Lets put some numbers to the requirements, just for grins..
Say you want to radiate a kilowatt (+60dBm) and you want to suppress that to
no more than 0dBm at the receiver input. I assume that 0dBm's not going to
raise too many problems with IM in the receiver (they use +17dBm or better
mixers, from the few designs I've looked at in any detail).
So.. to a first order, you need 60 dB isolation of the fundamental.
Physical separation of the antennas should give you 30 dB or so (assuming
you've some directivity, and you've got the room to separate a few
wavelengths), so you'd only need to knock down the carrier by another 30dB,
which seems plausible for fairly straightforward stuff.
However, I think that the real problem is that you need to suppress the
noise sidebands to something less than the ambient noise.
Say your noise floor is in the -100 dBm area. If your transmitter is +60,
and the sidebands are down 70 dB, you're at -10. Another 30dB for physical
separation, and you're only down to -40, which is nowhere near what you
need, and no amount of filtering on the receiver will help you. You need
another 60 dB..
Maybe a notch filter on the Tx output (accepting the loss?) might help, but
it would probably have to be tunable, to get the rejection you'd need
without undue complexity. As Tom says, it would be expensive.
I think the best bet is going to be some sort of fairly high level nulling
antenna scheme. A reference antenna, and a phasing/amplitude network into
a passive combiner in front of the receiver. The slickest approach might be
two reference antennas spaced a quarter wave apart and a pair of goniometer
type combiners to allow phase and amplitude adjustment with fairly broad
band.
As Mike points out, this is going to be quite tricky, and if you have to
deal with multiple signals it could get even worse. However, 60 dB is an
achievable null in a single frequency situation (although, if you have to
null out 3kHz BW, it might be a bit tougher..The phase shift network can't
have too high a Q or the phase slope will vary too much.
This IS sort of a classic system design problem, though. You can trade off
how much you spend on filters, antennas, combiners, nulling systems,
computers, etc.
A real question is what's the best way to cancel the noise sidebands from
the TX (I think you can knock the carrier down with more conventional
techniques). Since they are random, some adaptive canceller with a sample
from the TX is probably the best approach. Fortunately, they are low level
enough that you can do it with signal processing in the receiver, rather
than worrying about distortion products in the front end.
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "Michael Tope" <W4EF@dellroy.com>; "Jim Lux" <jimlux@earthlink.net>;
"Jim Brown" <jim@audiosystemsgroup.com>; "Tower Talk List"
<towertalk@contesting.com>
Sent: Monday, July 05, 2004 9:07 AM
Subject: Re: [TowerTalk] Narrow Band Filters
> > A phasing unit like the MFJ will work and have plenty of
> dynamic range,
> > but it would require a bit of work to get it setup and
> adjusted. It might be
> > practical for a really serious Field Day effort, but my
> guess is that you
> > would need one guy who did nothing else but handle the
> phasing unit
> > design, installation, troublshooting, and adjustment. Our
> Field Day usually
> > doesn't have enough workforce to make that practical.
>
> The MFJ unit has totally inadequate dynamic range for this
> application, and it is the best of all the units I have
> tested.
>
> The idea of an HF filter for in-band rejection of different
> modes is useless, unless very expensive. Even helical
> resonators would have inadequate Q.
>
> Even on 160 meters deep rejection at 40kHz requires very
> good components. I had to use multiple Q~500 inductors in my
> BPBR filters on 160.
>
> 73 Tom
>
>
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