Dear LF Group,
I think what Jan-Martin is saying is that by having a grounded center tap
on the windings of a loop antenna, you are effectively producing an
integrated 1:1 balun transformer, which forces the voltages on the loop to
be balanced, even if the output is single-ended. However, the balun effect
will not be entirely perfect because the coupling between the loop windings
is less than unity. But then, neither will a balanced preamp be perfectly
balanced. An advantage of a push-pull type of balanced preamp is that it
will give some cancellation of distortion products, but it's fair to say
this can be achieved in other ways too.
As far as I can see, the balanced, tuned loop with push-pull preamp has
changed little since the 1920s, except the bright-emitter triodes are
normally replaced with JFETs in modern designs :-) The idea then was to
produce an antenna with accurate and deep nulls in the MF range for
direction finding. By making the circuit physically and electrically
balanced, a symmetrical directional pattern was produced. It is a bit
different with an LF receiving loop, where the precision of direction
finding is usually less important than achieving high sensitivity, with
adequate nulling of unwanted signals. Also, especially where larger
diameter loops with relatively low inductance are used, the "antenna
effect" does not seem to be a very serious problem. The effect of the E
field pick up will be 1) to make the positions of the nulls asymetrical, 2)
if there is a quadrature component between the two induced voltages, the
depth of the nulls will be reduced, and 3) if local nose generates a
localised E field, the antenna will pick this up. For simple reception, 1)
is of little consequence, 2) in practice is not very bad - I find I can
usually get >25dB null even with no attempt at balancing, and the depth of
null is usually limited by how accurately the antenna can be positioned. I
also find that since local noise tends to be at least as much H field as E
field, 3) is not really an issue either.
I also find that external effects affect the depth of null you get - in my
field strength measuring experiments, I found some locations where there
was almost no null - This seemed to occur near overhead wires and power
lines, which were presumably acting as parasitic antennas, inducing signals
in the loop.
Cheers, Jim Moritz
73 de M0BMU
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