>
>
> "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
>
Hi Gary.
That is not the case .
The Bird 43 picks up current directionally. With a standing wave on
the line , current is washing back and forth through the line.
It still picks up signals directionally.
---
Ron
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