Dear Terry, LF Group,
Re: is litz wire useful? It depends on the individual antenna. Consider 3
examples:
1) Very large antenna in an open field environment (e.g. G3KEV). The antenna
might have capacitance of 1nF or more, requiring only a few hundred uH of
inductance for resonance, with a reactance of a few hundred ohms. The
antenna resistance is probably several ohms. In order to keep the proportion
of TX power dissipated in the coil to a reasonably small fraction of the
total, the coil resistance must be below a few ohms. This will be achieved
if the coil Q is < 100 or so. This is not a demanding requirement and does
not require special coil construction.
2) Small antenna in a back garden. Antenna capacitance is a few hundred pF,
requiring several mH of inductance, with reactance of several kilohms.
Antenna loss resistance is also high due to proximity to trees and
buildings, probably 50ohms or more. So provided the coil resistance is below
a few 10s of ohms, the coil will not greatly reduce overall efficiency,
which will be achieved with a coil Q of a few hundred. This is fairly easy
to acheve with the much larger coil neccessary than in case 1, without
requiring special coil construction.. For instance, a coil of 1mm enamelled
wire 150mm diameter should have a Q of well over 200. However, in order to
achieve anywhere near 1W ERP with the very low antenna efficiency, High TX
power of the order of 1kW or more will be needed. Using litz wire can reduce
the resistance of the coil by something like a factor of 3, which will
certainly help stop the coil overheating, even if it has little effect on
overall antenna efficiency. The high reactance also means high voltage
(>10kV not unusual), which requires a physically large coil to avoid
breakdown.
3) Small antenna in an open field site. Now the coil reactance must be
kilohms similar to the back-garden antenna, but the loss resistance of the
antenna can be much lower; in my experience, less than 10 ohms. So you now
require a large coil with a very high Q, of the order of 1000, so that the
coil does not seriously reduce overall antenna efficiency. Litz wire is
practically essential to achieve this, although even if a relatively low-Q
coil does compromise the antenna losses, antenna efficiency will still be
better than the back-garden antenna overall. Again, the voltage will be
high, so a large coil will be required anyway.
I have just seen an eBay ad for LITZ WIRE 10/44 250 ft at 15
USD including shipping to the UK (the seller has more than 10 available).
Is
this size any good for winding loading coils?
I guess this means 10 strands of 44AWG wire, which would be rather small for
winding a coil that has to support a few amps of RF current.
Cheers, Jim Moritz
73 de M0BMU
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