Chris,
Sounds intriguing.
You're near the portion of the near field referred to as the reactive field (sometimes as induction field which is less precise but pertinent here). At your 1km distance, coil diameter and frequency, the reactive (induction) field (which does not depend on radiation resistance) is larger than the radiative field. But neither field (radiative or reactive) should produce the SNR that you see without some help. One missing datum is your receive bandwidth. Is your CW signal QRSS? What speed or receive bandwidth?
It may be possible to say more once your receive bandwidth is known; but I should mention that with extremely small LF radiators, it's often necessary (when characterizing them) to aggressively filter the transmitter's connection to the AC line (power mains)to keep small components of the carrier frequency, impressed on the AC line by the load of the power modulator, and radiated by long segments of the AC power line, from exceeding the field strength from the extremely small radiator under test. There is also the aspect of re-radiation, not especially significant with small radiators used in practical communications but significant for extremely small LF radiators; but incidental re-radiation (i.e. re-radiation that is not purposely engineered into a system) is usually very inefficient and probably not likely to be a factor here.
Can you provide your QRSS CW speed or receive bandwidth?
73,
Jim AA5BW Sent via BlackBerry by AT&T
Date: Sat, 26 Oct 2013 23:42:50 +0200 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
Am 25.10.2013 23:58, schrieb hvanesce:
Chris,
I’ll
second Warren’s motion.The radiation efficiency of a 100-turn, 1-meter
diameter solenoid (of any reasonable length/diameter ratio) at 100kHz
is in the hundreds of nano-ohms, i.e. a very difficult starting point
with respect to dissipative losses directly and indirectly associated
with wire resistance and coil inductance. Radiation resistance of a
solenoid scales with turns squared, but with resistance scaling
directly with turns, large numbers of turns is not a reasonable
approach for practical wire diameters.
The
reactive field (induction field), which comprises most of the field
(for a solenoid that is presumably a small fraction of a wavelength)
does not suffer low efficiency, but reactive field strength falls by a
factor of eight for (practically speaking) for every doubling of
distance, starting near the aperture boundary. The reactive field
strength 1km from a 1-meter diameter solenoid (properly oriented away
from the null) would be 1E-9 that of the field strength near the
aperture boundary.
Regards,
Jim
AA5BW
From:
[email protected]
[mailto:[email protected]] On Behalf Of Warren
Ziegler
Sent: Friday, October 25, 2013 3:50 PM
To: rsgb_lf_group
Subject: Re: LF: air solenoids
Christoph,
A while back someone on the US longwave
reflector posted the result of an experiment. They took a receiver to
the site of a high power Tesla coil operating at LF, I believe it was
being driven at 175kHz. they expected to hear it at some distance but
were surprised to find out they could only hear the Tesla coil at very
short distances. It seems that a coil by itself is an extremely poor
radiator at LF! It is unknown to me if there was any earth connection
to the Tesla coil, perhaps only through the mains connection.
On Fri, Oct 25, 2013 at 4:20 PM, Christoph
Schumacher <[email protected]>
wrote:
no, cylindrical coils
Am 25.10.2013 22:11, schrieb Graham:
Sent: Friday,
October 25, 2013 8:53 PM
Subject: LF: air
solenoids
Dear all,
is there anyone who was concerned sometime with the self radiating
properties of single layer air solenoids without any piece of antenna
wire?
73 dl7saq, Chris
--
73 Warren K2ORS
WD2XGJ
WD2XSH/23
WE2XEB/2
WE2XGR/1
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