Hi Dick well if you have 140volts peak-to-peak, across the 50 ohm load, on
the scope that is 140/2.82 Vrms =49Vrms. then the RF power is 49*49/50
watt....so about 50 watt. Thus the current meter should read about 1 Amp
(I*I*R=50)
I cant remember the conclusion of the discussion but there was some
suggestion recently when JAllen in Yukon had similar problem sorting his
much bigger antenna. I cautioned about the possible reported odd effects
when a ferrite antenna current meter was placed close to the top of (i.e. in
the RF field of) a loading coil. I have no personal experience of this but
several early experimenters in the UK reported this effect and there was a
big search for old RF thermoammeters. Jim did comment on his thoughts on
what the effect might be. I cant remember the full story now. It might be
worth searching back through the messages. It turned out not to be the
problem in this instance more that the antenna was tuned well below 136 and
the thermoammeter was blown so showing no current.
Ok the drive frequency is 136, but if you hang a tuned circuit on the output
(accidentally) that happens to be tuned to a harmonic it will extract
harmonic power, which is bound to be there to some extent in any power
stage. Andy G4JNK reports blowing FETs because he fed his matching tap on
his tapped loading coil with a short length of coax. On calculation, I
believe the capacity of the coax and inductance of the tapped part of the
coil resonated at about 400khz., 3rd harmonic.
Cheers de Alan G3NYK
----- Original Message -----
From: Dick <[email protected]>
To: <[email protected]>
Sent: 13 August 2006 09:23
Subject: LF: PA4VHF tests/problems?
> Thanks Alan for the suggestion.
>
> Well while tuning the variometer everything seemed working as it should
be.
> The SWR nice dropped to zero, while the antennacurrent raised.
> With help of a separate receiver I was able to figure out that I was
> certainly not tuning on a harmonic.
>
> So, there really might be some extra capacitance produced by the
> coilwindings etc.
>
> About the antenna current, one reason might be an insufficient earth, as I
> just connected the ground tap
> to a groundrod with 5m wire to my groundrod 'somewhere' in the garden.
>
> I built up the transmitter again, but now connected to a 50Ohm dummyload,
> instead of the coil antenna etc.
> Used exact the same coaxial cables.
> SWR/PWR meter shows same value as with the loading coil/antenna connected,
> and also DC input power
> seems the same.
> So, that gives me reason to believe it's ok.
>
> Question however is the exact amount of power, hi.
>
> I am measuring 140 Volt top-top on a oscilloscope parallel with the 50 Ohm
> dummy load. (at the dummy load)
> DC input is 13.0V/7.5A (that's 97.5W DC input)
>
> For some reason I can't figure out the right formula to calculate the
power
> into the dummy load.
> I get wrong figures al the time.
>
> I'm more a practical man, hi
>
> 73
>
> Dick, PA4VHF
>
>
> my testfrequency as always is around 136.1kHz from an X-tal
> oscillator/divider.
>
>
>
>
>
>
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