I was trying out a simple construction idea for a shielded loop antenna
for SW and found that it worked well but it worked even better for BCB.
I have not tried it out on long wave but it might work well there also.

I made two antennas one 25 foot and the other 40 foot in diameter. Both
are vertical and are made of 50 ohm coax. The basic construction is to
use part of the cable as lead in to the radio and the rest is in a loop
shape of a square. It could be other shapes like circle or triangle. In
order to complete the loop the center conductor on the end of the coax
cable is connected to the shield of the coax about 10 foot from the end
connected to the radio. The shield on the far end of the coax from the
radio is not connected to anything. The part of the coax that becomes
the antenna is the part that is the cable end to the place that the
center conductor is connected to the shield.

I found I could make the response a little broader and impedance flatter
by connecting the shield on the cable end with 50 ohm resistor to the
same shield point as the center conductor was grounded to on the shield
with some loss of signal.

The loop has a real nice null. I found that I could null BCB stations
and even WWV at 5MHz with the 25 foot loop. The null at 10MHz was not
very good and the null was non existent at 15MHz. The nulling was tried
out on daytime signals.

These small loops picked up BCB stations very well and yet local noise
that I have around here aplenty is practically absent. The AM broadcast
band has always been noisy at my location and this shielded loop works
wonders on my table top radios. These loops need no amplification, no
tuning and are very easy to make.

--
Telamon
Ventura, California

So how are you handling the balanced/unbalanced issue?

These designs are in the ARRL Antenna book. Generally a second loop is
used to couple to the first loop to act as a balun. [Mutual inductance
between the loops couples the signal.]

The shield should be broken at the far end of the antenna, i.e. not the
end that goes near the radio.

Since you are nulling signals, I gather the loop is vertical, but 40ft
in diameter seems pretty hard to believe. We are talking 3 to 4 stories
high.

Telemon,

How does your Coax Cable Loop Antenna differ from this one ?

In article . com,
"RHF" wrote:

Telemon,

How does your Coax Cable Loop Antenna differ from this one ?
.
"Low Noise Receive-Only Coax Cable Loop Antennas
for 160 - 10 Meters HF Bands"
http://www.greertech.com/hfloop/mymagloop.html
The so called GREER TECH "Coax Loop" Antenna.

Three things:

1. My antenna is larger. My larger antenna will develop
more energy from a passing wave.

2. The shield opening is on the end on my antenna instead of the middle
of the loop. Technically the center opening should be better balanced
but I did not notice any difference.

3. My antenna has a termination resistor, which flattens the response.
You probably would not want it on a smaller loop.

Small point on the web page The comment "RF energy now travels from the
braid's outer skin to the braid's inner skin at the gap which then
couples to the center conductor" is not quite right. The shield
technically is not the antenna, the center conductor is the actual
receiving element. The coax becomes a magnetically sensitive antenna as
the shield blocks the electric part of the passing EM wave from it. The
shield develops a current from the voltage part of the passing EM wave,
which generates a secondary magnetic field that couples to the center
conductor in phase with the magnetic part of the passing EM wave. The
center conductor then sees a 2X magnetic field from a passing EM wave
where a local induction field will not cause this to happen. If the
coax is grounded on both ends then the current on the outer shield will
be opposing and cancel out instead of being in phase and having the
resultant gain.

Makes you appreciate that it is important to have a good ground on both
ends of a coax cable. If you have a bad ground on one end you have a
pretty good antenna.

The author of the web page is doing other things with the loop like
tuning them for transmit, which we don't care about here but he also
mentions a 2 turn loop. I defiantly don't recommend this as some of the
shield will end up floating because it must be separated on every turn.
For a shielded loop stick with one turn. Another reason would be cross
sectional area will be larger with one turn.

I neglected to mention loop gain is in the plane of the loop and the
null is perpendicular to it as it would be for any electrically small
loop.

--
Telamon
Ventura, California