Hi
Clemens,
thank you for the great idea. I will try to stimulate the lf coil
without a galvanic
contact and look what happens.
cu next weekend
73 Christoph, dl7saq
Am 27.10.2013 08:56, schrieb Clemens Paul:
Hi Christoph,
from the coil parameters you have given I would
estimate an unloaded coil Q of roughly ~500 on 137kHz.
I guess you've made your measurements with the measuring
equipment galavanicaly connected to the coil.
In this case *and at this quite high Q* it is very likely that every bit of conducting
structure of your *whole* test setup is radiating.
I would try the following procedure:
Couple the test signal *very loosely* inductively into the coil by using a coupling coil.
For measuring series resonance connect the ends of the coil together,
for parallel resonance leave the ends open.
73
Clemens
DL4RAJ
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of
Christoph Schumacher
Sent: Saturday, October 26, 2013 11:43 PM
To: [email protected]
Subject: Re: LF: air solenoids
Hi Graham, Warren, Bob, Rik, Jim
thanks for your comments. What I found (by chance) is a series
resonance in the impedance spectrum of solenoid air coils, a
little underneath the high impedance parallel resonance. The
series resonance could derive from the coils main inductance
and its capacity to the surrounded electric field. But it fits
only very vague. However on that frequency the coil is a very
efficient radiator especially related to its shape.
Stimulating the coil on the high impedance parallel resonance
does not effect any far field radiation.
For instance: my lf coil for 135kHz has app. 5mH inductance,
42cm (17 inch) diameter, 27cm (11 inch) length. The high
impedance parallel resonance is 600kHz and the low impedance
series resonance is 500kHz. The resistance on that frequency
is 180 Ohm and the SWR=2 bandwidth is 2kHz. On 500kHz the coil
is a very efficient radiator related to its mechanical length
of lambda/500ppm. Using an input of app. 7mW (DDS-gen.) into
that coil I could read cw code at 1km distance via active
antenna on my car roof (maybe 6dB SNR).
I become suspicious on whether the radiation resistance model
is still valid for such a tiny radiator. It is not my
intention to introduce a new lf radiator because at that
resonance the voltage across the coil must be gigantic. I only
would like to understand the physics. Do you have an idea?
Best regards
chris dl7saq
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