Shirt & Mug:
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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.
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.
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?
Quote from: madsb on December 17, 2016, 10:02:41 am
Remember that a low SWR does not indicate an efficient antenna. Otherwise I would immediately swap my dipole for a 50 ohms resistor since the resistor is both compact, lightweight , easy to set up and has a consistent low SWR across all the HF bands :)
Sendt fra min S60 med Tapatalk
Quote from: gil on December 13, 2016, 06:01:50 pm
Interesting, I have never heard of this... Mine is working better now that I slipped ten ferrites over the coax, close to the middle, about 11m from the BALUN. I had to change the BNC connector so it was the perfect opportunity. Now the SWR is lower on all bands.
Quote from: gil on December 13, 2016, 09:17:19 am
Do you mean in parallel with the BALUN?