Topband: Shunt fed Tower Question
wa5rtg at gmail.com
Mon Dec 9 08:36:34 EST 2019
I have my tower on Cayman Brac Shunt fed for both 160 and 80 using separate coaxes, capacitors and shunt feed wires. Works great, except that I don't like the way it behaves. It appears to me that when I am on 160 for an extended length of time that something may be going on with the 80m setup where the capacitor is getting hit by all the RF coming down the shunt feed wires to the capacitor.
I can take the 80m cage and connect it to the 160m shunt feed cage, even though the tap point is about 15 feet lower, eliminating the 80m capacitor and coax and retune it for just 160 and it works perfectly. I can also tune that setup for 80m if I want with a different capacitance value.
What happens when the 160m RF comes down to the 80m capacitor if I have separate coaxes for each band and how can I easily create a situation so I can have separate coaxes for SO2R or M/S setup without some complicated setup to activate relays when transmitting on one band or the other?
Grounding the shunt feed or disconnecting the coax on the unused band when transmitting on the other would solve the problem but seems like a difficult thing to accomplish particularly if you wanted, for example, to dual CQ, back and forth, between those bands.
Possible to put a hairpin type coil from where the capacitors connect to the shunt feed wire to ground?
The very best idea I have had, is a matching network that would match the antenna with a single wire cage for both bands with one coax. Might need some AC6LA or other type help with determine whether I could accomplish what is described here.
I know I can have a separate 80m antenna that is not connected to the tower and that works well but I want to use the tower which is effectively about 1/2 wavelength on 80 and may be the best antenna I have ever used based on the CQ WW experience on that band.
If any of you have any ideas I would appreciate knowing them.
Thanks... Stan, ZF9CW
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