To Bob, ZL2CA and others
.
Bob wrote:
My first reaction to this IS one of surprise. My first thoughts are that
in
a series resonant circuit (the loading coil in series with antenna
capacitance and system resistance) the loop current would be in-phase (an
I
current) with the transmitter drive.
The loop current, translated as voltage over the resistor in the earth
connection, IS in phase with the transmitter outpuit voltage when the
system is tuned for maximum current in the aerial, as explained in my
e-mail..
My second thought on your observed
phasing is that an input current Q at the cold end of the loading coil
turns
up as a similar magnitude quadrature current Q at the hot end???
Because the impedance at the cold end op the coil is resistive when the
system is tuned for maximum aerial current there is no Q current going into
the coil.
Or is it
that the very low capacitance coupling of your scope probe near the top of
the coil means that it is equivalent to a constant current source, with
current leading the voltage by 90 degrees (the capacitive reactance being
many times the load impedance on the probe)???
In my view the probe acts as an E-field probe. The E-field is in phase with
the voltage at the top of the coil. So the phase of the voltage on the probe
is the same as the phase of the voltage at the top of the coil. (The
distance between probe and coil is so small, expressed in wavelength, that
this causes negligible extra phase shift).
73, Dick, PA0SE
> > Using the idea of I+Q current going into the cold end of the coil, I
> suggest
> > there are two tuning conditions of interest:
> > - when the antenna is tuned for maximum current out the hot end (into
the
> > antenna wire), for maximum radiation, then the condition at the cold
end
> is
> > not purely resistive
> > - when the loading coil is tuned so the current at the cold end is
> > resistive, the radiation current (hot end) may not be peaked.
>
> I checked this with great care. In my earlier e-mail I reported that the
> output waveform of the transmitter voltage was somewhat distorted due to
> harmonics. I now reduced the drive to the final amplifier to a level
where
> the output voltage became a pure sinewave. The current into the bottom
end
> of the coil was displayed as the voltage over a 0.78 ohm resistor
carrying
> the current.
> I very carefully tuned the system for maximum aerial current. Because
> voltage at and current into the cold end of the coil were now both
sinewaves
> I could superimpose them on the scope by varying the amplification in
one
of
> the two channels. The two sinewaves completely coincided, showing that
> maximum aerial current occurs when the impedance at the cold end of the
coil
> is resistive.
OK, that point is cleared up.
> I think I now have done the maximum that is within my measuring
> capabilities.
> But suggestions are still welcome.
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