"Richard Clark" wrote in message
...
On Sun, 15 Feb 2004 21:49:12 -0000, "Richard"
wrote:

Consider, if you could accumulate half a dozen of these you could then
build your own phased array to take care of your interference (some).

Yea! :c)

No, what I decided was that I needed to use phase nullinhg/cancelation, I
mean adjusting the phase and amplitude of the undesired local signal (got
from a dipole or omnidirectional antenna) and combining it with the
intefering signal from a directional beam. Which is a different tack than
nulling out using nulls in antenna lobe patterns.

I could so with an FM omnidirectional antenna, but the one that is on
ebay,
I don't think it is an FM antenna, despite the description. Looks
possibly
a scanner type. But, I don't know.

Rich.

Hi Richard,

My suggestion is the classic approach and more aggressive than the one
you describe (which uses the same logic).

Basically you set up the several many different antennas and combine
them at one point. Difference is, that before you combine them, you
introduce a delay in each. The delay selected then combines with the
others to perform the cancellation (or addition, depends wholly on the
delays involved).

In some, fixed designs, it is simply a matter of where the many
antennas are located (their physical distance imparts some portion of
delay) and the length of their transmission lines to the combining
point.

Delay is simply a matter of wavelength. For the FM band we are
talking about 3M representing a total of 360 degrees of delay. If two
antennas are separated by 1.5M and combined, they add with 180 degrees
of difference iff their transmission lines are the same length. They
would cancel. If two antennas are separated by 3M and combined, they
add with 360 degrees of delay and add (again, iff their lines are
equal length).

Now, if we took two antennas separated by 3M and combined them through
lines that themselves show 180 degrees different length, then their
combined signals would cancel. So, through the manipulation of line
length with fixed antennas, you can either add or negate signals by
choice (phase cancellation through line length).

The boxes for sale you describe are simply virtual lines that can be
changed electrically instead of physically. This allows you to
combine any two antennas with any line lengths and eventually cobble
together enough delay to accomplish the job.

So, delay can be obtained through physical separation of antennas,
AND/OR varying the length of their lines, AND/OR inserting variable
tuning components before the combination point.

If you invest in more actual antennas you also boost your flexibility
(and complexity) and deepen the null, or enhance the gain. If you get
particularly good at this through the use of electronic tuning of the
delay, you can then construct a "steerable beam" antenna.

Hence my oblique comment on garnering several many of these antennas
(far cheaper to build them from brazing rod and SO-238 chassis
connectors).

Understood.

What do you think about the antenna on ebay? I've not yet had anyone say
what they think it is, which I'm finding curious. The seller does not know
what it is depite the description, because I've talked to him. He just put
it down as an FM antenna, but I have my doubts. But if it is not, maybe I
could modify. What does it look like to you, a scanner antenna, a CB
antennna, a marine antenna, a 2 meter antenna? Model is CA-815, but you will
not find a match on Google. Rich.