[TowerTalk] Taming an 80 m beam

[Chris Burger]

When I started contesting in the early Eighties, even 40 m beams were not
that commonplace.  The few big guns who used them were LOUD--PY5EG, I2VRN
and a few others.

As they proliferated, my awe-struck teenage eyes shifted to 80 m beams.
There were but a handful again--I5NPH, OH1RY and W2HCW spring to mind.  I
promised myself at age 20 that I would have an 80 m beam before my 40th
birthday.

I'm now 36.  Fortunately for me, N6BT and others came up with some design
philosophies that helped to bring it within reach.  Thinner diameter
elements, using thick walls and high-grade materials, have reduced the wind
load of practical 80 m elements to the point that a much flimsier tower will
hold one aloft.

I remember the legend that an 80 m beam is a life-long maintenance
commitment.  However, I'm a firm believer that if things are done right,
they will survive.  Doing it within an amateur budget is another matter,
though.

I finally got my 80 m beam flying around my 34th birthday.  It's a Force 12
Magnum 2/3, with 2 el on 80 and 3 el on 40 collocated on the same boom.
Although it's loaded on both bands, it's still a monster by most standards.
The boom is something like 15 m long; boom diameter is something over 150
mm, and the whole thing weighs over 100 kg (50', 6" and 220 lbs if you
insist).  However, because of its loaded elements and relatively light
weight, it doesn't demand a megabuck tower.  In my case, my tower is 42 m
(138') tall, with this monster at 36 m (118').  The tower has a face width
of over 1 m (3'), and weighs in at over two tonnes.  It is guyed at three
levels and in three directions.  It is not a toy by any standards, and
really taxed my engineering design, construction and erection skills to
their limits.  

Tuning this beast was an overwhelming undertaking.  Each element went up and
down over a dozen times, to get the four band segments tuned exactly right.
The entire beam went up and down three times before the feedpoint was
properly matched.  The whole process took two guys (me and ZS6AWK) around 20
hours.  This doesn't sound like much, unless you try and squeeze it into
your work schedule!  I couldn't get the 40 m portion working at all;
resonance was easy to achieve, but at high SWR.  Low SWR was easy to
achieve, but not in the band.  It's still inoperative to this day, although
it has not been a high priority because of lots of other remaining work and
the availability of another decent 40 m beam.

Using the thing was awesome.  Signals that could not even be imagined on the
wire antennas and the loaded tower were audible.  Given our high QRN level,
anything that was audible was also workable.  My biggest surprise came when
I was calling several audible mults after sunrise in CQWW CW.  The beam
could still hear signals at least half an hour after the other antennas had
lost the last ones.

The first problem came with the rotator.  The OR2800P simply broke under the
sideways load imposed by the cable drive.  The cable drive was based on a
W3LPL idea.  The sideways load was less than that imposed by a load as
specified, mounted exactly on the mast clamp (i.e. best case).  The brittle
fracture appears to be due to a casting fault.  Modifying the tower for a
chain drive took considerable creative engineering, as a split sprocket had
to be machined to fit around the mast without removing it.

After fixing the rotator, I had more than two years of service from the
beam.  It's a wonderful performer, both on Phone and CW.  I expect I had
less than 50 hours of use all told, as that is simply how little operating I
get done.

The beam suddenly died one day, and then resurrected itself intermittently
for some time.  Fixing the problem involved taking the beam down and
replacing the balun.  The original Bencher BY1 had unsoldered itself.  I've
learned a lesson here:  Don't try and save on baluns.  Having to haul an 80
m beam up and down is worth quite a few dollars.  After fixing the balun and
hauling the antenna back up, the resonant point was about 10 kHz low.
Clearly, more work will be required.  I'm hoping to find the time before
CQWW this year.

Was it worth it?  Yes and no.

On the one hand, I just had to try it, and there isn't another one around to
try.  I also had to keep my promise to myself.  It really is an awesome
performer.

On the other hand, given the magnitude of the project, it's hardly a sane
choice.  Paying that price for something like on S unit over a vertical is
not a reasonable decision.  If you really need the gain, a vertical array
will give you the same thing with instant direction switching, and far less
cost.  The vertical array's only disadvantage is the space required, but
even that disadvantage can be ameliorated to some extent by using
close-spaced triangular arrays and creative radial arrangements.

We all know that these decisions are not taken fully on reason, though.  You
can't knock an 80 m beam unless you've tried it!

To summarise:  I've had two failures in about three years.  The rotator
failure was probably due to a faulty shaft, but could have been avoided by
using a three-bearing arrangement such as is in place at the moment.  The
other failure was due to an unreliable balun, something that could have been
avoided if I'd had the luxury of walking into a dealer's premises and
touchy-feeling a few different types.  I had to order blindly from paper.  I
guess if I'd had the benefit of experience with either of those subjects
before, the beam might well still be operating.  Whether this kind of
reliability could be achieved in less benign climates, though, is anyone's
guess.


Chris R. Burger
ZS6EZ

PS:  You can see some pictures on http://zs6ez.za.org.


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