"Richard" wrote in message
...
Steve Nosko wrote:
I believe he is talking about adjacent channel signals which are much
stronger than the desired and trying to see if there is a way to "fix"
that. ...
Capture effect:
http://en.wikipedia.org/wiki/Capture_effect
It's an intersting phenomena, and potentially useful.
But I believe not here. Only for co-channel interference.
If I could phase null
(at the antenna) the local stronger signal ... if you cannot get the
inteferring signal below the
This is a sticky problem if the undesired is only 100kHz off center with
the standard 200kHz system design. I this case you can have significant
energy within the Rx passband coming from the undesired. I can't say how
low the undesired needs to be.
Pity that somehow you cannot design circuitry which recognises a 100Khz
difference between the wanted FM station and the unwanted FM station and
proceed to demodulate the wanted one only. If it were possible, it would
have been done by now. Maybe you could do it digitally,I dunno.
As you have surmised, the "null in the antenna pattern" is probably your
best bet. This is done in some rather dificult situations. One is on a
radio equiped copter where the undesides TX is on board! ( the TX output
itself can be sampled) The antenna with a null would be my first path. I
also have an FM band adjacent channel problem I'd like to solve and will
(when I get time) try a nulling antenn.
The "adcock" type would be one of the easiest. Two vertical dipoles
(probably folded, but not necessary) fed out of phase. There are two nuls
toward the "flat side" or broad-side. Point the null at the undesired.
The "two vertical cardioid" is just as easy. I think it is two vertical
dipoles spaced (I think) 1/4 wave apart with a 3/4 wave coax (electrical)
between and fed at one of the dipoles. The 1/4 wave (free-space) physical s
pacing gives 90 degrees and the 3/4 (in coax) gives another 90 for a total
of 180 - thus cancelling. 3/4 coax is needed because 1/4 will be shorter
that the free-space 1/4 spacing of the dipoles.
I recently saw both of these in the Ham transmitter hunting book. They are
probably both on the net if you know where to look. Try amateur radio DF or
ARDF.
http://members.aol.com/BmgEngInc/Adcock.html
Probably the transformer is not needed.
I took a quick look and can't find more, so Here's a web ring on DF:
http://m.webring.com/hub?ring=foxhunt
Other DF sites, but I don't know if they have for U. Some have more links.
http://www.panix.com/clay/ham/rdf.html
http://www.ardf-uk.co.uk/index2.html
http://members.aol.com/fdecker/rdf.htm
http://members.aol.com/homingin/index.html
Dual null & cardioid figures, prpbably not helpful.
http://members.aol.com/homingin/hfinderfix.html
Lota' links
http://members.aol.com/homingin/links.html
Another technique would also handle the afore mentined multipath problem.
This I will call the "Secondary Antenna & Canceller" (SAC) technique. It is
used in the military and commercial arena to solve some sticky strong signal
problems.
A secondary antenna is used and fed to an amplifier which has phase and
amplitute adjustments (under computer control for automatic tuning). The
output of this is fed back into the RX line (where the desired and undesired
signal are) and tuned for minimum interference. This amounts to a custom
antenna null. MFJ makes two for HF
http://www.mfjenterprises.com/contact.php
MFJ-1025 1.5 - 30 MHz Noise Canceling Antenna
MFJ-1026 1.5-30 MHz Deluxe Noise Canceller
http://www.mfjenterprises.com/produc...rodid=MFJ-1025
http://www.mfjenterprises.com/produc...rodid=MFJ-1026
--
Steve N, K,9;d, c. i My email has no u's.