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LF: Re: Loop antenna problem

To: [email protected]cksheep.org
Subject: LF: Re: Loop antenna problem
From: "Niels Rudberg J?rgensen" <[email protected]>
Date: Sat, 21 May 2005 10:34:05 +0200
Delivery-date: Sat, 21 May 2005 09:35:01 +0100
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----- Original Message -----
Sent: Saturday, May 21, 2005 1:52 AM
Subject: LF: Loop antenna problem

Hello LF Group,
 
I've finally finished the construction of my LF loop and I appear to have a
problem with it regarding the connection to the receiver.  The loop itself
is in the form of an octagon, with an overall diameter of 10.5 feet, and it
consists of 15 turns of number 12 THHN stranded wire.  The spacing of
the turns is 0.25 inch and the tuning is done with a 3 gang, 400 pF. per
section, variable capacitor with all sections connected in parallel.
 
Checking with a weak signal source, the tuning range is roughly 100 to
300 kHz.  This agrees with Reg's program for multi-turn loop antennas.
A portable radio covering the LF band really comes to life when I bring
it close to the loop, and by turning the loop in azimuth I can peak the
signals and/or null out noise.  This would indicate that it is working as
I would expect.
 
The problem is with the pickup loop, which is of the classical shielded
construction as described in many antenna books.  Mine is made from
RG-213 coax and it's positioned roughly six inches inside of the main
loop windings, but in the same plane.  The station receiver, a TS-440S,
gets better reception of LF signals when it's connected to my 40 meter
delta loop than it does with the LF loop!!  I know the feedline is OK, as
I've checked it with an MFJ-259  antenna bridge and a 50 ohm load at
the antenna end.
 
The one suspicion I do have is that the cable clamps I've used to hold
the pickup loop to the frame are made of metal and are shaped like a
capital letter P.  I'm wondering if these look like shorted turns to the
pickup loop and are attenuating the signal?  Before I get out my big
stepladder and replace them, I thought I'd ask the opinions of those
of you who have gone before me.  Any/all suggestions are welcome.
 
73, J.B. Weazle McCreath, VE3EAR - VE3WZL
 
Dr om!
Quite a big loop you have made there!
I would not recommend the use of a pickup loop at all. It is rather
difficult to predict the degree of coupling between the loop prober and
the pickup loop.
From your description I guess that your loop inductance is about 2 mH,
as for the Q of the loop I would say it lies between 20 and 50.
Why not try to scrap the heavy RG213 loop and instead use a
capacitive divider . If you try a capacitor of 10 to 22 nF in series
with your variable capacitor, you might have success.
As for the question of balancing etc. in order to avoid capacitive signal
pickup, I suggest you try to solve that problem with an isolating
transformer. But let that be a second exercise!     
73
Niels/OZ8NJ+
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