[Top] [All Lists]

Re: Topband: 4sq receive antenna side filters?

To: Grant Saviers <>, "" <>
Subject: Re: Topband: 4sq receive antenna side filters?
From: Lee STRAHAN <>
Date: Mon, 11 Sep 2017 23:58:48 +0000
List-post: <>
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 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 [] On Behalf Of Grant Saviers
Sent: Monday, September 11, 2017 7:26 AM
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
Topband Reflector Archives -
Topband Reflector Archives -

<Prev in Thread] Current Thread [Next in Thread>