Topband: Rx antennas

[Tom Rauch]

> Mike Tope's comment this morning is dead on.  After
thinking a bit
> about this last night I came to the same conclusion. The
near field
> characteristics of the receiving antenna when transmitting
are not
> particularly relevant.  What matters is how the antenna
behaves in the
> near field
> of the noise source if the noise source is close enough to
matter.

1.) The "near field", at least for field impedance, is
clearly shown to be about one wavelength for the small loop.
Not 1/8th wl.

2.) At distances smaller than 1/8th wl loop response
(receiving or transmitting) is magnetic dominant because
field impedance is less than 377 ohms. It STILL has an
electric field response (it must or it won't receive
anything).

3.) At distances over 1/8th wl the field impedance
(receiving or transmitting) is higher than the freespace
value that ALL antennas eventually settle on. For any
distance between 1/8th and a full wave the antenna responds
to electric fields a bit better because the field impedance
is higher than the distant value.

4.) Noise has no specific dominant response, it might be
anything and change in any way when it is close to the
antenna. Anything we do or assume is just a guess because
even if we know the loop's characteristics we really don't
know anything about how the various nearfield noises behave.
They could be low impedance or high impedance. Most likely
they are low, since noise sources generally come from long
conductors with many loads distributed along them.

The real problem I was trying to point out is people assume
a loop, through some magical property, excludes noise by
virtue of responding to "magnetic fields". That would mean
all or most noise would have to be high impedance fields,
and signals low impedance fields. That's just a silly
concept, since the most problematic noise we hear comes from
long wires (power lines) with multiple grounds and loads. Of
course an exception might be if an isolated arc to a short
conductor is very close to the receiving antenna.

For distant noise, it doesn't matter a single bit if the
noise is from something with a low impedance induction field
near the source or a high impedance. It doesn't matter if it
is a spark, an accidental oscillator, or something
intentional like a transmitter. It ALL looks exactly the
same to the antenna.

This is why the answers by people who have tried loop
antennas and who respond to questions while never fully
consistent mostly report no improvement.

1.) Nearfield noise is a random unknown.

2.) Distant signals and noise all "look alike".

3.) A loop responds to magnetic AND electric fields. It only
has overwhelming magnetic response well within 1/8th
wavelength, but even in that region it still responds to
electric fields.

My only purpose in pointing any of this out is to dispel the
common myth that "good signals" are magnetic and "bad
signals" are electric, and the misplaced notion that at any
distance a small loop (or any loop) has some mystic power
that prevents electric field response.

Since none of us know what happens in the nearfield of any
random noise sources, why does any of this matter?  It
doesn't.

The bottom line is the only predictable thing is farfield
pattern, and anything in the induction field or within the
Fresnel region (when the pattern is not fully formed) is
mostly a matter of experimenting and luck.

A good number of loops aren't even built correctly. The
shield (that isn't really a shield, it really is the actual
antenna) isn't grounded or constructed properly, and in many
cases actually creates problems. Many antennas work in spite
of what we do wrong because they are used in an environment
where we can't forecast results. Success is through
Edisonian engineering methods. If you try enough filaments,
something is bound to light up.

73 Tom