[Elecraft] Using your tower as a vertical - 160 or 80

[Erik Basilier]

Years ago, after putting up a heavy-duty crank-up tower with several
antennas on a tall mast at the top, I was interested in loading up the whole
metal tree for 160 and/or 80. However, I didn't like the prospect of
transmitted power getting back into the shack via the existing feedlines,
causing all kinds of problems including losses. From a surplus vendor I
obtained four square slabs of ferrite (no spec's) and taped them together to
form a box-shaped common mode choke around the existing feedlines (and rotor
control cable etc). I didn't have radials at the tower base, but a couple of
long and wide copper strips buried and connected to ground rods to which the
tower was grounded. I opened up the tower ground connection and I had a feed
point. Finding resonance was not as easy as using my antenna analyzer. The
signal from the analyzer was overwhelmed by picked-up broadcast signals,
rendering the analyzer unuseable. I got by using a custom measurement setup.
It turned out that the entire metal tree resonated in the broadcast band and
was inductive at 160. I tuned it with a serial variable capacitor, and found
the antenna worked very well on 160. However, I didn't continue using it, as
I didn't feel safe not to have the tower grounded. I pondered schemes to add
some kind of gamma-like matching device, but never got around to it. Also I
never tried it on 80, but I suspect it would not been ideal for low angle
radiation. 

 

Later I added more antennas to the mast, and with the added cables, the
whole bundle would no longer fit in the makeshift ferrite choke. Out of
curiosity I once again tried ungrounding the tower to check on its
characteristics as a vertical antenna. I could no longer find the resonance
I had seen and used before. Apparently, the ferrite choke had been a crucial
part of the scheme.

 

At this point I still don't have an antenna for 160 or 80. (I did try an
inverted vee off the tower for 80, but it caused terrible de-tuning of the
40m part of my beam on the tower, so I gave up on that. Maybe I should try a
sloper.)  I am thinking of putting up a dedicated vertical, but on my small
lot it would couple to the tower. Perhaps it would be better to give the
tower another look as my low-band vertical? My source of ferrite slabs dried
up years ago. I wonder if anyone else on the list has used a similar
approach and found a good way to choke off RF on a bundle of feedlines?
Individual chokes don't seem very attractive to me as I have many cables,
but if one has to go that route then it would make sense to look very
carefully at the choice of chokes. I would also be interested in knowing
about others' experiences with feed systems that leave the tower grounded.

 

Thanks in advance for any useful ideas!

 

73,

Erik K7TV