>
> While the receive-side jumper may work well at any random
> jumper length, the
> transmit-side jumper is a different matter.
My experience has been just the opposite. Unless you sweep-tune a receiver
front end that uses helical resonators, the chance of having a Z even
remotely close to 50+j0 are pretty slim if you just go with the
manufacturers' tuning instructions. In addition, many 'stock' preamps have
rather poor return loss, especially GaAsFET's that are tuned for minimum
noise figure with no regard for input return loss. There was a lengthy
discussion on this subject a few months back.
> I think we agree that the
> TX-to-duplexer jumper can be optimized, and many installers
> do take the time
> to try several different cable lengths. One installer I know
> takes a box of
> a few dozen RG-400 cables to each new install, each cable
> being cut and
> connectorized at 1/2-inch intervals from about 18" to about
> 40". He will
> keep trying cables until he finds the optimum length.
What's "optimum" using this tech's approach? Maximum power output? Best
efficiency? Cleanest signal?
> Regarding the tuning, I always use a network analyzer to tune
> a duplexer,
Me too.
> and I know that it is optimized for a 50-ohm source and load.
> The PA is
> another matter, and it seldom provides a 50 ohm match to the duplexer.
You're looking at the PA being a load rather than a source. The PA can't
detune the duplexer if you're looking at signal flow from the PA into the
duplexer. On the receive side it's a different ballgame.
> That's where a Z-matcher is a valuable tool, but- as can be
> seen by several
> dissenting posts- not everyone thinks it is necessary!
Depends on where you draw the line. If I can't make a system perform right
with all of the passives (and the actives I have control over) work with
everything tuned to 50+j0, I go back to the bench, warm up the VNA, cool
down a few 807's, and spend time making it right. If you can't tell by now,
I'm no fan of band-aids to fix fundamental problems. Sure, many a tech has
come up with an ad-hoc iterative technique to make a box play in order to
get out of a site in time to be home for dinner, but let's not confuse "what
worked this time" with sound engineering.
--- Jeff
