>
>>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.
end
|