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Tuesday, 10 March 2015

My SWRs got no nose - How does it smell?

Well,

You may recall the 6 Element LFA 2M Yagi I installed back here:

http://g0mgx.blogspot.co.uk/2014/08/this-is-even-more-bonkers-than-before.html

plus the masthead amplifier I built and installed back here:

http://g0mgx.blogspot.co.uk/2014/12/how-can-something-so-simple-be-so-tricky.html

There has been something very strange indeed going on with this antenna. Here's the evidence as I see it:


  • The SWR varies - not while in use, but sometimes it's not as good as it was when I left it!
  • It is either good or bad, I have never seen it go from good to bad while in use;
  • When it's bad the Gemini linear trips with high SWR (which is good)
  • The bad SWR does seem to be more present when it's cold
  • When it's bad the SWR seems to be about 1.8:1
  • Having had the antennas down many times to look at this problem, the only thing I am suspicious of is a "back to back" N-Type connector that's up in the air. Everything else seems OK to me.
When I installed the pre-amp, I didn't want to hard wire it in so it was a permanent installation - I therefore decided to hard wire it to the cable tail that connects to the antenna, and fit an N-Type connector to the other lead from the pre-amp. This other lead is connected to the Westflex 103 that feeds back to the shack by means of a "back to back" N-Type plus the N-Type fitted to the end of the Westflex - got it?

So - the connectors are all expensive (not that this means they are good quality, but they look good to me) and they are compression fittings. No amount of wiggling these connectors will make the SWR change. 

I have concluded that the only thing that might be causing my problem is the back to back N-Type connector. To remedy this I have purchased a Female N-Type in-line connector which I can fit to the end of the pre-amp tail, thus eliminating the need for the back to back connector. 

Today I brought the  antennas down and found that by adjusting the 2M antenna, I could reduce the SWR further than it was when I started. Now, due to my suspicions, this was the return loss trace from the 2M beam last time I finished fiddling with it:


So, according to this I had an SWR of 1.55:1 last time I finished with the antenna, after my fiddling today, today my AA-170 analyser is saying this:



which is a return loss of 22.68 dB and an SWR of 1.16:1. My return loss bridge on the spectrum analyser is saying this:


which is that the return loss is 22.12dB and an SWR of 1.17:1 - which is pretty damn close to the analyser or perhaps the AA-170 is close to the SA. What I am trying to say is that the results are astonishingly similar!!

This suggests that I hadn't adjusted the antenna properly when I finished with it last time - perhaps I just got bored? Who knows!

I haven't fitted the female N-Type and will wait and see what, if anything changes going forward. If I have any recurrence of this jumping SWR the pre-amp will have to come down and the female N-Type connector fitted.

Odd, egh?  

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