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LF: Re: Radiation from loading coil causing difference in current at top

To: [email protected]
Subject: LF: Re: Radiation from loading coil causing difference in current at top and bottom?
From: "Vernall" <[email protected]>
Date: Mon, 14 May 2001 08:05:38 +1200
References: <[email protected]><[email protected]> <[email protected]> <[email protected]> <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
Dick PA0SE and others,

Thanks for trying out the dual trace scope method.  You did not comment on
the phase at the hot end of the coil compared to the cold end.

Bob Vernall wrote:
> Finding out the phase information of interest does need a dual trace
> oscilloscope or a vector voltmeter.

I followed up Bob's suggestion using a dial trace oscilloscope.
snip
Thus as far as  can be judged from oscilloscope traces voltage and current
at the bottom end of the coil were in phase. If there is an extra
capacitive
current  flowing from the coil to surrounding objects it must be too small
to
show up in this way. This is also confirmed by my earlier experiment which
showed that moving the coil upwards and sideways in the shack made
no difference at all in the currents at top and bottom of the coil.
The current and voltage at the cold end of the coil can be arranged to be in
phase by a certain tuning of inductance.  Using the vector currents in my
suggestion, when the antenna is tuned for resistive input (the situation
Dick reported on) then for input currents I+Q, that particular tuning
introduces a current -Q, so the effective current is then I at the cold end
of the coil.

There is still no information on the possible phase difference between
currents at the cold and hot ends.  Using a scope probe to do the hot end
test needs a very low power test signal, even a signal generator, so as to
not exceed the voltage rating of the probe.  I can give this warning, as I
fried a nice 10:1 probe when I forgot to remove it after QRP testing, and it
also fried the input attenuator to the scope :o(

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.

So my conclusion is that the difference in current must be due to
radiation
from the coil.  The late Klaas Spaargaren, PA0KSB, once told me that in
an article on mobile antennas it was stated that a coil of L metre length
produced a radiation equal to a straight wire of that same length.
I am not in disagreement with that, for an unshielded coil (as they
generally are for mobile whips).  If the coil was in a shielded enclosure
then it does not have a far field to consider, so could not be allocated an
effective height component.

For unshielded loading coils, the height of the coil should be included in
the overall antenna system.  For a top loaded vertical, the difference in
current from ground level to the node where top loading is (horizontally)
connected, the reduction in current with height is far less than for a pure
vertical (with no top loading), however I agree that it is some value and
contributes to the observation that less current comes out the hot end than
goes in the cold end.

However, I believe there is additionally an I+Q situation as the stray
capacitance to ground for a loading coil is far more than when the coil is
substituted by a straight wire for considering the radiation current
distribution.

73, Bob ZL2CA



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