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[TowerTalk] Measurring at the antenna feedpoint

To: <>
Subject: [TowerTalk] Measurring at the antenna feedpoint
From: (Eric Gustafson Courtesy Account)
Date: Wed, 10 Jun 1998 14:16:59 -0700

To: <>
>Date: Wed, 10 Jun 1998 10:00:44 +0000
>From: Tom Rauch <>
To: <>
>> Date:          Tue, 09 Jun 1998 16:35:32 -0700
>> From:          Eric Gustafson<>
>Hi Eric,
>> For example, when the Autek is in place (and free floating),
>> the system is relatively well balanced and the readings are
>> correct for that configuration.  But when the feedline is
>> connected, the system balance is disturbed and readings taken
>> this way are correct for this configuration as well.  But
>> different from the readings taken right at the antenna.  It is
>> a bit hard to say for sure without some more information about
>> exactly how the quad is fed and how the feedline is routed.
>That can not possibly be the problem. The Autek is a small
>instrument, and as such has a case and circuitry that occupies a
>very small fraction of a wavelength.

Hi Tom,

I think you must have misread what I said in the last post.  I
was accepting that given my new understanding of the instrument,
the system was not unbalanced when the Autek was connected at the


>Understand this does NOT prevent the feedline from being a
>problem, and sheer dumb luck from curing that problem when the
>feedline is connected to a grounded source. Let me give a
>working example of that case....


>Of much much greater concern is the antenna getting "goofed up"
>by the improper use of an unbalanced feedline to feed a balanced
>antenna, and that the builder is compensating for this "goof" by
>detuning the antenna. Then when the Autek is connected the "goof
>up" is removed and the antenna goes to a correct operating mode
>that changes the frequency.
>This MAY NOT be the case, but it is a likely event. 

Right.  That is exactly what I was attempting to get at.

>1.) The Autek is "messing up" through some internal design or 
>component flaw         

Possible but I'm discounting this since he says it works for
other measurements.

>2.) The unbalanced feedline feeding a balanced antenna (both of
>which have low common mode impedances) is screwing up the
>antenna, and when it is removed the Autek reads the correct
>antenna impedance that WOULD have been obtained with a proper
>feedline or feedline interface.

This is the one that gets my vote.


>> The bottom line is that the quad element is a balanced
>> antenna.  If it is directly fed with coax, there are probably
>> antenna currents flowing on the shield.  Antenna currents on
>> the shield will disturb the pattern and also make measurements
>> iffy unless feedline routing is carefully controlled.
>Even feedline routing won't cure the absolutely improper use of
>a unbalanced feedline to balanced antenna!

No.  It won't.  But it can stabilize readings taken at the user
end of the feedline so that the system appears consistent.


>IF that line was grounded 1/2 wl away, that would be the worse

Right you are.  But I thought that grounding it 1/4 wave out might
suddenly cause his readings to agree with the "at the antenna"
measurements.  It would almost certainly cause a change in his
readings in some direction or other which would not be apparent
if the line was sufficiently decoupled from the radiator.

>Worse case for an UNgrounded line is 1/4 wl long. By the way the
>velocity factor inside the line does NOT figure into this
>equation, since common mode current flows only on the outside of
>the coaxial line.

Right.  But all I was trying to do was to get him to fool around
with the location and connection status of the feedline shield to
see if he got variations in his readings.


If it is convenient, please do the test with the feedline that I
mentioned and let us know what you found.

73, Eric  N7CL

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