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

[Rich - K1HTV]

Erik,
I've been feeding K3 RF to my house bracketed, 75 ft Rohn 25 tower for years with great success on 160 Meters. The tower is topped with HF and VHF yagis which act as a capacitive top hat. The 50 Ohm coax feedline is connected to a tap on the coil of a parallel tuned circuit. The top of the LC circuit is connected to a #16 THHN wire which runs parallel to the tower, about 18" outside one of the legs and connects to a bolt around 10 ft down from the top of the tower. At the base of the tower I simply tune the capacitor and find the best spot on the coil to tap to obtain the lowest SWR. There are 16 quarter wave radials around the tower, most of them on the tower side of the house from south to north and a few wrapping around and under the back deck toward the northeast. There are around a dozen 8 ft ground rods connected to the tower with #6 wire and the tower also is connected to a perimeter ground system surrounding the house.

The shields of most, but not all, of the coax cables leaving the tower are grounded at the tower base. Before all coax and control cables enter the shack they all are grounded at the steel entry box around 30 feet from the tower base.


So how does a shunt fed 75 ft tower work with a barefoot K3 feeding it? With 100 Watts I've confirmed 229 countries on the Topband and have worked 89 countries on all continents with QRP 5 Watts. You can check out the 160M QRP WAC cards at:
http://www.qrz.com/db/K1HTV


If you haven't tried shunt feeding your grounded tower, give it a try. You may be surprised at how well it can work on 160 Meters.


73,
Rich - K1HTV


= = =


Erik, K7TV wrote:

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