I was asked by a DXpedition leader if it was practical to bandpass
filter the input (antenna side) of the DX Engineering 4 sq's they plan
to use (I have one). Given the few acres of the island and 5 feet ASL
there isn't space to get adequate antenna separation and of course both
transmit antennas are verticals. The concern is with QRO transmitters
on 160 and one on 80m at the same time that the coupling will cause
overload and IM. Beverages would not work above the salt water
intrusion. A direction switchable DHDL is planned for 40m.
Any thoughts on if this is possible, been done before, or how to do it?
A deeper question is what impedances are possible for a 20' whip and the
input of the amplifier for a filter design? Sacrificing noise
performance, some gain, and frequency range are all reasonable to solve
the problem. Probably more than 40db of rejection is needed. Switching
a pair of filters with an outboard box on each whip looks to be the easy
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