"Ian White, G3SEK" wrote:
> >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
Thanks Ian,
This is the point that I have been trying to get across. A bird meter acts
as a directional coupler with isolation between forward and reflected only
when the load is a pure 50 ohm resistance. Any other load and the isolation
between directional couplers starts to deplete. It is no longer a
directional coupler and can not distinguish between forward or reflected
power.
73
Gary K4FMX
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