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Re: [TowerTalk] Comtek 4sq box question

To: "reflector -tower" <>
Subject: Re: [TowerTalk] Comtek 4sq box question
From: "David Robbins K1TTT" <>
Date: Sun, 20 Aug 2006 19:01:44 -0000
List-post: <>
That is probably because the actual loads on those ports are not 50 ohms,
they are something else, likely with a reactive component also.  Following
the comtek procedure... you initially adjust a single vertical to 50 ohms,
you then replace the feedline with 1/4 wave of 75 ohm.. that transforms what
was a single 50 ohm vertical into 100 ohms at the box end.  then you raise 3
more of them which creates coupling that will further change the load
impedance, and then you feed them at different phases which changes the feed
point impedances even more... and it will be different on the front, back,
and side ports.  For more details of what the impedance of the 4 verticals
might be try checking out on4un's low band dxing book and run through some
of the calculations.  When I last did that here I ended up with one
impedance of 3.4-j12.5, the side ones were 39.4-j175. and the back was
63.4+j45.5... definitely not your stock 50 ohms.

David Robbins K1TTT
AR-Cluster node: 145.69MHz or telnet://

> -----Original Message-----
> From: [mailto:towertalk-
>] On Behalf Of ON4IQ
> Sent: Sunday, August 20, 2006 17:03
> To:
> Subject: [TowerTalk] Comtek 4sq box question
> I got a 2nd hand 4sq box (Comtek) a while ago and decided to put it on the
> networkanalyser.
> The 4 antenne ports are terminated with 50Ohm suhner dummyloads, the load
> port goes to the network analyser and the xmtr port to the analyser
> aswell.
> I inject a 3.5 to 4MHz signal into the comtek box and see on the load port
> (where your dummy load needs to go) a level -7.5dB down on the xmtr level.
> This seems pretty high to me as I found in on4un's book notion of -21dB as
> a value for the dumped power when terminated in 50ohm.
> Is there reason to suspect the seller sold me a bad unit?
> 73  Johan
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