Wednesday, December 18, 2019

Omega match shunt fed tower

Final match, I removed the ladder of course...
I got a insight of how to construct this when watching VE6WZ videos on this subject. I miss a good working full size vertical antenna for 60m and after my successful efforts with a top loaded shunt fed unipole vertical made from my antennatower at the start of this year. I could have repeated building it but currently have my autotuner feeding my vertical all band antenna on top of the tower. So I still search for another solution. This describes my experiment with shunt feeding my tower with a omega match or gamma match. I took a 80cm alu corner strip and tighten it to the tower with ty-raps. Measured with a ohm meter if I had a good connection of course. A single wire is going down from the end and on the ground around the mast in star formation are about 10 radials of 15m each. Well, I guess you look at the video first to understand what I'm talking about.



I followed Stephen's advice and searched for a 50 Ohm point on my tower with the antenna analyzer. Started too hight of course and found 50 Ohm on 4,4MHz. So I went on a little lower in a few steps for a 50 Ohm point at roughly 5,360MHz. (You can click the pictures for a larger one.)

A variable capacitor should make the SWR 1:1 but in practise I got a 1:2 match only. I got a better match when I moved the wire further from the tower.


 The final match is 1:1 SWR with almost a R=50Ohm and X almost nil. Good enough for me.

And now the final test, does it work?
Well not surprisingly it does but not as good as my inverted-V. It works like the vertical I made last year alongside the tower and it works just as well compared to the 1/4 wave endfed. About 3-4dB less signal as my inverted-V both on receive as transmit. Compared to my vertical on top of the tower it is about 2dB better. Yes, even on DX it doesn't work as well as my inverted-V.

Is it disappointing? Well it is of course, after each experiment you hope to find something that works much better. But unfortunately this is real life, and not every experiment will succeed. If you have a free standing tower like me and it is at a reasonable distance from any building (with iron inside) it could work well with a system like this. But I guess a vertical antenna for me is out of the question due to the metal structures near the tower.

Now a few other antenna experiments are on my wish list. MW1CFN John suggested I should try a sloper configuration. This will be a next experiment. I tried a sloper before at my previous location. It was a 3 band, 160m, 80m and 40m and it worked well. But at that time I didn't have my 2x20m inverted-V with open line feed, so I couldn't compare. Another experiment would be a single shortened dipole in the tower. I've had the idea from PA2S Henk, but think it is really a compromise antenna to turn away the noise. A magnetic loop outside the house probabely does equally. And that is a experiment I wish to do as well but it has to wait for summer. A large magnetic loop only for 60m, will it work? Too many ideas....and not that much time.

4 comments:

  1. Hello Bas - my own experimentation with vertical antennas has shown me the reality of what all the books say - that they are far better at transmitting than receiving. I end up being copied by stations that I can't hear. This was most evident on 630m, but on 160 as well. For me, a lowband vertical would only make sense if I could have a separate rx antenna. So until that day arrives, I use a mediocre tx antenna with the knowledge that I can receive anyone who can hear my puny signal!

    73 - John

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    Replies
    1. Hello John, unfortunately I got less good results on TX as well. Tried it with several "DX" stations. I think the iron in the buildings around the tower is having too much influence on the radiation pattern of the radiating tower. At least that is what I think. Anyway, i'm just a experimenter and not a expert. By the way I just missed you on 60m yesterday. I was on till 23:30 UTC and then went to sleep. But left my station monitoring. I received 2 of your CQs at 23:40 UTC....I'll keep an aye on the cluster...next time better...73, Bas

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    2. At least my dipole is working then! That was my Anan-10e and HR50 at about 55 watts. I won't be on again in the evening until Saturday but I have your callsign set as an alert in WSJT-Z's filtering.

      73,
      John

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  2. Well, as you say Bas, you learn a lot more from people's blogs of things they've done than from magazines!

    That was a very interesting post and embedded video. Always the people who know their stuff are the calmest...

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Thanks for your comment. Bedankt voor je reactie. 73, Bas