Mel Farrer wrote:
> In my experience, using a tracking generator and spectrum analyser
> works fine on "realitively" modest Q circuits. The swept response
> display picture, is however a function of the detector response and
> the sweep speed of the generator. That is, the detector display will
> be deeper as the sweep speed is reduced and the effect is compounded
> when the Q of the circuit is very high as in a cavity. A synthesized
> generator and crystaled receiver will always give you a deeper null
> reading and will always allow you to find the "real" sweet spot.
Since most amateurs don't own or have access to a tracking generator and
spectrum analyzer there's a huge contingent out there using some other
form of signal generation and detection to tune duplexers. You hear
everything from station monitors with receivers to handi-talkies and
dummy loads.
For the average repeater owner I believe it is important to remember
that a duplexer is best tuned with receive levels rather than make any
adjustments using your handi-talkie or other transmitter. Primary
reason is to keep from ruining your transmitter during moments of high SWR.
The MOST important lesson to learn is to isolate the signal source and
signal detector (receiver) from the duplexer using 50 ohm attenuators of
3 db or more to help remove the impedance variations experienced during
tuning so you end up with as near the desired 50 (ish) ohm impedance
input and output on your duplexer. Connect to and tune only one cavity
at a time so you can keep signal levels within a reasonable range of
generation and detection. Make sure that the best pass on each cavity
will result in the notch occurring the desired distance away. Also make
sure that if you have adjustable coupling loops, that they are adjusted
per the manufacturer's specification for the loss you expect for the
band and separation you want. Use double shielded cable for all
connections during the process.
It is possible to achieve very good results with simple equipment (a
Mastr II exciter makes a good signal source if it is housed in a
shielded box and the output fed through attenuators with low leakage).
If you are going to own or maintain a repeater, take the time to put
together the basic equipment you need to do the job well. You don't have
to spend a fortune, just be wise in your selection of the things you use.
73, Tony W4ZT
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