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From: "james moritz" <j.r.moritz@herts.ac.uk>
To: <rsgb_lf_group@blacksheep.org>
Date: Tue, 18 Jul 2006 13:24:34 +0100
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Subject: LF: RE: RF Ammeter
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Dear J, LF Group,

Well, it's a pity the meter died, but a relief as far as the antenna =
system
is concerned!

The antenna matching system you have at the moment actually looks pretty
good. The main effect of the 1:1 transformer will be to act as a shunt
inductor of around 50uH, together with the series inductance of the =
loading
coil. This all-inductor L network can only achieve a match if the load =
has a
resistive part less than 50ohms, showing that your antenna has loss
resistance well below 50ohms. A rough calculation, assuming the =
transformer
is also using a 10-3/4" former with the same size wire, coupling between
primary and secondary near unity, and factoring in some 25pF stray
capacitance for the loading coil, gives the following

Loading coil XL =3D 613ohms
Shunt inductor XL =3D 42.6ohms

For match to 50ohms,
Total antenna+coil loss resistance Rloss =3D 21ohms
Antenna capacitance Cant =3D 1900pF

For 100W to the antenna, the 21ohms of loss resistance would result in
antenna current =3D 2.2A.

As far as splitting ferrite cores to make an RF ammeter goes, my =
suggestion
would be not to split the core, and just feed the wire through it! I =
can't
see this would be any harder than connecting your broken meter into the
circuit, and avoids the trouble and uncertainties involved in splitting =
the
core. I think the "weird results" that sometimes occur are likely to be =
due
to stray capacitance between the low-current parts of the ammeter =
circuit
and the high voltage parts of the antenna tuner. Putting the complete
ammeter circuit inside a metal case, connected to one end of the current
carrying wire forming the transformer primary, prevents this problem.

Cheers, Jim Moritz
73 de M0BMU=20

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of J. Allen
Sent: 18 July 2006 03:06
To: LF (RSGB)
Subject: LF: RF Ammeter

Scott and Friends,

The ammeter check results are in.... Dead Meter.