This is related to the "Slinky Beverage" thread currently being discussed
on the list. My experience(s) may help others
considering a beverage of less-than-optimum length. And, maybe someone can
give me a hint or two to help me improve my
setup.
I have three beverage antennas at my QTH. One is 870 feet long, terminated
and aimed at about 40 degrees. The second is
540 feet long, reversible, constructed of 300 ohm twinlead, firing due E/W.
The third one had to fit into a less than optimum
space in order to cover JA/Asia in one direction and S. America in the other.
The shortened beverage could only be 300 feet long. Clearly this is not
enough room for a proper, conventional
1-wavelength beverage. When I was planning this one, I first looked at the
slinky option, but seeing as how slinkies are
made of steel (poor conductor of RF) I didn't like that idea. My second
idea was to wind a continuous length of #14 solid
-copper, insulated house wire into a copper slinky. I had everything set. I
had rigged a winding device using a 10-foot length
of 3-inch PVC pipe, and had elicited a promise of help from a friend to
turn the crank while I carefully formed the winding.
Then, pipe and all was to be slipped over the support rope ( loose at one
end) and the pipe extracted, leaving the coiled
wire close-wound on the rope. The free end of the rope would then be
re-attached to its support and the turns of the coil
spread out and fixed in place every ten or so turns with small black cable
ties. I had tested this construction method with
short runs, and it worked well. All of this went according to plan until I
determined the proper pitch of the coil (from
ON4UN's book) and calculated the amount of wire needed. Turns out I would
need just shy of 1400 feet of wire! That
would weigh about 40 pounds! I judged the whole affair would be too
expensive (given the extra wire and high cost of good
support rope) and too difficult to keep aloft due to the extra weight. I
would be fighting a never-ending battle to keep the
support rope taught.
The third option was to load the short beverage to make it electrically
1-wavelength long. Since I wanted to verify the
proper loading coil values and because the lowest frequency that my Autek
RF-1 will tune to is about 1130khz, I initially
constructed the short beverage as two shortened, loaded dipoles, each
having two loading coils (each about 71 microH)
placed in the center of each leg. (Four coils total) I inserted strain
insulators at each dipole's temporary feedpoint, another to
separate the two dipoles, and insulators at each end. Resonance checked out
fine without the need for any adjustment, so I
then jumpered all the insulators and attached short "pigtails" for feeding
either end. I use relays to allow either end to be
attached to either a matching transformer and feedline or a terminating
resistor. This gives me coverage in both directions.
The system works well and just as expected on 160. I worked CX5BW the first
night I had it up, and Pedro's signal had the
best S/N ratio on this antenna at all times, though it does have a lower
overall output than the longer beverages.
However, this antenna does NOT work well on 80 or 40. It appears that the
loading coils act less like loading coils and
more like RF chokes at these higher frequencies. When I have time, I plan
to replace this antenna with another shortened
beverage using 8 smaller coils instead of 4 to see if the distributed
inductance (reactance) will extend its useful upper
frequency range.
BTW, I temporarily jumpered all four of the loading coils to see how a
plain, terminated 300-foot wire would perform. The
short answer is NOT GOOD. It became almost omni-directional with very low
F/B ratio.
Further thoughts, anyone?
73
Charles - K5ZK
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