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Re: [TowerTalk] Antenna Traps?

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Antenna Traps?
From: "Joe Subich, W4TV" <lists@subich.com>
Date: Mon, 2 Jan 2023 18:15:41 -0500
List-post: <mailto:towertalk@contesting.com>


At 7.025 MHz you can think of this antenna as a shortened, resonant, 3/2-wavelength dipole.
Does the pattern show the typical narrow main lobe with substantial
quartering lobes of a 3/2 wavelength dipole at 7 MHz?

73,

   ... Joe, W4TV


On 2023-01-02 5:51 PM, Brian Beezley wrote:
I tried optimizing a loaded dipole for 3.525 and 7.025 MHz. I used an RLC load to properly represent a self-resonant coil. After optimization, the load wound up out 40.5' on a 53.9' wire (one side of dipole). Load loss for a 3" coil of #12 copper wire was < 0.1 dB on both bands. The free-space impedance was about 60 ohms on each band with negligible reactance. SWR = 2 bandwidth was 100 kHz on 80 and 210 kHz on 40. The antenna looks quite useful as a dual-band CW antenna.

I then optimized the same model using an RL load with no shunt capacitance. The idea was to show how the RLC model was essential for the operation of this antenna. The wire lengths changed slightly, but the RL model worked just fine. The shunt capacitance has nothing to do with antenna operation!

This image shows the wire current at 7.025 MHz:

https://i.postimg.cc/ydh1DDc5/dipole.gif

The red dot is the feed point. The green dot is the coil. The yellow trace is phasor current. Current changes phase just before the coil. At 7.025 MHz you can think of this antenna as a shortened, resonant, 3/2-wavelength dipole.

The longer I stare at that explanation, the less sure I am that it offers any insight. But it's the best I can come up with after being humiliated by the RL model.

Brian


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