----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
> I was thinking about the same thing, after trying to get a few PSK31 Qs in
> Sunday morning on 14070 while Marty N6VI was hammering away on 20m CW in
> the next tent. Likewise when the GOTA operation happened to be on the
same
> band as the main operation, even second harmonic was a problem between 20
> and 10 (although we didn't bother to figure out if it was 2nd harmonic of
> the Tx or some IM problem in the receiver) Here are my initial thoughts:
>
> Making a suitably narrow band filter shouldn't be a challenge, except:
> 1) most LC filters are quite sensitive to their terminating impedances.
If
> the antenna you're hooked up to isn't a nice 50 ohm resistive (and none
> are), you might need some way to tweak the filter to tune it after
> installation.
For receive only, yes. For transmit, don't count on it being an
easy design. Even if you can get the inductor unloaded Q's up
above 500, you are still going to have pretty high insertion loss
(3 to 4 dB) if you want meaningful rejection of the adjacent mode
segment. You might be able to do it with hi-q bandstop sections,
but even then I'll bet you its a non-trivial design (unless you don't
care about insertion loss). Also when the loaded Q gets high,
you are going to have to contend with high circulating currents
and voltages even at the 100 watt level.
> 2) You do need filters on the TX as well. Even transmitters with very
good
> phase noise will have significant power a few tens of kHz away that needs
to
> be notched/filtered out.
The Ten-Tec Orions are -140dBc/Hz at 2 KHz offset. I don't know
if that would be enough, but it would be a good start as compared
to some of the other rigs out there. At larger offsets where PA noise
probably dominants, there might not be much difference between
the Orion and other rigs. An interesting test would be to measure
broadband noise for various rigs at 50 to 500 KHz offset to see if
any rigs would be standouts for this kind of same band different
mode operation.
> 3) The usual field day operation requires some amount of tuning around, so
> the very narrow band high-q approaches used, in, e.g. 2m repeaters won't
> work.
600 KHz at 2 meters is equivalent to 60 KHz at 20 meters, so you really do
need pretty high Q to get any meaningful rejection of the adjacent mode
segments on HF.
> 4) On receive, it might be possible to use an interference cancelling
> approach (sort of like the MFJ (and other) widgets that use a secondary
> antenna. I'm not sure about bandwidth, though. Maybe some sort of true
> time delay using coax would work.
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.
> 5) An active canceller/filter would be the hot ticket, but dynamic range
> might (WILL) be a problem. It would give you the tunability, etc. though.
>
> Unfortunately, field day isn't a great opportunity to make detailed
> measurements, and we didn't shlep a spectrum analyzer up there to really
> look at the problem.
> Jim, W6RMK
Yes, the problem with Field Day is that about the time you get everything
working, its time to tear it apart and through it in a box until next year
:)
73 de Mike, W4EF................
>
> ----- Original Message -----
> From: "Jim Brown" <jim@audiosystemsgroup.com>
> To: "Tower Talk List" <towertalk@contesting.com>
> Sent: Saturday, July 03, 2004 8:26 PM
> Subject: [TowerTalk] Narrow Band Filters
>
>
> > I'm thinking about building some narrow band filters (as opposed to a
> bandpass filter that
> > covers an entire band) for use on Field Day. The object is to minimize
cro
> sstalk between CW
> > and sideband stations on the same band. Has anyone tried this? Is it
> practical to get enough Q
> > to pick up enough rejection to make it worth the trouble? Certainly
80/75
> would be the most
> > practical, since it is the greatest percentage bandwidth. Is anyone
making
> a commercial
> > product?
> >
> > Jim K9YC
> >
> >
> > _______________________________________________
> >
> > See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
any
> questions and ask for Sherman, W2FLA.
> >
> > _______________________________________________
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>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
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>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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