----- Original Message -----
From: "Michael Tope" <W4EF@dellroy.com>
To: "Jim Lux" <jimlux@earthlink.net>; "Jim Brown"
<jim@audiosystemsgroup.com>; "Tower Talk List" <towertalk@contesting.com>
Sent: Sunday, July 04, 2004 10:19 PM
Subject: Re: [TowerTalk] Narrow Band Filters
>
> ----- 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.
Exactly what I was thinking of...
Maybe lots of sections, each fairly broad, low loss. A very tricky problem
overall. Loss and breakdown problems would be a challenge. (Hey, we have
trouble separating 7 and 8 GHz with low loss and power handling capability,
and that's a LONG way apart compared to 14.1 and 14.3)
>
> > 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.
I seem to recall seeing such measurements somewhere (ON4UN's book?)
>
> > 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.
And that low loss cavity on 40m is going to be quite the challenge.
>
> > 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.
Come on, Mike.. surely we can spare another person... But you're right.. it
would need to be darn near automatic..
>
> > 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
See.. we need a field day every couple months..
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