[Amps] Re: [Amps] Re: [Amps] Re: [Amps] Re: [Amps ] Re: [Amps ] BirdŽ 43 Ma nual

[Tom Rauch]

> 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)

OK, just as it should. You have proven the meter is reliable for 
reading true power when reflected is subtracted from forward power. 
 

 > 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.

If you prefer circuit theory rather than wave mechanics, consider it 
VAR  (volt-amperes-reactive) power.  VAR power is simply current 
times voltage or apparent power.

The reactive part causes heating and loss, and does no useful work. 
 
> 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?

At any impedance, assuming the coupler is designed and 
calibrated perfectly.

In measurements I have made, a Bird meter works pretty well even 
with a 300-ohm load.
73, Tom W8JI
W8JI@contesting.com