That is the antenna I use on 80 meters. It has a double-humped SWR
response and stays below 2:1 between 3500 and 3900kHz. Up at 4000kHz it
is a little higher than 3:1. The 1/4 wave 75 ohm section is a
transmission line transformer. The 1/2 wave 50 ohm line repeats the
impedance at the antenna feedpoint.
Mine is an inverted Vee with a 120 degree angle at the apex.
Since the 75 ohm section is 1/2 wavelength on 40 meters, the same
feedline can be used on 40. So I also have two radiator wires for at
right angles to the 80 meter radiator. Being at right angles, there is
no interaction.
73,
Don W3FPR
On 3/14/2019 2:51 AM, Jim Brown wrote:
Here's a VERY old solution that W6NL has taught to his EE classes. When
I asked him about it, he says it's not his original idea, far older than
him.
Tune your 80M dipole to 3675 kHz, feed it with a half-wave of 50 ohm
coax starting at the antenna, then a quarter wave of 75 ohm line. Line
lengths are at 3675 kHz. Add 50 ohm coax as needed to reach the shack,
or coil up the excess if those two lengths are too long. The result will
be SWR below 2:1 from 3500 kHz to 3900 kHz. With good RG213 and RG11,
Line loss is about 0.5 dB at the resonant point and about 0.7 dB at the
limits.
If you want to do some engineering, you can probably tame that 40M
antenna too. Here's a tutorial applications note.
http://k9yc.com/PacificonSmithChart.pdf
73, Jim K9YC
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