Dear Paul, LF Group,
I have yet to try a transmitting loop on LF, but from experience with
loading coils and various other LF inductors, you can expect that replacing
normal wire with Litz of similar size gives a reduction in RF resistance
due to the wire itself by a factor of the order of 3. Obviously this would
reduce the losses in an antenna, but by how much depends on what proportion
of the loss occurred in the wire, and how much was due to other causes. If
the total loss resistance of the antenna was dominated by external factors
such as losses in the ground, using litz would make little difference. This
will depend on the particular antenna.
...To what do you attribute the changes in tuning using the higher Q litz
wire?
I mean when the wind blows?. Is it movement in the antenna wire itself? Or
is it an effect of the air/moisture,temp. change passing over the coil?
An antenna with a higher Q will have a narrower bandwidth - or
equivalently, an impedance that varies more rapidly with
changing inductance or capacitance - therefore the effect of any slight
change in inductance or capacitance caused by movement in the wind or water
droplets, etc, will have a greater effect on the antenna impedance for a
high Q antenna than for a low Q one. The tuning can be made less critical
by reducing the Q; however, for electrically small antennas, the efficiency
will be reduced in exact proportion to the Q reduction. So halving the Q
will give you twice the "SWR bandwidth", but only half the radiated power,
for a given transmitter power and antenna dimensions. I believe some
low-power MF beacons actually deliberately introduce resistive loading to
achieve this, but in these cases efficiency is not a major issue; if you
want the maximum possible efficiency from a given antenna, you will have to
live with high Q.
My question all stem from the root question of is it possible to build a
small multiturn loop, say 3 meters on a side,
that would radiate reasonable well if it were elevated from the ground, and
out in the open with no trees.
Also if great care was taken in the design to eliminate losses. Thus the
litz wire questions etc.
What gives the best results can only be found by experiment in each
individual case - however experience shows that the prime factor in
maximising LF antenna efficiency is to maximise the critical dimensions of
the antenna; effective height in case of a vertical, effective area in the
case of a loop. Other improvements like reducing losses in wire or
environment can be significant, but are unlikely to be as effective as
making the antenna as big as possible in the first place. An inevitable
consequence of reducing the size of an antenna is that the Q required to
achieve the same efficiency as a larger antenna is increased - so even if
it can be achieved, it will lead to practical operating difficulties.
Making a small, efficient, wideband antenna is the holy grail of antenna
specialists, but none have done it - although a very good small antenna can
sometimes give better performance than a very poor big antenna.
Cheers, Jim Moritz
73 de M0BMU
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