The hour is growing late and the urge to tell a story is upon me. Note,
that unlike others, I never imbibe alcoholic beverages. This is a true
story.

A large, expensive, rotatable LPDA was erected on a tall tower and
tested in an informal way with a receiver and short-wave broadcast
transmitters as signal sources. Nulls were where they were supposed to be.
F/B was roughly what was expected. Front lobe seemed about right.

A month went by and it was time to perform more careful measurements. A
180 swing while listening to a distant carrier suggested that there was more
gain off of the back than off of the front! Wise men pondered that this
must be a measurement error.

Impedances were very close to expected. The scheme that was used to
determine that the antenna needed to be brought back to earth were these:

(Keep in mind that this is a LPDA and not a Yagi.) A 6db pad was put in
the line to ensure that the nonexistent impedance changes with no change in
frequency would not affect things. An accurate, step attenuator was then
added in the line. (Should sound familiar.) Antenna was pointed towards a
known very high power broadcast station with a known location such that the
expected receiving take-off angle was about 6 degrees. Bandwidth was made
very narrow and centered on the carrier. Enough attenuation was added to
get the S-meter to average either side of a mark. The attenuation was
varied by one db up and down with several minutes at each level. Then the
antenna was rotated 180 degrees and the change in attenuation to return to
the same indication was noted. Ouch. Repeated several times it became
clear that the antenna had reversed direction and (are you listening?) a
number for the difference in gain (at the angle involved) and the
uncertainty in that number was crafted. Lord K. should have said - and may
have said - that all measurements comprise at least two numbers: an
estimate expressed as a number and an estimate of the uncertainty in the
first number expressed as a number.

The evidence was conclusive that something had happened. A search for
causality took place with the assistance of NEC. With a good deal of
speculation and noting how the largest elements were constructed, it was
speculated that one of the rearmost elements (or a part of one of the
rearmost elements) may have become detached electrically. An NEC simulation
was produced that fit the data. No, this is not proof, but it did furnish
additional support for the need to lower the antenna. More support came
from measurements at the high end of the frequency range where no anomalies
were noted and where the long elements would not be expected to play much of
a role.

At significant cost and effort, the antenna was lowered and it was found
that one of the mechanical connections of a sleeve with normal screw had not
been tightened in a rear element. The conclusive proof was that the screw
had not "cut." Apparently, a month of very sight corrosion had isolated
part of an element.

So, be my students and find the lessons in this tale. Make a list.

73 and I am off to bed, perchance to dream of antennas, Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
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