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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