Gary Schafer wrote:
>
>
>"Ian White, G3SEK" wrote:
>
>>
>> There are a few loose ends, like the need to prove that it works the
>> same at any position along a transmission line, not just at voltage
>> maxima and minima where E and I are non-reactive... but I'm content to
>> believe that's provable too. Also Maxwell uses a term Zc which he
>> describes as the "line impedance", which doesn't cover the case where no
>> line exists, either inside or outside of the directional coupler... but
>> it still works if Zc is described as the impedance for which the
>> directional coupler was designed (to give zero reflected indication when
>> matched) and at which the power calibration was made.
>>
>>
>>
>> --
>> 73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
>> 'In Practice' columnist for RadCom (RSGB)
>
>I just read the original Bruene description and it doesn't give any
>information about open termination's either. He only discusses impedance's
>close to design impedance..
>
>
>Well I just drug out an old Heath HW 101 transceiver. Hooked a short cable to
>a Drake W4 watt meter and no connection to the output of the watt meter.
>Tuned up on 20 meters. I was able to get the watt meter to read 200 watts
>forward and 200 watts reflected. All this with the plate current on the
>6146's just a little above the 50 ma idle current. With 800 volts on the
>plates that works out in the neighborhood of 45 watts input to the finals!
>
>Now either I have one hell of an efficient HW 101, to get 200 watts out with
>45 watts input, or there is a flaw in this business that the directional watt
>meter reads properly at all impedance's.
Bruene's fault, Tom's fault, and mine: we each should have said: "All
impedances except infinity, that is."
>Yes the net power delivered to the load is zero as the meter would show
>subtracting reflected from forward. However I would conclude that there is
>not 200 watts forward power and 200 watts reflected on the line.
The actual reading was meaningless. There are no watts delivered into an
infinite load - obviously, because there is no current.
Remember that your "wattmeter" does not actually measure RF power. It
actually detects RF voltage and current, and it only indicates power
correctly under a limited range of conditions. Infinite load impedance
is not one of them.
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'
'In Practice' columnist for RadCom (RSGB)
New e-mail: g3sek@ifwtech.co.uk
New website: http://www.ifwtech.co.uk/g3sek
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