SWR?????

[Cheech]

Okay here is the deal I have a set of astatic 600s inline with my radio and something has got me confused although I know there is an explanation to it. The swr at the meters reads about 1:5 on the regular 40 and flat match or close to flat match at the radio. Now if I go down to my lowest chanel 26.065 the swr at the meter is 1:3 and 1:8 at the radio. I thought the meters may be off so I took the 600s out of line and put the coax directly to the radio and it shows exactly what the 600s did. I thought swr should not change anywhere along the coax line, or atleast the readings should be close?

 

[Captain Kilowatt]

If the antenna is a true 50 ohms with no reactance then the SWR will be the same any where along the line,a perfect 1:1. Anything differant from this value will show up as variations in impedance at various points along the line.That is why some people think they can tune the coax length to get a better antenna match.All they are really doing is in effect moving the radio to a point along the coax where the SWR is lower.You can prove this by inserting a jumper inline and noting the change in SWR.

 

[Switch Kit]

I hear this size thing all the time , which I understand in theroy , I have a GEO Metro and I use a Wilson 5000 roof mount ,at first I was using only 10 foot of mini 8 and had a flat match on it , I then got a new amp and was having trouble with the relays sticking on the high side and I was only using a 3 foot jumper of the same gage , I then but a 18 foot piece from antenna and my match went to a 2.1 )-: ......Alrightty then !!! So then put a 24 foot of mini from the antenna and the match shot right back down to 1.1 , using the same 3 foot jumper from radio to amp , was a flat match again with a match 1.3 with the amp on high and there was no longer any sticking relays. So in theroy this is all wrong right ? Well I've even had a few fancy type meters on my stuff where it only said I had a watt or two reflective power at the same 1.3 with the power on High , I have left it like that for 4 years now , I have had no problems what so ever ......but yet in theroy it should all be wrong ? Well ,wrong or not ? My system works and works very well for me ,theroy or no theroy. In my case .what more could I really do that I haven't already did ? I only have not had one of those high dollar antenna analizers on it ,maybe that machine might tell me different ? but then again , I still don't know what more I could do there after if the analizer told me different. Good luck to you Cheech. In my case up to this point ,If it works ,I need not fix it.

 

[DXman]

I thought swr should not change anywhere along the coax line, or atleast the readings should be close?

This is true. It should not change like QRN mention.

Try putting a 14' jumper from the radio to the meter and check it again or if you have around a 9' piece then try that. It could also be your meter that could be causing the problem and don't rely on the meter in the radio.

 

[Cheech]

Yeah im going to change jumpers and see of that changes anything.

 

[Captain Kilowatt]

I hear this size thing all the time , which I understand in theroy , I have a GEO Metro and I use a Wilson 5000 roof mount ,at first I was using only 10 foot of mini 8 and had a flat match on it , I then got a new amp and was having trouble with the relays sticking on the high side and I was only using a 3 foot jumper of the same gage , I then but a 18 foot piece from antenna and my match went to a 2.1 )-: ......Alrightty then !!! So then put a 24 foot of mini from the antenna and the match shot right back down to 1.1 , using the same 3 foot jumper from radio to amp , was a flat match again with a match 1.3 with the amp on high and there was no longer any sticking relays. So in theroy this is all wrong right ? Well I've even had a few fancy type meters on my stuff where it only said I had a watt or two reflective power at the same 1.3 with the power on High , I have left it like that for 4 years now , I have had no problems what so ever ......but yet in theroy it should all be wrong ? Well ,wrong or not ? My system works and works very well for me ,theroy or no theroy. In my case .what more could I really do that I haven't already did ? I only have not had one of those high dollar antenna analizers on it ,maybe that machine might tell me different ? but then again , I still don't know what more I could do there after if the analizer told me different. Good luck to you Cheech. In my case up to this point ,If it works ,I need not fix it.

:?: :?: :?: :?:

Unless I missed something you proved my point.You changed the coax length and the SWR changed.My guess is that you were reading a good match at the end of that 10 foot length of cable but have no idea what the true antenna impedance was.Obviously it was not a pure 50 ohms but the coax transformed whatever impedance it was into something close to it.The only way to insure the impedance is a real 50 ohms is to measure the SWR through a 1/2 electrical wavelength of coax.When I set up my 80m dipole I measured the SWR while feeding it through 87 feet of RG58 which is an electrical 1/2 wave of that type of cable.After tuning and pruning I connected it to my permanent feedline which is about110-120 feet long and the SWR was exactly the same.I even added a 20 foot piece and it still remained the same because the antenna itself was a true 50 ohms.Yeah I know a dipole is supposed to be near 72 ohms but it is only 40 feet high and is more of a vee configuration.

 

[DTB Radio]

You can download a simple coax length calculator from my website, towards the bottom of the front page. It will give you the correct length to use for the most accurate SWR readings while tuning.