See additional information below Eric's well written dissertation,
>Actually, if the duplexer insertion loss is really 2.2 dB, you should get
>120.5 watts out for 200 watts in. Once you consider the small additional
>loss in jumpers, connectors, adapters, etc., you're not too far off the
>mark. The tolerance and linearity of your power measuring instrument may
>contribute a small error.
>
>The major difference between your two conditions is that in one, the PA is
>feeding a resistive load, and in the other, it is feeding a highly
>reactive load- the duplexer. If you experiment with the length of the
>jumper connecting the PA to the duplexer, you will probably see a
>significant change in output power- which may be better or worse. You can
>also try an impedance matcher ("Z-Matcher") at the output of the PA, to
>see if you can optimize the match between the PA and the duplexer.
>
>Bear in mind that when a duplexer leaves the factory with precisely-set
>tuning and insertion loss, it may suffer considerable vibration during
>shipment. That's why I always check duplexers and bandpass cavities on a
>network analyzer before installing them. It's rare that such devices
>arrive in perfect adjustment.
>
Very much agreed, and in addition, the Duplexer Factorys uses equipment
that presents a perfect 50 ohm source/load impedance to the duplexer.
Many of us hams use converted commercial equipment for our repeaters.
This equipment, when operated outside its intended tuning frequency, may
not present a perfect 50 ohm impedance anymore, and the use of custom
cable lengths and Z Matchers become more common to match the duplexer to
the radio set.
The other thing.... All of the RF power being produced at the 200 watt
level may not be on the operating frequency. Remember that a watt meter
reads total power, not just the power on the repeater frequency. If the
transmitter or PA or both are spurious or dirty to some degree, you may
actually have less power (on your transmitter frequency) than you think,
even though the power 'reads' higher. Since the duplexer provides
filtering, the power leaving the duplexer may show less due to the
removal of the spurious energy.
Kevin Custer
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