Dear Richard, LF Group,
It is not generally the case that the output impedance of a PA is equal to
its design load impedance, and so maximum power delivered to load is not the
same thing as load impedance being 50 ohms or whatever. This is especially
the case with switching type amplifiers, which will usually deliver more
power to a load below the design value.
With a switching type amplifier, assuming nothing in the PA is getting hot,
most of the DC input power is being delivered to the load as RF (80%+
efficiency is normal). So checking DC supply voltage/current is similar to
what it is when the PA is driving a dummy load will give a fairly good
indication if the actual antenna load impedance is similar to the dummy
load, or whether the output power is more or less than what is being
delivered to the dummy load.
Thermocouple-type RF ammeters are prone to be a bit erratic in their old
age - a check on calibration is to measure the current into a dummy load,
and compare with the current in the load calculated from the RF voltage
measured with an oscilloscope or diode voltmeter, etc. Alternatively, DC or
(preferably) 50Hz AC current can be used to check calibration.
The definitive place to measure RF current is at the feed point of the
antenna itself - this then allows a reasonable calculation of ERP to be
made, in conjunction with calculating the radiation resistance from the
antenna dimensions. As has been pointed out, there is generally some
impedance transformation occuring between the TX feeder and the antenna feed
point, so the currents will be different.
The only real way to be sure whether the load impedance is what you expect
is to have some sort of instrumentation to measure it. A suitable LF SWR
bridge will tell you if the impedance deviates from the design value of the
bridge. I use either a simple "scope-match" circuit in conjunction with an
oscilloscope to indicate voltage/current/phase at the TX output, or my LF
tuning meter design (see G3YXM's "features" page) to do something
equivalent. Another possibility is to measure the impedance with an RF
bridge.
Cheers, Jim Moritz
73 de M0BMU
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