To: <towertalk@contesting.com>
>Date: Mon, 08 Jun 1998 21:07:25 +0000
>From: Tom Rauch <10eesfams2mi@mail20.MCIONE.com>
>
To: <towertalk@contesting.com>
>> Date: Mon, 08 Jun 1998 11:07:48 -0400
>> From: Paul Ferguson <Paul.Ferguson@pobox.com>
>
>---------------
>
>> 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".
Tom, Paul,
I am unfamiliar with the exact configuration of the Autek device
Paul is using. From what you have now said, it might be that it
is the balancing device problem in reverse.
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.
I admit to being a bit more sensitive to the balance issue than
the typical ham needs to be when working at HF. Most of my work
related antenna measurements these days are on antennas that are
2.2 inches long. And _these_ antennas are very easily disturbed
by unbalanced connection to instrumentation.
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 measuremants iffy unless
feedline routing is carefully controlled.
A simple experiment to determine for sure whether this is the
source of the mystery would be to put a 1:1 balancing device at
the feedpoint and take the Autek readings at the end of the 1/2
wave coax section that Paul tired already. If balancing the
connection to the antenna suddenly makes the readings at the
bottom end of the feedline agree with the readings taken at the
antenna, then the problem was the feedline unbalancing the
antenna.
You can get an indication of whether this is the case even
without the balancing device. Just hook up to the 1/2 wave
feedline as before. Note the readings. Then move the feedline
around to a different angle relative to the antenna. Do the
readings change even slightly? Have someone grab the feedline at
the 1/4 wave point. Do the readings change? Ground the feedline
shield to the tower (or any convenient massive conductor) at the
1/4 wave from the feedpoint distance. Do the readings change?
If any of these things cause the readings to change, the feedline
shield is part of the radiating structure.
>
>> 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.
>
The above discussion applies equally well to a dipole fed
directly with coax.
73, Eric N7CL
>> 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
>w8ji.tom@MCIONE.com
>
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