Dear Dimitris, LF Group,
There are 2 important reasons to have a high Q loading coil. The first, as
you have already calculated, is to improve overall efficiency of the coil +
antenna combination. To make a significant improvement, the coil resistance
reduction must be a significant fraction of the overall loss resistance.
This will typically be the case with relatively small, low-capacitance,
antennas that have low loss resistance - these require a high loading
inductance, which for a given value of Q will have a high loss resistance.
The figures you quote say your antenna falls somewhat into this category, so
a modest efficiency improvement is achievable with feasible coil Q. Many of
us are using relatively long antennas with higher loss resistance, where the
loading coil Q has to be really bad to make much difference.
The second reason is if you intend to use high transmitter power, a high Q,
low loss coil will dissipate less power. Also, since high Q coils tend to be
large, they are capable of dissipating more power without excessive
temperature rise. Taking your figures of 9ohm coil resistance and antenna
loss resistance of 18ohm, about 17W of your total 50W TX power is being
dissipated in the loading coil. If you were to increase your radiated power
tenfold by increasing TX output to 500W, the 170W dissipated in the coil
could be a serious problem. Making a coil with twice the Q would reduce this
to 85W, dissipated in a physically larger coil, which would certainly be
helpful.
The loaded Q of the overall resonant antenna system would only increase by
about 20%, so tuning would not be greatly affected.
In my experience, using large litz wire intended for LF use (729 strands,
about 4mm overall wire diameter including insulation) on a 150mm dia former
of PVC pipe gave a Q of about 600 at 500kHz, so your target of Q = 500 is
certainly achievable. It is possible to optimise the Q with a given
inductance and wire size by having multi-layer coils with air spacing
between the adjacent turns and between layers, using "basket weave"
constructions and so forth, but these are much harder to make and use than a
simple single-layer solenoid, and only give modest increase in Q. So the
simple single layer coil is usually the best bet. For a fairly
high-impedance coil such as yours, dielectric loss in the distributed
capacitance of the coil can significantly reduce the Q, so PVC insulated
wire is not such a good idea, although a thin-walled PVC tube former does
not seem to make a big difference.
Alan is partly right about increasing losses with a high-Q coil, in the
sense that it is easy to convert it into a low-Q coil if it is poorly
installed. With very high-Q coils, any conducting object or lossy dielectric
material nearby can significantly reduce the Q, so keep the coil several
diameters away from the ground, trees, walls, wiring, etc., and give some
thought to possible losses in the coil housing.
Cheers, Jim Moritz
73 de M0BMU
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