Moving from LF to MF for a bit, here's a problem for the topband
operators and loading coil winders.
At the weekend I made a loading coil to resonate the LF Tee antenna on
topband. 34uH was needed and the only suitable former to hand was 40mm
plastic drain pipe - the white type with no significant RF loss.
First off I used 27 turns close wound of 1mm enamelled wire (which was
all that I had of that wire), and completed the coil with 20 turns of
0.7mm diameter tinned copper with turns wide spaced at 3mm to allow
tapping points.
This worked fine but I noticed the coil got a bit warm with 100 Watts
and decided I needed a higher Q....
So, I made some Litz wire by twisting 20 strands of 0.25 mm diameter
enamelled, which gives a copper X-sectional area a bit more than 1mm
diameter equivalent. Overall Litz wire diameter about 1.8mm.
40 turns of this, close wound with a few taps gave near enough the same
inductance value for a winding length of 72mm
Q was measured by connecting a 680pF cap in parallel, exciting the
circuit with a one turn coupling coil driven from a synth, very loosely
coupled and monitoring voltage across the coil with a x10 scope probe.
Peak response frequency was noted and then the points either side where
the response fell to -3dB (0.707) to get the bandwidth. Then Qu = CF /
BW
Now for the interesting bit. When it came to measure the unloaded Q of
each coil, the original one was a fair bit higher at Qu = 140, compared
with the Litz wound coil with Qu = 95. Both coils were the same
diameter, same inductance, and roughly the same length in total. So
why was the one made of plain wire better ? Self capacitance ? Q
was only measured at 1 MHz, perhaps I should try measuring gain at a
lower frequency.
Puzzled and curious..................
One interesting aside to come out though. Rayners formula for
inductance predicted the values of each coil to better than 10% and
bears out observations I've made over the years - such a simple
formula for wound inductors is too good to be true, working over a range
in excess of 1000000 : 1
L(uh) = (D.N) ^ 2 / (460.D + 1020 * length) D = mean
diameter, mm N = turns
Andy G4JNT
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