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 = 613ohms
Shunt inductor XL = 42.6ohms
For match to 50ohms,
Total antenna+coil loss resistance Rloss = 21ohms
Antenna capacitance Cant = 1900pF
For 100W to the antenna, the 21ohms of loss resistance would result in
antenna current = 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
-----Original Message-----
From: [email protected]
[mailto:[email protected]] 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.
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