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LF: Re: Re: Re: Radiation from loading coil causing difference in curren

To: [email protected]
Subject: LF: Re: Re: Re: Radiation from loading coil causing difference in current at top and bottom?
From: "Vernall" <[email protected]>
Date: Tue, 15 May 2001 07:30:30 +1200
References: <[email protected]><[email protected]> <[email protected]> <[email protected]> <[email protected]> <[email protected]> <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
Dick PA0SE and others,

Thanks for doing further tests for phasing.
> You did not comment on
> the phase at the hot end of the coil compared to the cold end.

I cannot measure the phase difference between currents in and out of the
coil. I don't have a current probe with my scope but even if I had one the
high voltage at the hot end necessary to obtain a readable current on the
thermo couple ammeter in the aerial wire would make the applicatiion of a
current probe impossible.

I did look at the phase difference between the current at the bottom end
and
the voltage end the top end. Holding the voltage probe of the scope near
the
top of the coil was sufficient for this.
Not surprisingly the phase difference was exactly 90 degrees, as far as
this
can be judged from a scope display.
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.  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???  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)???

> 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.
Have you any comment on the 90 degree observation mentioned earlier?

73, Bob ZL2CA



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