>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.
--
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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