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From: "Vernall" <vernall@xtra.co.nz>
To: rsgb_lf_group@blacksheep.org
References: <LitYtBAGJB+6MwCK@telemetry.demon.co.uk><LitYtBAGJB+6MwCK@telemetry.demon.co.uk> <3.0.1.16.20010510084513.2c6f9b82@mail.cc.kuleuven.ac.be> <004f01c0d9a2$3334c6c0$2cb21bca@rvernall> <002c01c0db96$045db000$fa8274d5@w8k3f0> <001c01c0dbe8$33b0e320$36b51bca@xtr743187> <001e01c0dc83$8b612580$9da674d5@w8k3f0>
Subject: LF: Re: Re: Re: Radiation from loading coil causing difference in
 current at top and bottom?
Date: Tue, 15 May 2001 07:30:30 +1200
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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