[BC] Long Wire

[Craig Healy]

Interesting thread on the skirt fed towers...

A few things I've run into on these.  The first one I was involved with was
the 1110 Providence station increasing power and going to directional
daytime.  The main tower is around 5/8 wave at their frequency and has a
diplexed FM on top which makes a bit of top loading.  It had to be skirt fed
for grounding purposes for the FM and other cables.  My goal at that time
(1981) was maximum bandwidth.  I disconnected the existing feed point and
set it to the top of the skirt which was maybe a quarter wave or slightly
less up the tower.  Then did a measurement of the resistance and reactance
across 30KHz each side of the main frequency.  Then moved the connection
point down ten feet and redid it.  I continued this until the feed point was
maybe fifty feet up.  In the end, the best bandwidth point was chosen which
happened to be somewhat under a quarter wave up.  After the project was
done, they brought in a consultant who remarked that the common point
bandwidth was about the best he'd seen.  Another thing was the resistance
when at the upper connection point was quite high.  The end result was just
under 200 ohms which is just within the 4:1 ratio I try to keep for the ATU
impedence transfer.  I wonder if the overall tower height affects the best
point for the feed?  A quarter wave up was not as good as the end result.

The second project is one that I only observed, rather than worked on it.
This is the 1170 station in Norfolk, MA.  Two tower daytime directional.
The owner decided to rent space to cell companies to make some money.  To do
that they had a consultant come in to built the skirt feed so the base could
be grounded.  First problem was he connected it to the top of the tower
which was a fair bit taller than quarter wave.  The resultant feed
resistance had come down to around ten ohms rather than the higher and
better number if connected lower.  Second problem is that he used 1/8"
stainless steel cable for the skirt.  I found a length of it on the ground.
 It measured about .7 ohms DC for the ten feet piece.  Given the resistance
and current required for the new feed point resistance, the tower radiation
efficiency went into the toilet.  They refused to fix it.  The coverage of
that station dropped considerably.  The phase was close to what it was, but
the ratio of the other tower dropped by more than half to get the monitor
points to comply.  I suppose the tower radiation efficiency was made to be
non-compliant.

Third thing is a question.  If the best feed point is a quarter way up, what
works best for a shorter tower?  Feed at the top?  I have another client
with sixty degree tower height.  At some point it may be worthwhile to
research a change to skirt feed for a few reasons.

Last question...  If the skirt feed is brought out from the base and then
back in, would it increase the bandwidth?  There is a military antenna which
has the feed wires sent out from the base at about 45 degrees, and then back
in to the top.  It makes a big fat radiator with a significant bandwidth.

Craig Healy
Providence, RI