"Ian White, G3SEK" wrote:
>
>
> The actual reading was meaningless. There are no watts delivered into an
> infinite load - obviously, because there is no current.
>
> With an infinite load impedance the circuit can only detect the RF
> voltage, because the current. The voltage signal is nondirectional, so
> it will be the same in both the "forward" and "reverse" switch
> positions. It happened to be the same voltage as you'd get for 200W into
> 50 ohms, so the meter happened to read 200W.
>
> --
> 73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
That is what I said from the very start, "there can't be any power if there is
no
current in the line".
Yes I understand subtracting reflected from forward power readings and agree
with
that. What I am questioning is how to determine reflected power in the line.
I just tried another test with a :
transmitter-- watt meter-- tuner-- drake watt meter-- capacitor-- 50 ohm load.
By adjusting the capacitor in series with the load I can get a complex impedance
seen by the drake watt meter. I then adjust the tuner so the first watt meter
shows zero swr. With that setup with a particular capacitor setting I read 30
watts into the tuner and on the drake meter I read 100 watts forward and about
65
to 70 reflected. Subtracting reflected from forward does give the approximate
forward power. (30 watts)
But the question is how much reflected power is there really. There can't be 70
watts because the transmitter is only putting out 30 watts.
This whole thing started with the discussion of how much power got reflected
back
to the finals in a transmitter when there was a mismatched load. It was said by
some that if the transmitter had 100 watts output and there was a mismatched
load
to cause, say 25 watts reflected power, that the watt meter would read 125 watts
forward and 25 reflected. Then subtracting the reflected from the forward would
give the power delivered to the load. However, the statements were that the
extra
25 watts of forward power that the meter was reading were the result of the 25
watts that were reflected from the load being re-reflected from the
transmitter
matching network back toward the load again. And this was why the watt meter
read
125 watts forward.
I am guessing that as the line impedance gets nearer to the watt meter design
impedance that the directional coupler part of it starts to have enough
isolation
to give a valid reading, but where does that start?
73
Gary K4FMX
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