>> Could it be that my co-ax loop is rejecting something in
>> its very near
>> field,(the TV of my neighbour that I suspect is about 10
>> meters
>> away from my
>> RX antennas!)or is it a case of unbalance in the Flag
>> antennas,(that would
>> make sense)I do not have to rotate the loop to get rid of
>> the
>> offendingnoise, it simply is not there!I also tried K9AY
>> (various
>> ones) with the same
>> problem as the flags.
Raoul,
I'm not going to debate the shield issue on this reflector.
It is well past worn out, and it is very old established
technology that no field (electric or magnetic) penetrates a
conductive wall that is more than several skin depths thick.
If you look at this page
http://www.w8ji.com/radiation_and_fields.htm
you'll see an explanation of fields.
The largest thing that separates nearfield performance of
antennas is balance of the antenna and the field impedance
around the antenna. A closed loop has a confined electric
field very near the gap in the antenna. That field diminshes
in intensity away from the gap, although it exists in
significant level at any point. The magnetic field dominates
close to the antenna (within 1/10 wave or so), so a small
closed loop has a low field impedance near the antenna.
The problem is we never know what the local noise field is
like. Noise generally radiates from RF noise currents in
conductors, and those conductors (since they are the
antenna) determine field impedance of noise. If the antenna
is in the nearfield of the noise and the field impedances
greatly mismatch, there can be some reduction of noise. But
it is a luck of the draw what the field impedances are. And
whatever they are at one point they will be something else a
small fraction of a wavelength away.
This is why some people swear by small loops, and why others
swear at them.
All the satisfied terminated elongated loop users, be they
"flag" antennas, "K9AY" antennas, or an EWE (the ground
return closes the loop) users don't have a shield, and we
can build a totally "unshielded" loop that has exactly the
same response in every manner as a "shielded" loop.
What we can't do is build an antenna that does not respond
to the electric field, and even if we could there aren't any
noise sources that are electric field only. Factually the
field impedance of noise is random close to the source, and
most commonly of moderate impedance. House wiring and power
cords are not very small very high impedance conductors with
very high voltage and very little current.
So what happens?
The largest single problem is balance and feedline
radiation. Tiny antennas have very small area and a very low
radiation resistance. The antenna itself is very insensitive
because the electrical area is so small. This means they are
very sensitive to any common mode currents on the feedline.
That 50-foot feedline looks like quite an antenna compared
to a closed 5 foot loop, and if you follow that feedline's
path it goes right to the power mains.
This is actually the big problem that causes the K6STI
antenna to be such a flop. In the model, the antenna has no
feedline. It is two very close out-of-phase dipoles. These
dipoles have terribly low radiation resistance and very poor
sensitivity. In a model without the feedline, they have a
wonderful pattern. They look beautiful. In the real world it
is almost impossible to get them balanced. The feedline
connected to the antenna receives more signal from common
mode excitation than the actual thing we call the antenna,
so the system flops.
Loops of all types are no different.
If you carefully build a loop, shielded or not, so the
halves are balanced and the coaxial feedline has no common
mode current the loop will be very directive. The thing
doing the receiving will be the thing you call the antenna,
not the feedline. Get the slightest bit sloppy, and many
systems are very sloppy, and any noise in the house follows
the cables out to the antenna and pollutes the antenna.
The largest single problem with a K9AY, Flag, the K6STI
antenna, or even a small loop is that feedline connection.
The K9AY works against ground, and if the feedline is
"pumping" the ground at the antenna up and down with noise
voltage that noise voltage will get into the antenna. It
will with a small loop also....but not as bad.
The key to a good receiving antenna in a limited space next
to noise is having the antenna be the antenna, and keeping
anything on the feedline isolated from the antenna.
There isn't any magic in what you see and it does not have
to violate Maxwell's equations or the laws Faraday, Lentz,
and others discovered. Odds are a dozen different antennas
would work just as well if you decoupled the feeder properly
at the antenna. The key is balance, and a bit of luck if you
are trying to mismatch field impedance to a low or high
impedance VERY local noise source.
73 Tom
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