I already posted a diagram of the 64:1 transformer in a thread here: http://radiopreppers.com/index.php/topic,1292.0.html in case you want to build one.
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Show posts MenuQuote from: caulktel on June 11, 2017, 06:58:42 PM
Hi all,
I normally use EFHW antennas for all my portable ops with a 28ft fiberglass pole, but they are all mono band, so I got this idea to make a multi band 20,40 meter EFHW using these traps from, http://www.sotabeams.co.uk/pico-traps-kit-pair/ I understand how to tune the traps, but I'm not clear on how to determine where to place the trap on the wire. I assume being as I want to use it on 20 and 40, that it needs to be approximately 66 feet long total length, but where do I insert the 20 meter trap, at 33 feet? I have done a lot of searching, but have not found a definitive answer on how to figure it. Do I build the 20 meter section first with the trap at the end and tune it to length, then add the 40 meter section? Any help would be appreciated.
Joel
N6ALT
Quote from: gil on May 28, 2017, 04:59:48 AM
Thank you. I wish we had 60m here in France...
Gil.
Quote from: Quietguy on March 23, 2017, 09:11:28 PMQuote from: DJ6KR on March 23, 2017, 10:00:54 AM
It is all about the reflector on the ground under the Antenna, I think.
Yes, I have a copy of Terman's book that is referenced in the article. Terman discusses a "Half-wave Antenna with Reflector" starting on page 903 and derives the gain. He shows plots of gain compared to radiation resistance as a function of antenna to reflector spacing. Radiation resistance (and efficiency) goes way down with small spacing, while gain stays around 8 up to a spacing of about 0.1 wavelength (about 13 feet on 40 m) before falling off with increased spacing. He says:
"In order to prevent incidental loss resistances from making the antenna efficiency very low, the spacing S should accordingly be at least 0.05 wavelength, and preferably 0.1 wavelength."
The article uses 7 feet, which is about 0.05 wavelength at 40 meters, but Terman's chart shows you would have better efficiency without losing gain if you made the antenna 14 feet high.
Wally
Quote from: gil on February 24, 2017, 01:20:58 PMQuoteUse an FT140-43 core and wind 18 turns of RG174 cox on it
Good one... I have slip-on ferrites that are just the right diameter for RG-174. If that doesn't work I'll try the toroid.
Gil.
Quote from: cockpitbob on December 21, 2016, 09:53:31 AM
That tuner has been on my list for a while :). /jealous
I hadn't thought about protecting the finals while it finds, or fails to find a tune. I can't turn down the power on my MTR and I run it at 12V so it's in the danger zone for a bad mismatch. Would you worry about using this tuner with your MTR and for example a random wire with 9:1 unun?