[TowerTalk] Antenna Traps?

[Joe Subich, W4TV]

> 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