The two antennas behave as one, a director with a reflector, with a null in
the radiation pattern. Varying the phase and magnitude controls have the
effect of rotating the antenna pattern and varying the spacing between the
two elements.

Orientate the noise antenna in a different direction to the main antenna and
if you know where the noise source is located then maximise noise pickup
from the noise source. There will be one null only if there is one noise
source. And usually you can consider yourself lucky if you can find it.

Another tactic is to arrange for the noise antenna to pick up a minimum of
the wanted signal if you know from where it is coming. If the main antenna
is outdoors and high in the sky and the noise source is very local, ie., you
are in the noise near-field, then install the noise antenna indoors at
ground level.

A null is impossible if signal and noise arrive from exactly the same
direction. If from nearly the same direction then the wanted signal will
decrease almost as much as the noise. But the situation can usually be
improved yet again by changing orientations of one or both antennas.

If the main antenna picks up stronger noise than the noise antenna then a
null can be obtained only by increasing gain in the noise receiver.

Its all a matter of phase and magnitude.
---
Reg, G4FGQ

"Reg Edwards" wrote in message
...
The two antennas behave as one, [...snip...]


How come no one told me what an SEM is??

There will be one null only if there is one noise
source.

Reg explains well, but I don't believe this item is, in general, true.
One example case could be two identical verticals where you wind up with
them fed 180 degrees out from each other. There will be two nulls broadside
to the antenna and 180 out, in direction, from each other (an Adcock).
Change the phase and both nulls tilt toward the direction of one antenna or
the other...untill they coencide and you have the cardioid.
With at least one directional antenna, the "other null" may be where
there is little pickup from the directional antenna and therefore non equal
signals and the "noise sense" antenna signal would dominate, in essence
hurting the directivity of the "main" antenna. (Because there isn't enough
from the main antenna to cancel what comes from the noise antenna.) Ya
gotta have both to cancel.
Therefore, it's a bit more complex as well, but killing the noise may
far outweigh other problems. But as Reg says, you want to maximize the
niose pickup in the noise antenna and minimize the desired signal in the
noise antenna.

Automatic systems like this are used on "radio gun ships" Lotsa radios
trying to interfere with each other. Sample the offending transmitter at
its output and cancel it at the problem receiver.


--
Steve N, K,9;d, c. i My email has no u's.