[TowerTalk] 50 MHz beam height

[Chris Burger]

My neighbour, Hal Lund ZS6WB, has a beam on my high tower (41 m or 135'
a.g.l.), and another on a separate tower (18 m or 60').

The high beam is a 7 el with a medium boom and the low beam is a 9 el
monster on a long boom.  I forget the exact length, as I get vertigo above
30 MHz.

Remember that we have virtually no Es, and mainly long-range stuff.  Of the
1000 stations in my 50 MHz log, less than 150 are within 8000 km (5000
miles) from me.  We have a very low incidence of Es, and the country is too
small (1800 km or 1100 miles) for decent Es in any case.  In other words:
Low-angle is more useful than high-angle.

In this context, even the high beam is not too high.  It suffers from more
noise, as it can apparently "see" more noise sources than the low beam.
However, it very often wins in signal strength.

My own beam is a modest 5 el short-boom design at 18 m (60').  If Hal is
working stuff that I can't hear, I can count on it being on the high beam.
It is often up to 10 dB better than the low beams, and occasionally it's
weaker.

Hal's high beam, with a gentle downhill slope to most directions, has duped
the locals into believing that we somehow have superior propagation in the
Pretoria area.  However, I know it's not true.  I live only 35 m (115') from
him, and there's no doubt that the high beam plays.

Bottom line:  I doubt if there's such a thing as "too high" when you're
dealing with long-haul DX.

The other side of the coin, of course, is that if you intend to do mainly MS
or Es stuff, you require higher angles.  You can achieve this by low beams,
an approach that doesn't work well in the urban clutter, or by using a stack
with variable phasing.  W2PV's book from the Eighties shows sample patterns
for many different phase differences, and concludes that a 180 degree phase
shift provides the cleanest high lobe.  Several vendors (including WX0B)
sell "BIP/BOP" switching arrangements, that allow "both in phase" or "both
out of phase" operation, as well as separate operation of either beam.  I've
used this arrangement on HF, and it really plays, both for short-haul and
long-haul stuff.  Mechanically it's not too hard at 50 MHz, and it also
provides resistance against rain static, as the lower beam in the stack is
at least 20 dB quieter than the top beam when it's raining.

Chris R. Burger
ZS6EZ

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