Topband: 4sq receive antenna side filters?

[Grant Saviers]

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 
part.

Grant KZ1W