the air you will notice a big difference. (Or make your own out of some wire and a fibre glass fishing pole, a half wave di pole is very effective and not that big (2 quarter wave lengths of wire (Taped to the pole) joined in the middle with some coax). Happy CB'ing : ) Steve |
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| Krupp Stahl | Sunday, 23. October 2016, 21:14 Post #3 |
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26 Charlie Tango 2248
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Hi Steve, Many thanks for your informative reply. Reading your post suggests to me that my current set up is far from ideal! My antenna (as well as being very small) is currently situated indoors and the surface it's sitting on is non-metallic (it's wood). It would be fairly straightforward to relocate it to the back garden and I could certainly try to source some sort of metallic surface for for the magbase to sit on. I'm going to have to look into the ground plane principle I think! The antenna would still be located fairly near to the house however, due to the limited length of the coax which again I guess would be quite limiting. I can see that a much larger outdoor antenna and asscociated flex is already on the cards if I am going to get anywhere with this. |
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| Smokey Jay | Monday, 24. October 2016, 09:41 Post #4 |
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In that case you are working with antenna with no ground plane!, and I'm amazed the SWR reading isn't off the scale. The magnetic mount achieves grounding (to a metal surface) through capacitive coupling - It will not work on wood. There will be no ground and if the antenna is on the table with you, I recommend not getting to close to the antenna whilst transmitting as you are in danger of getting RF burns. I strongly recommend getting an outside mounted antenna meant for the job. Edited by Smokey Jay, Monday, 24. October 2016, 09:42.
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| Krupp Stahl | Monday, 24. October 2016, 19:23 Post #5 |
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26 Charlie Tango 2248
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Hi Smokey, Thanks for your reply I've relocated the antenna to a large rectangular radiator for now and, touch wood, it seems to have rendered the SWR relatively stable across the spectrum for now. I am going to look into an external antenna. I can see that what I've got at the moment is a long way from the ideal and is limiting the set's potential. Thankfully, I've visible no scorch marks as of yet - there's not nearly enough traffic around here for that! However, I get the distinct impression I'm not receiving much of what's out there as I have encountered several 'half' conversations. The way forward is clear as you say. |
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