Dear all,
according to my experience radio transmitters with good efficiency and
meeting all relevant data are usually designed for a load (!) of 50
ohms
only, nothing else.
But this does not mean that the source impedance of the transmitter
output feeding the cable will also be 50 ohms! Remember that the supply
voltage of the amplifier remains constant whilst the current within the
output device may vary.
The condition, that a power amplifier must also have a defined output
source impedance I have found in the field of broadband cable driver
amplifiers only, in that case mostly 75 ohms. But such a defined output
source impedance can only be achieved in class A amplifiers and by
using
a lot of feedback in or around the output amplifier, a solution that
degrades efficiency (not acceptable for RF power amplifiers). These
amplifiers also use feedback at their input to achieve an imput
impedance of 75 ohms.
One typical solution for 75 ohms input and out impedance in this field
has been a three stage grounded emitter amplifier, with resistive
feedback from last collector to the first base. As such feedback will
result in an input and output impedance of about zero ohms, 75 ohms
series resistances wee added at the input and output to get 75 ohms
input and output impedance! Losses did not count in this case. To get
the feedback stable, the frequency limits of the transistors involved
had to be much higher than the highest operating frequency of the
system, of course. Similar solutions are found in the application of
modern HF operational amplifiers.
I hope this helps to clear the situation. Therefore I feel that
impedance measurements into the output of a typical radio transmitter
would be questionable.
I rely on good SWR in the feed cable and maximum RF-voltage across the
cable impedance, and this should also correspond to maximum output and
aerial current at LF.
HW?
73 Ha-Jo, DJ1ZB
"Richard Newstead" <[email protected]> schrieb:
> Rik Strobbe wrote:
> > Hello Richard,
> >
> > is you antenna matched to 50 Ohm ?
>
> Not sure but the optimum power transfer suggests that it is matched to
> the output impedance of the transmitter.
>
> > If you just bring it to resonance (loading coild adjustment) the
> > antenna impedance can be anything between 10 and 100 Ohm.
> > And some 10m's length of coax can cause some odd impedance
> > transformations, even on 500kHz.
> >
> >
> There is no coax involved (well only 30cm) here so it is not that!
>
> Strange!
>
>
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