Hello All
Recently I purchased a tuned passive magnetic antenna
for receiption of utility stations in the 70-150 kHz range.
Initial messurements on it showed, that sensitivity was only
slightly lower than required for 137 kHz amateur signals and
bandwith much too broad.
The antenna consists of a ferrite rod 285 mm in length and
28mm in diameter (made from 19 staggered toroids). The
antenna winding consisted of 175 turns made from
0.335 mm diameter stranded annealed copper wire, additionally
insulated with silk filament. The coil former has 30mm inner
diameter, 50mm outer diameter and about 10mm in length.
The coil can be characterized as a flat or short multilayer solenoid.
coil without ferrite rod
L0=1.3mH
R(137kHz)=9.1 Ohms
Q0=124
coil with ferrite rod
L0=7.1mH
R(137kHz)=70 Ohms
Q0=87
In order to reduce losses of the complete antenna system
I decided to re-wind the coil with larger size of conductor.
I had 2mm diameter annealed copper wire available. The
coil then was made 90mm in length, 4 layers, 42 turns per
layer. The coil can be characterized as a long multilayer solenoid.
After assembly and on air testing I had to
learn, that bandwidth increased instead of
being reduced.
Measuring the new coil without ferrite rod:
L0=0.3mH
R(137kHz)=11 Ohms
Q0=24
confirms that the new coiling scheeme was not
good to reduce coil losses.
Questions:
How do I form a low loss coil for 137 kHz
using mentioned 2mm annealed copper wire?
Shall I use other form factor?
Shall I use greater distance between
wires?
Any suggestions and hints are welcome.
Best 73
Gamal Soegiono (SWL)
50N01 / 08E27 / JO 40 fa
[email protected]
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