James Moritz wrote:
Dear Piotr, LF Group,
Considering only the conductor losses for the moment, in principle
increasing the number of turns of the loop by a factor N increases the
radiation resistance by approximately N^2, while the loss resistance
of the conductors increases by approximately N times, giving an
improvement in efficiency of a factor N. However, if you instead
connected the same N turns in parallel, you would have a single-turn
loop with N parallel conductors, with a loss resistance 1/N times that
of a single conductor loop, but with the same radiation resistance.
This would also give rise to the same N times efficiency improvement.
So from the conductor loss point of view, increasing the number of
conductors in the loop should improve efficiency by much the same
amount, it would not matter much whether the turns are connected in
series or parallel.
However, the multi-turn loop inductance is also approximately
proportional to N^2, if the turns are fairly close together. The
current in the multi-turn loop will be reduced by a factor of N to
achieve the same radiated power. Most of the loop impedance is due to
its inductance, so the overall voltage across the loop will increase
by a factor of N for the same radiated power.This means a greater
electric field strength around the antenna, and so increased
environmental losses due to the multiple turns - dielectric loss
increases as the square of the applied electric field. So assuming a
constant value of environmental loss (1.5ohm in your calculation) is
not realistic, if a significant proportion of this resistance is due
to dielectric losses, which is probably the case for most practical
antennas. Probably the main virtue of the transmitting loop is the
reduced environmental loss due to it being a relatively low-impedance,
low-voltage device. so to maintain this advantage, it would seem to be
a better idea to have multiple parallel turns rather than series turns.
Cheers, Jim Moritz
73 de M0BMU
Dear Jim, LF Group
I do thank you for your comprehensive answer. In particular, I did
like your remarks concerning the contribution of increased
dielectric losses ( as one increases the number of turns) in the
overall environmental loss. In some way it is still an open issue but
as you've said, from the point of view of practical antenna
design , each tenth of Ohm counts...
73 de Piotr, sq7mpj
qth: Lodz /jo91rs/
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