> The other ways to correct the problem, other than using a different
> transmitter that is not bothered by reactive loads as much,
> is to use a Z
> match or try different length cables that make the
> transmitter happier.
But if the transmitter is bothered by the bad Z at frequencies outside of
the pass, any matching device you put in line is only going to throw off the
Z at the pass frequency. You can't have a duplexer that presents 50+j0 at
the pass frequency, and then add a matching device between it and the PA and
still have 50+j0 at the transmitter at the pass frequency. Sure, you can
give the PA 50+j0 at some other off-channel frequency by doing this, but at
the expense of messing up the pass performance, this seems like it's only
creating new problems.
> Both
> of these transform the impedance/reactance presented by the
> duplexer to
> something more palatable to the transmitter and allow it to
> produce the
> power intended.
If you have a PA that is happy with a good load at the carrier frequency,
but isn't happy with the Z at other frequencies where it shouldn't be making
power, I'd have to say you have a problem with the amplifier that needs to
be fixed. The antenna system shouldn't have to be the cure for the
amplifier's ills. I've never had a good PA (emphasis on good) that got
unstable if it was looking into a matched load at the carrier frequency
regardless of what was going on off-channel.
> There is no sin involved in using different length cables to
> make the system
> work properly. It is not a band-aid approach to mask other
> problems.
Well, I guess we'll have to agree to disagree.
> The
> real problem is that some transmitters, because of the way they are
> designed, do not like reactive loads.
They should like a good load at the carrier frequency, and be tolerant of
strange load Z's off-channel. A PA that has a tendency to "run away" just
because the load Z at frequencies well removed from carrier isn't perfect is
an accident waiting to happen. Even if you mess with cable lengths and
Z-matchers and walk away from the site with the PA running clean, there's a
good chance the next time the antenna ices up or the HVAC fails that you'll
be getting calls about spurs coming from your box.
If I have a PA go spurious, for ANY reason, whether during commissioning or
sometime during operation, it comes out of service until it can be fixed.
That's just me.
> Close spaced duplexers will be the worst with
> off frequency
> reactance as the impedance has to change quickly as you move
> away from the
> wanted frequency in order for the duplexer to do its job.
Mostly agree, except for notch-only duplexers with tend to have a fairly
wide range of tolerable load Z outside the notch, and notch duplexers are
fairly common in close-spaced situations.
--- Jeff
