Hello Jim, LF,
indeed, class D amplifiers will (within reasonable limits) "adapt" to
the load impedance.
At least as long as the load is resistive. I have noticed that my PA
(G3YXM design clone) reacts "alergic" to a complex load.
My problem is that the antenna feeding point is about 35m away from
the house, I need a 40m coax between TX and antenna. This complicates
tuning if the antenna (+ loss) impedance is different from 50 Ohm. Not
because the TX output is 50 Ohm but because the coax is 50 Ohm and
will transform any impedance different from 50 Ohm resistive.
If the antenna is brought to resonance (at 502.5) the impedance is 40
Ohm resistive but this is tranformed by the coax to a complex
impedance (at 502.5) at the TX end.
In fact I have to tune the antenna several kHz above 502.5 (measured
at the antenna feeding point) to get a resistive load on 502.5 at the
TX end of the cable.
As so far I don't have remote tuning of the loading coil, getting the
antenna "tuned" is a bit labour intensive ;-)
73, Rik ON7YD
Quoting James Moritz <[email protected]>:
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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