On 7/19/2011 3:47 PM, Richard Clark wrote:
On 19 Jul 2011 20:55:42 GMT, Allodoxaphobia
wrote:

In BCB SWLing you want the null(s) deep (and steerable)
to kill the strong, unwanted station.

This characteristic demands the SAME considerations as required for a
sharp beam of high directionality.


Actually, no.
You can form a deep null with a much simpler antenna system than for
high directionality.

All you need is to "cancel" the signal from the undesired direction.

If you had two verticals spaced some distance apart, fed by equal length
transmission lines, and one is polarity reversed, you get a fairly sharp
null on the line perpendicular to the line between the antennas.
(e.g. a signal coming from broadside will exactly cancel in the combining)

A loopstick antenna on a ferrite core or a multiturn loop, as popular in
direction finding, is another example with a fairly sharp null.



Now.. if you want one null, and one null only, that gets a bit trickier
in a small space. I don't know that you can do it with only two
elements (haven't thought about it much).

And if you want to steer the null.

One traditional approach in direction finding is to use a goniometer or
an adcock array.


If we were to select the antenna that matches your quoted specs above,
it would be called the Cardiod (example available in EZNEC). Problem
there is the 30dB null isn't steerable (not enough elements). And it
is large (too large for the back yard):
1. Too tall for most to build.
2. Elements too far apart (out of necessity for, dare I say it?
Phasing).

that's the "one null and one null only" problem.