At 2/8/2006 10:38, you wrote:
> > I'll take most of what you said as being sarcastic, but your point
>is taken.
>
>I guess so, just pointing out the apparent absurdity.
>
> >If so, great, you're done. If not, fix it.
>
>In the end though, the function of the pass adjustments is simply to
>transfer energy at the pass frequency with minimum loss.
>
>IF everything were really 50 ohms resistive, then we could tune in
>the lab, and we could mix and match cables and equipment without any
>concern or adjustment. Unfortunately, as someone pointed out a ways
>back, real equipment is rarely so idealized. The measures
>for "fixing" it may induce more loss than what we are getting with
>the non-ideal system.
>
>On the transmit side, power is cheap and a little extra loss
>(fractional dB) isn't an issue, so I think the idea of tuning the
>pass pretty close with whatever you have is probably workable. Either
>an SA and tracking generator, or HT and bird, simply adjusting the TX
>pass for maximum power out.
The problem is "whatever you have" is probably not 50 ohms, so your pass
cavity or BpBr duplexer cavity becomes a matching transformer. I've seen
some cavities actually appear to have "gain" because by slightly detuning
them from the nominal peak one would see if everything was 50 ohms, they
matched my test TX to the 50 ohm load on the wattmeter.
I suppose this is why Jeff says
>Don't tune pass or pass/reject cavities with a tracking generator and
>spectrum analyzer which shows only insertion
>loss. Don't. Don't. Don't. You need to look at return loss.
Well, I'm going to go out on a limb & disagree with Jeff, perhaps for the
first time ever. At least when it comes to the "Don't. Don't. Don't."
part. A while back I put a TX/RX UHF duplexer on an 8510 VNA & measured &
saved all 4 S-parameters (lucky for me - usually I only save
S21). Anyways, I compared the S11 (return loss) minimum frequency to the
S21 (transmission) maximum frequency & found only the slightest
disagreement between the two. My conclusion is that if whatever you're
using to measure amplitude is sensitive enough & everything is close to 50
ohms, tuning for max. trans. is good enough. A 10 dB/div. log scale on a
spectrum analyzer doesn't provide enough sensitivity for this
measurement. However, a 2 dB/div. log scale, or better yet a linear scale,
is good enough. Of course, a Bird 43 wattmeter's scale has enough
resolution as well, but again you must be sure to use a 50 ohm source. The
easiest way to do this is to put an isolator on your test TX.
>On the receive side, the power isn't free, it can be considered very
>expensive, and we would like to save as much as we can, especially if
>it's only cost is an adjustment. But we don't have such a problem
>here, this can be adjusted with the real cables and receiver
>attached, so you are tuning the system, not just the cans themselves.
Yes, you can tune your RX cans to maximize power transfer into your
RX. But then what happens to your notches which you've just moved as well?
>The notches are no problem to tune at full power, so not an issue
>either way.
>
> > With the cavities tuned to resonance, cable lengths are no longer
>an issue.
Cable lengths will always be an issue if there are any mismatched ports in
the system. This is one of the reasons why I encourage use of isolators on
repeater TXs. They may not be necessary at non-comm-sites, but if you
start having weird duplexer problems (isn't that the title of this thread),
they can help solve a multitude of troubles.
> > Needing to use "magic" cable lengths should be a red flag that
>you've got a
> > Z mismatch somewhere.
>
>Well, in the real world, we always do, to some degree.
>
>
>The method that I'm using at the repeater now, is to tune the pass at
>low power using an HT as signal source. At 5W, I can't believe that
>we are anywhere near the damage threshold. I can also tune it using
>the SA and generator, slightly different point, but after a lot of
>back-and-forthing, I found that the HT method gave me slightly more
>output when connected to the real transmitter than the SA method.
>Still, we're talking small percentages of difference.
All that means is that the RFPA in your HT is a closer match in Z to your
repeater TX's RFPA than the 50 ohm sig. gen. is. Lucky.
>Receive pass, and notches are tuned while injecting a signal using a -
>50dB tap between the antenna and the bandsplitter. This is similar to
>the "broken tee" method, except that I am using a calibrated tap that
>is flat well beyond 2M and 440.
Sounds like a nice piece of test equipment to have around: a 50 dB
non-directional coupler.
> I am getting good numbers (-126 ish dBm) for copyable signal, and the
That's pretty darn good for 2 meters. Around here the 2 meter noise floor
is higher than that.
>notches and receive pass align very closely to where they do when tuning
>with the SA and generator, but not exactly the same place. (as expected)
>
>I go up today to replace the water-damaged GP-9 which is apparently
>more and more a dummy load as time goes on, and to supress some nasty
>feedback that's happening whenever the UHF system is active.
>
>I'm very tempted to build a calibrated signal source, it would be a
>lot more portable than my HP generator. But I am not sure it's really
>worth the effort.
I spent a lot of time looking for a cheap service monitor so I could have
some sort of calibrated signal source at my sites, & I'm glad I found
one. A little beat up, oddball make with no documentation whatsoever &
only does 12.5 kHz steps on UHF (a problem here in 20 kHz SoCal), but it
gets the job done.
Bob NO6B
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