>
> > Because the meter and its cable's electrical length will change the
> tuning.
> > You can read it at the antenna port of the duplexer if
> youre using one.
> This
> > has already been covered in previous posts. 73,Lee
>
> I must be dense . From what I understand you have the transmitter (no
> tuning in the amplifier stages) , then the swr meter ,the
> Z-matcher , and
> then the duplexer. It will not change the tuning of anything
> if you tune
> fhr z-matcher for minimum reflected power and then remove the
> swr meter.
> It will only change what the transmitter sees and it is not
> tunable anyway.
> That is all that the matching section does in the duplex
> transmitter. It
> does not tune for maximum efficiency if you follow the GE tuning
> instructions.
This discussion has gone full-circle and is now to the point of confusion.
The original premise was that a Z-matcher was needed on the output of some
PA's because the PA itself was *not* properly matched when terminated in a
50 ohm load. Wasn't that the original discussion? The goal wasn't to
minimize VSWR looking into the cavities. With that in mind, the "best
match" isn't necessarily that which produces the least reflected power. The
"best match" is the load Z that the amplifier is most happy transferring
power to, and the only way to know that is by looking at PA efficiency.
GE's procedure which is based on the least-reflected-power method is only
correct IF we assume that the output stage of the Mastr II PA is "best
matched" when looking into a perfect 50+j0 load. But that's not what we're
trying to accomplish here. The original premise was that many PA's,
especially when operated outside their design range or at reduced power,
were NOT best-matched when terminated in a purely-resistive 50 ohm load.
So, if you tune your GE Mastr II matching network for least reflected power
as indicated by the test pin on the onboard directional coupler, you're not
accomplishing what you set out to do.
Contrary to popular belief, and over-simplification by manufacturers, least
insertion loss and maximum power output aren't always the right answers when
tuning amplifiers, filter cavities, and other devices in the transmission
system. Maximum return loss in filter cavities and antenna systems, and
maximum efficiency in PA tuning, are steps in the right direction if the
goal is to have a transmission system that is stable and the least immune to
external influences including temperature.
Now, if you have a PA that runs away when presented with a known-good load,
then you've got a problem that needs to be fixed IN THE PA. Using a
Z-matcher, or the crude equivalent of feedline pruning, to help tame an
unstable PA isn't a fix, it's a band-aid that will eventually come off...
--- Jeff
--------------------------------------------
Jeff DePolo WN3A - [EMAIL PROTECTED]
Broadcast and Communications Consultant
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