Topband: Noise and reception

[Steve Ireland]

W8JI replied:

>> 1.  A simple half wave dipole about 10' high, fed in the centre with a
>> voltage-type 1:1 balun.  It will be virtually omni-directional but
>> discriminate against the predominantly vertically polarised noise.
>
>I think you mean a current balun Steve. A voltage balun does not 
>ensure equal currents, and so the feedline might radiate. 
> 
>> 2.  A horizontal full wave loop, about 10' high, again fed via a
>> voltage-type 1:1 balun.  This needn't be particularly symmetrical - it
>> can be squarish, rectangular, circular-ish, whatever you like.
>
>Again I think you mean a current balun.
>
>Horizontally polarized antennas can help discriminate against 
>noises propagated in from a distance along the earth. As long as 
>that noise is ground wave propagated from some distance.

G'day Tom and all,

This is an interesting one.  Tom's point about horizontal polarised
antennas discriminating from noise at a distance is the key one - if the
noise source is relatively close-by (i.e. within a wavelength or two) they
usually don't help much or at all.  However, as an rx antenna for people
who have a lack of space and a predominantly vertically polarised tx
antenna, they are a very useful antenna for discriminating against general
atmospheric noise.

Some of the old-timers on this reflector may remember the excellent signals
on 160 SSB from Bill, VK6AS, who operates from Esperance.  Bill actually
had a full-sized quarter vertical antenna for transmit and settled on a
full-wave horizontal loop for his receive antenna.  The loop was just high
enough for Bill to drive his tractor underneath it - and helped him to work
98 countries on topband SSB - a huge achievement from Down Under.

With regard to the baluns, I actually did mean a voltage-type balun.

I would never use/try a voltage-type balun on any other type of antenna but
a 'balanced' resonant receive type - based on a particular experience.
Some years ago, I was playing around with a low dipole for rx purposes and
swapped the W2DU current balun I was using at the feed-point for air-cored
voltage one - and to my surprise, the signal-to-noise ratio on the antenna
improved markedly.

I didn't take any measurements other than finding the noise level dropped
about 1 - 2 'S' points on the meter while the signal levels stayed about
the same.  Probably this was just because of the particular environmental
'conditions' at my QTH.

I guess the way to go would be to initally use a current balun as Tom
suggests, as this SHOULD produce the best results.  However, a voltage
balun, wound in bifilar/trifilar fashion on a piece of plastic tube or a
ferrite rod, as per G6XN's HF Antennas for All Locations book, is easy to
make and try.

Once again, I guess we come back to: "let's try stuff." 

I should add that my inverted vee dipole at 90' is a much better rx antenna
than my Marconi-T, on which the amount of noise it picks up tends to
totally swamp weak DX signals.

Vy 73,

Steve, VK6VZ