----- 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+
