Yesterday, I reported that I had increased my mast-head
inductance so that the antennna was almost self-resonant. The new
coil introduced resistive losses and the current reduced, but reports
were better than before.
Last night, Jim, M0BMU measured my field strength compared to a
measurement taken two weeks ago. It was 0.9dB up.
This does not account for the greatly improved reports, but it does
show that with less current and more resistive losses, the only way
the ERP could have increased is if the 'effective height' had
increased. Field strength is of course directly proportional to current
squared multiplied by effective height (h) squared.
At last here is real evidence that the elevated coil really does
increase the 'h' part of the equation. Several of us were sure that it
did, and several have noticed improvements in our signals when
using elevated coils, but the evidence has always been anecdotal.
Before you all rush out to put coils at the tops of your masts, the
improvement is really only significant if you have a small top section -
mine is three parallel wires 0.5m apart and 18m long.
My ERP is now estimated at 165mW.
Mike, G3XDV (IO91VT)
http://www.lf.thersgb.net
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