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From: "Clemens Paul" <cpaul@gmx.net>
To: <rsgb_lf_group@blacksheep.org>
References: <526ACC49.6030208@t-online.de>	<29208E66335B49E08158AB7E38F57C7C@AGB>	<526AD2A4.9060704@t-online.de> <CAMFjj70u0=iFN67i8JEYFvVRhP6oMM6e9LEZ6Q5s2spwhHYBRA@mail.gmail.com> <0a4101ced1cd$728cbd10$57a63730$@comcast.net> <526C375A.7060304@t-online.de>
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Date: Sun, 27 Oct 2013 08:56:30 +0100
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Subject: RE: LF: air solenoids
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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: owner-rsgb_lf_group@blacksheep.org 
>[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of 
>Christoph Schumacher
>Sent: Saturday, October 26, 2013 11:43 PM
>To: rsgb_lf_group@blacksheep.org
>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
>