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Message-ID: <5.1.0.14.0.20021220113920.00af1d00@gemini.herts.ac.uk>
Date: Fri, 20 Dec 2002 12:19:23 +0000
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
In-reply-to: <2D280BB697C7EE4C979C5C89925418EF1BF5A5@TNS-FBU-2E-003.corp .telenor.no>
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Subject: LF: Balanced vs. Unbalanced loops
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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