[Amps] Re: [Amps] Bird® 43 Manual

[Richard]

>
>>Page 4-1 section 2 of the Bird 43 manual:
>>
>>"Where appreciable power is reflected, as with an antenna, it is 
>>necessary to subtract reflected from forward power to get load power"
>
>A large part of this problem is the assumption that the Bird 43 measures 
>forward and reflected power.
>
>It does not.
>
>Look at what's in the slug. It is a short pickup line which is both 
>inductively and capacitively coupled to the main 50-ohm transmission 
>line. Depending on the orientation of the slug, these two coupled 
>components either add or subtract, which gives the instrument its 
>forward/reverse directivity when the slug is rotated. The resulting RF 
>voltage is rectified and produces a DC current that moves the meter 
>needle.
>
>There is nothing in there that responds *directly* to RF power. The only 
>thing being measured directly by a Bird 43 is the DC current through the 
>meter. Everything else is indirect, assumed, implied, inferred.
>
>The RF "power" is computed indirectly by the scaling of the meter, but 
>this scaling is only valid when the instrument is placed in a very 
>special environment, namely a matched 50 ohm system.
>
>Also the instrument is imperfect. The meter scaling isn't completely 
>accurate (up to 5% error at full scale). The directivity - ability to 
>separate forward and reflected waves - is not perfect either. Even when 
>terminated with a perfect 50 ohm load, the meter will indicate some 
>reflected "power" that simply isn't there.
>
>The whole subject of transmission lines and "what happens to reflected 
>power" was done to death in rec.radio.amateur.antenna a few months ago. 
>I certainly don't claim to understand the subject in detail. I only know 
>that a Bird 43 won't teach you to understand transmission lines - it's 
>definitely the other way around.
>
€  Well put Ian.  This is why I prefer to measure RF power with a 50-ohm 
termination and a freshly-calibrated** oscilloscope.  [P=E^2/R].  
However, since this produces a peak watt value, one must divide P by 2 to 
arrive at the (standard) RMS-watt value.    ** I use a 10.00V Motorola 
standard to calibrate the oscilloscope and probe.  

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
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