> Well, I'm going to go out on a limb & disagree with Jeff,
> perhaps for the
> first time ever.
That's OK, we'll still let you hang around our tent and drink our beer at
Dayton :-) Are you going again this year Bob?.
> 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.
My take on the situation is that I'd rather have everything as close to 50
ohms as possible. If there is any variation in the system outside the
duplexer/cavities that you can't control (such as Z changing due to antennas
icing up), the transformation effects of the cavities should be minimal.
Also, by keeping all of the passives at 50 ohms in and out, it eliminates
much of the uncertainty when adding additional cavities (e.g. pass cavity
ahead of a receiver) or when adding an isolator that was bench-tuned with 50
ohm loads. I'll take consistent performance over optimum performance when
we're talking a one or two tenths of a dB in insertion loss difference.
As far as using cavities as matching networks to eek more power out of a PA,
without actually looking at the efficiency at different load Z's, there's no
telling what the PA is truly happy with. Just because you can squeeze an
extra dB out of the amplifier by providing it with an some odd load Z
doesn't mean that's the ideal load Z to operate it at. If you have to burn
up 50 more watts in heat to get an extra 10 watts out of the PA, that's bad.
So, if you lack test equipment and have no choice but to use high-level
signals for tuning the pass, you should still be tuning for minimum
reflected power.
> 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?
You can also tune Rx front ends with a network analyzer or SG/TG and RLB.
You will also see that the "window" response of the front end of your
typical two-way radio (Micor, M2, whatever) can be tuned to favor the side
away from interference sources (e.g. your transmitter or other co-located
transmitters) without any significant detriment to insertion loss at the
desired Rx pass frequency.
> Sounds like a nice piece of test equipment to have around: a 50 dB
> non-directional coupler.
Bird 4274-025 non-directional coupler element. I keep one in each of my 43
cases. Handy to have.
--- Jeff
--------------------------------------------
Jeff DePolo WN3A - [EMAIL PROTECTED]
Broadcast and Communications Consultant
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