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[TowerTalk] Measurring at the antenna feedpoint

To: <>
Subject: [TowerTalk] Measurring at the antenna feedpoint
From: (Tom Rauch)
Date: Mon, 08 Jun 1998 21:07:25 +0000
To: <>
> Date:          Mon, 08 Jun 1998 11:07:48 -0400
> From:          Paul Ferguson <>

> I made the mistake of trying to measure impedance/resonance right at the
> feedpoint with the Autek  RF1.  

This should work absolutely fine. The only precaution is to use 
battery supplies only, and move your mitts off the case to take the 
actual reading. 

The Autek, if left floating, would NEVER have low enough impedance 
from the case to the antenna to upset balance! Don't sweat the 
balance issue, it isn't a consideration at all IF you let it "hang in 
the air". 

>I knew from experience with the 40
> dipole that that's a bad thing to do. I'm not sure quite why. 

Nor am I. I regularly use a MFJ-259B  to measure balanced antennas, 
even on ten meters and even with 400 ohm transmission lines. The 
Autek, being smaller, should be even less of a problem.

>  So I did two
> other things. I had an old Johnson Directional coupler.  I stuck it right at 
> the
> feedpoint (6" coax actually) with the signal lines running down to the ground
> and got readings that made much more sense.  I also cut a 1/2 wave piece of 
> RG8x
> and put the Autek at the end of that.  It essentially reproduced the coupler
> results.

Maybe something else is amuck with the Autek, like parasitic 
oscillations from inductive field feedback when the unit is used in 
the nearfield of an antenna. There is absolutely no other reason why 
the feedline would make a difference...except..... 

 > Body capacity effects I guess. --------------------------

Now that might do it in some unusual cases, what happens if you back 
off a bit?? Does it read ok??

That's a real weird problem, since every other instrument I've ever 
used works fine at the feedpoint! Are you running it off batteries?

73, Tom W8JI

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