Here in UK , like probably elsewhere in the world, FM stations need to be
seperated 200Khz to avoid mutual inteference. A good RX ought with good IF
filtering ought to be able to seperate two 200 Khz spaced FM signals I
think.
But of course stations are only 100Khz apart so it nigh on impossible to
seperate stations under these conditions as far as I know.

Well, if filtering won't sort out the problem, can you seperate the stations
any other way? Can you make a circuit that responds only to the wanted
station when it is just fractionally stronger than the unwanted station?
That's the only thing I can think of, apart from antenna solutions. Do we
have technology these days to do what we thought was impossible in this
case?

Richard:

Stations can be separated with a directional antenna.

Roger K6XQ

"Richard" wrote in message
...

Can you make a circuit that responds only to the wanted
station when it is just fractionally stronger than the unwanted station?

That's called "capture ratio". Better receivers have lower numbers -
smaller difference required to suppress unwanted station. They have numbers
like a VERY few dB.

To get this you need very flat IF within the passband - no ripples in
response.

A wideband limiter and wide and very linear discriminator.

The National "Criterion" series of tuners from 50 years ago were among the
first to attempt this. IIRC, there were HH Scott tuners like this as well.

All bets are off if you're trying to recover stereo.

73 de bob w3otc

I believe that you are asking about narrowing the I.F. bandwidth. If you use
narrow filters, this is quite possible. Since FM stereo stations occupy a
150kHz bandwidth, you would be clipping the sidebands, resulting in lost
information. If you are only receiving in mono, this wouldn't be an issue.

Pete

"R J Carpenter" wrote in message
...

"Richard" wrote in message
...

Can you make a circuit that responds only to the wanted
station when it is just fractionally stronger than the unwanted station?

That's called "capture ratio". Better receivers have lower numbers -
smaller difference required to suppress unwanted station. They have
numbers
like a VERY few dB.

To get this you need very flat IF within the passband - no ripples in
response.

A wideband limiter and wide and very linear discriminator.

The National "Criterion" series of tuners from 50 years ago were among the
first to attempt this. IIRC, there were HH Scott tuners like this as
well.

All bets are off if you're trying to recover stereo.

73 de bob w3otc

I believe that you are asking about narrowing the I.F. bandwidth. If you use
narrow filters, this is quite possible. Since FM stereo stations occupy a
150kHz bandwidth, you would be clipping the sidebands, resulting in lost
information. If you are only receiving in mono, this wouldn't be an issue.

Pete

"R J Carpenter" wrote in message
...

"Richard" wrote in message
...

Can you make a circuit that responds only to the wanted
station when it is just fractionally stronger than the unwanted station?

That's called "capture ratio". Better receivers have lower numbers -
smaller difference required to suppress unwanted station. They have
numbers
like a VERY few dB.

To get this you need very flat IF within the passband - no ripples in
response.

A wideband limiter and wide and very linear discriminator.

The National "Criterion" series of tuners from 50 years ago were among the
first to attempt this. IIRC, there were HH Scott tuners like this as
well.

All bets are off if you're trying to recover stereo.

73 de bob w3otc

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 does not apply for this situation. Capture is a co-channel effect
and is better the wider the whole system is, not just the IF.

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


"R J Carpenter" wrote in message
...

"Richard" wrote in message
...

Can you make a circuit that responds only to the wanted
station when it is just fractionally stronger than the unwanted station?

That's called "capture ratio". Better receivers have lower numbers -
smaller difference required to suppress unwanted station. They have
numbers
like a VERY few dB.

To get this you need very flat IF within the passband - no ripples in
response.

A wideband limiter and wide and very linear discriminator.

The National "Criterion" series of tuners from 50 years ago were among the
first to attempt this. IIRC, there were HH Scott tuners like this as
well.

All bets are off if you're trying to recover stereo.

73 de bob w3otc

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 does not apply for this situation. Capture is a co-channel
effect and is better the wider the whole system is, not just the IF.

Capture effect:

http://en.wikipedia.org/wiki/Capture_effect

It's an intersting phenomena, and potentially useful. If I could phase null
(at the antenna) the local stronger signal then it would dissappear as
inteference if it was a few db below the wanted signal.

Only problem is, is that if you cannot get the inteferring signal below the
wanted one, it's works very bad for you, because you cannot listen to an
inteferred with signal. Which is better than nothing I guess in some cases.

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.

"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.

Steve Nosko wrote:
"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.


In the UK, where I am, the spacing between channels is 100Khz. And that is a
real problem, because, as you say a lot of RF energy from the undesired
station gets into the IF passband.

And so basically I was wondeting if the technolgy exists for the RX to know
that energy from the undesired station, 100Khz off frequency, is to be
ignored in the demodulation. I don't think it can be done, but if it
could somehow the electronics would have to associate the unwanted energy
with a non wanted station, and in a sense, ignore it. Just wondering if the
technoloy is around to do that. Probably not. Only likely possible thru
digital computing methods maybe.

Steve Nosko wrote:
"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.


In the UK, where I am, the spacing between channels is 100Khz. And that is a
real problem, because, as you say a lot of RF energy from the undesired
station gets into the IF passband.

And so basically I was wondeting if the technolgy exists for the RX to know
that energy from the undesired station, 100Khz off frequency, is to be
ignored in the demodulation. I don't think it can be done, but if it
could somehow the electronics would have to associate the unwanted energy
with a non wanted station, and in a sense, ignore it. Just wondering if the
technoloy is around to do that. Probably not. Only likely possible thru
digital computing methods maybe.