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

To: [email protected]
Subject: LF: Radiation from loading coil causing difference in current at top and bottom?
From: "Dick Rollema" <[email protected]>
Date: Sun, 13 May 2001 12:00:57 +0200
References: <[email protected]><[email protected]> <[email protected]> <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
To All from PA0SE

Bob Vernall wrote:

Further to the item I have recently sent,

I have since realised that the suggestion of observing two ammeters may
not
show different peaking conditions, as the reactive current in the stray
capacitance (current Q) may not vary much over the band of testing,
whereas
the radiating current (current I) will rise sharply and have a resonant
"nose" (for any decent high Q antenna).  This observation of currents I
and
I+Q are likely to show the same tracking across resonance.

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.

My transmitter normally is connected to a tap on the loading coil and I felt
this might confuse the issue.
So I connected the coax from the TX to the bottom end of the coil and the
shield of the cable via a resistor of 0.78 ohms (actually three 2.5 ohm
resistors in parallel) to the earth wire. So the dual trace scope displayed
the
earth current as voltage over the resistor on one trace and the output
voltage of the TX as the other one.

To make sure that no current could escape via the mains  I inserted a choke
in the mains cord to scope and TX..

The total resistance of the aerial system is only 30 ohms (lucky me, living
in a country with my feet almost in the water ...) so the TX was not
properly
matched  and the aerial current was lower than before.
I measured 1.6A flowing into the aerial and 1.8A in the earth lead. The
latter value was read from a thermo couple meter and confirmed by the
voltage of 2V peak over the 0.78 ohm resistor.

The output voltage of the TX  was somewhat distorted due to harmonics,
showing up as some ripple at the positive and negative peaks of the
trace but  the zero crossings were at exactly the same points as the voltage
over the resistor.
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.

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.
As reported before that would in my case produce a current difference of
only 1.55%, as shown by computer modelling; much less than the measured
difference.
Perhaps the statement in the article is not true, or not applicable for the
sort of coils we
use on LF?
Could that explain the succes of the helical antenna?

73, Dick, PA0SE





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