Mike,
I am happy to clarify various points:
ZL2CA wrote:
snip
> If I recall, you said that the changed current distribution was an
> explanation for the reduced value of current at the feedpoint, which
> was incorrect. Current is dependent on "loop resistance" at
> resonance.
Can you explain this concept, please. It is foreign to me.
It could be a term from MF broadcast antenna practice, and it means adding
together all the series resistance factors in the "antenna equivalent
circuit". So it includes coil loss, ground loss, radiation resistance,
feeder resistance, insulation loss (series R equiv) and transmitter output
resistance. At resonance, the circulating current is is a function of this
loop resistance. In the case of replacing one loading coil with another
that has higher resistance, the overall loop resistance increases. This may
also impact on impedance matching as seen by the transmitter, but that is
likely secondary. For a short vertical antenna, lower loop resistance means
more radiated power for a given amount of applied transmitter power.
> > Last night, Jim, M0BMU measured my field strength compared to a
> > measurement taken two weeks ago. It was 0.9dB up.
>
> That is very similar to some tests I did some time back, at 181 kHz,
> with a temporary top loading.
OK, so you agree that toip loading can make an improvement.
> > This does not account for the greatly improved reports,
>
> Quite so. 0.9 dB hardly shifts the S meter.
But would you reject a 0.9dB improvement?
No, but 0.9 dB is only 0.9 dB.
If I had done so at each
stage over the last few years I would still be struggling to hear my
own signals a few km away. Also, 0.9dB makes a big difference at
the 'knee' of the readability curve, for instance when trying to work
VE.
Yes, 0.9 dB could make all the difference for very marginal copy. If an
amateur station is below the radiated power limit for their licence, then I
suggest that 0.9 dB is "easier" and "cheaper" to obtain by increasing
transmitter power. Receive conditions are dictated by external field
strength to QRN ratio, so the main issue in being copied by a DX listener is
in maximising radiated power (regulations being the upper cap).
snip
> > 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.
>
> And still appears to be the case. The same "gain" should be observed
> on transmit and receive (being wary that local noise could be
> different at each end of a path).
I am not sure why you are discounting Jim's measurement. Why is
the evidence still anecdotal?
Although I agree with your argument about reciprocity, my antenna
is matched to the Tx and not the Rx so I do not think a receive
measurement would be valid.
I was not discounting the 0.9 dB measurement. It continues to be my opinion
that only 0.9 dB of CONFIRMED improvement is hardly worth the mechanical and
electrical overhead of having top loading inductance. Bumping up
transmitter power is more attractive, in my opinion. If a station is at the
regulatory limit for radiated power, then any change to the antenna is a
matter of shuffling one factor for another, and setting the transmitter
power to suit.
73, Bob ZL2CA
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