Amateur Radio Pages
updated November 2014
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LF and MF Experiments etc - 14
LF Tuning meter and ScopeMatch - continued
Having written “the unit has been able to work with the high RF voltages present” on the previous page , some time later during prolonged testing of a developing new weak signal mode the same RF voltage sensor got very hot and was destroyed. I e-mailed Jim M0BMU whose design it is and here is an extract of his reply :-
“I suspect this is not due to insulation breakdown, but to overheating of the toroidal core. The voltage transformer has the full TX output voltage applied to its primary, and it turns out a surprisingly large core is needed to produce a low enough level of flux in the core when using
fairly high TX powers / low frequencies, even though the actual power throughput of the transformer is minimal. This is mentioned in my "LF tuning aids" article.

As it says in the article, the options are basically:-

1 - Increase the number of turns on the transformer
      (e.g. from 50:1 to 100:2 - not easy with a small core)
2 - Use a larger ferrite core (I used an RM10 ferrite pot
     core satisfactorily)
3 - Use a capacitive voltage divider instead of the
     transformer - see the article.”

Of course the capacitive divider was the easiest way to go now as the other method had failed twice before. A link to Jim’s article is on previous page.
LF & MF experiments 15 LF & MF experiments 13