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Thursday, 11 April 2013

My antennas got no nose; well no decent SWR anyhow!


Further to my musings last time about SO2V, I've been making some changes here with a view to getting a second HF general purpose antenna up in the air - the idea being that this can be connected to the second V!

I took down the 30M (10MHz) dipole that I had here and put up a half size G5RV - this was something I had kicking around anyhow and it's fed with 400 Ohm ladder line. It's good and clear and up quite adequately high. I was rather horrified and/or surprised to find that signals on 40M were way stronger on this G5RV than on my 40M/80M dipole - something was not right!

So here's what I did:
  • Checked the antenna SWR to find it close to infinite everywhere....
  • Replaced the feedline (RG213) between the shack and the ground level under the dipole centre.
  • Replaced the feedline (RG58) between the end of the RG213 and the dipole centre.
  • Checked the SWR with the analyser at the end of the RG58 feeder and there is a clear SWR curve around 7 MHz ish
  • Then checked the SWR at the shack end of the RG213 cable and its 10:1 flat….
  • Went for a curry
  • Decided to look again at the dipole centre so I dropped the antenna again
  • Centre looked OK, so tried to pull the antenna back up and broke the bailing twine in the pulley at the top of the pole at the bottom of the garden
  • Gave up completely and went to bed 

Today I've replaced the pole at the bottom of the garden and made an alternative antenna, here is the design I came up with:

The inductors ("L") in the design calculate out at about 80uH.

The first thing I did was to just construct the two 33' legs of the antenna and get them tuned to the RTTY/Digital segment of 40M - that was a doddle. Then I added the coils and the stubs, the calculation was aimed at 3.050MHz and here (using my new fancy analyser thingamabob):

So not bad at all! Here's the plot for 40M now I've added the stubs - clearly there has been some slight de-tuning of the 7MHz part of the antenna, but I can live with it for now and fiddle some more with it later:

The only other thing I have to report was that following my being a good boy at work I have bought myself a gift:

The picture doesn't do the radio justice; it's quite nice looking and has a shiny finish (not the normal Yaesu matt). The good things are that the radio includes a CW decoder, plus in-built decoding for PSK and RTTY - you can also send all three modes without any external equipment.

All in all I think it will be an excellent second radio for me - it has replaced the TS-590 - too early to report fully but early signs are good!

Fun, egh?

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