Shunt Loaded Towers

Today I finally finished the tune-up on the 80m shunt.

I have had a shunt on my VHF tower for a couple of years. It seems to play ok. In
the past I have used door knob capacitors to get it into the ballpark. The challenge
is that then the antenna is locked to a small portion of the band. 80m is a big band
from 3.5mhz-4mhz. That is big place.

Earlier this year I decided that I wanted to able to use the antenna in the bottom
of the band 3.525-3.585mhz for CW, RTTY, and PSK in addition to being able to work
the DX window for SSB up around 3.8Mhz. Additionaly I did not want to have to use
a tuner since I like to build QRP rigs which do not normally have a tuner so I need
a decent match.

The answer was to use a biasT to power a remote relay at the feed point that allows
me to switch between two variable capacitors. One is tuned to the CW portion of the
band and the second one is tuned to the SSB DX window. A simple flip of the switch
from the shack and I can now move from the bottom of the band to the top and I am
back in business. (No running outside in the winter with my analyzer to retune it
i the dark.)

Thanks to N0FP for helping design the biasT network on a piece of paper and letting
me borrow some junk box components. He also provided me a nice spreadsheet a couple
of years ago that helps to figure out the turns ratio for winding transformers. I
wound a custom transformer to help get the feed point between 50-75 ohms. This is
my third generation feed point and I am extremely happy with the results.  (I
also have a shunt for on 160m on the taller HF tower.)

The magic:

On/Off switch + BiasT + relay

Two variable capacitors roughly 75-150pF each

A custom wound transformer for the feed point impedance match

A custom wound balun

Please ping me if you would like more info or pictures.

73 de NG0R in EN25ue