Topband: 4sq receive antenna side filters?

[Lee STRAHAN]

Grant, and others.
    It is sort of possible to filter high impedance type RX 4-squares. The source impedance of a 2 foot long 1 inch dia. tubing vertical is a  few ohms resistance depending on the ground situation and approximately a 60 picofarad series capacitance. For the Hi-Z systems the amplifiers, their input impedance is approximately 43Kohms parallel with ~15 picofarad. I cannot speak to the DX Engineering 4-square values, only the Hi-Z systems.
   We have routinely supplied filter sets for the front end of the Hi-Z systems that work for the reduction of broadcast AM frequencies with a notch. They all have to be carefully matched components so the system phasing is not disturbed. There is no known degradation of the system performance using these filters.  I am also aware of a high pass filter system that is in use at a contest station that provides attenuation of frequencies above 40 meters also at the high impedance level. 
   I do not know of any single filters that would operate well without switching for different bands. I believe that it may be possible to build a switchable system that could provide your 40 dB of rejection of a lower band frequency band than the band operating on.   
   If you would like to discuss this in detail please e-mail me at k7tjr at msn.com and we can set up a telephone session or e-mail exchanges. I would be happy to share my work with you.
Lee  K7TJR   OR

-----Original Message-----
From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of Grant Saviers
Sent: Monday, September 11, 2017 7:26 AM
To: topband at contesting.com
Subject: Topband: 4sq receive antenna side filters?

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
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