On 25 Oct 2005 17:31:02 -0700, "K7ITM" wrote:

You need first to realize that the "shield" IS the antenna. The whole
point of the "shielded loop" is that you can make it very symmetrical,
which is just what's needed to reject strong local electrical fields.
The symmetry does nothing to reject electromagnetic signals. BUT you
can make an "unshielded" loop which is as symmetrical as a "shielded",
if you are careful, and get the same advantages. If you really want to
build one like a classical "shielded loop" and maintain high Q, just
build the "shield" out of copper pipe and put the capacitor across the
gap. The wire inside the pipe is just the center conductor of a short
piece of coax connected to the feedpoint.

If you don't understand this, please see King, Mimno and Wing's
"Transmission Lines, Antennas and Waveguides." It's explained quite
nicely in the "antennas": chapter. It's also explained reasonably well
in Johnson and Jasik's "Antenna Engineering Handbook."


Hi Tom,

No, I don't understand this. I thought a shielded loop meant the loop
antenna wire was shielded by the copper (non-ferrous) surrounding the
wire. The shield tends to protect the wire from electrical field
inputs and allows it to only respond to magnetic field variations.

I thought the capacitance between the wire and the surrounding shield
material represented a loss in Q, therefore a loss in output voltage.
So, a loop that might have a Q of 100 in free space would have a much
lower Q if the loop wire was enclosed in a non-ferrous pipe.

There are countless horror stories about those attempting to use
surplus hardline as shielded loops on LF and VLF, all with
disappointing results. The predominate attitude was that the
capacitive coupling between the wire and the shielding material was
the cause. I don't say the predominate attitude is correct.but, if it
is a false assumption, then I am not the only one who needs
revision)

If the copper pipe IS the antenna, then why have the wire inside it at
all??

I must say I'm more confused now than I was before reading your
message.

I'm sorry, I have to leave now. The director of the asylum is
calling.......

T