In reference to an auto transformer matching network for a half wave
vertical:
In John Devoldere's book, Low-Band DXing, he describes a balun intended
for this purpose. It was designed by W1FC, and it consisted of a two
stacks of 10 to 15 half-inch cores, Amidon T502-2. The cores are
arranged in two rows so they look like a two hole balun. Primary 2
turns, secondary 8 turns. He describes this as a high power balun
(doesn't say how high) for use to match 600 to 10,000 ohms.
For 200 watts, you will need good insulated wires. If the impedance is
4000 ohms with no reactive part, expect 894 volts. For 1500 watts that
jumps to 2450 volts. If it is truly a half wave antenna, the impedance
could be much higher, consequently much higher voltage.
I haven't used a balun for this purpose, but I did once build a 5/8
vertical for 30M and matched it with an LC network at the base. This
was a high impedance antenna. I found the matching to be very touchy,
and it changed every time it rained. Because of the high impedance,
stray capacitance is very significant. I never used this antenna at
high power (it was a 30M antenna).
John's book, Low Band DXing is a very good book (my favorite antenna
book) and I would highly recommend it. I don't have the latest edition,
but it is on order. Incidentally, he makes some interesting conclusions
about half wave vertical antennas, which are contrary to most people's
thinking. He concludes that the ground currents around the base of a
half wave vertical increase as you move away from the base, instead of
being at a maximum at the base in the case of a quarter wave antenna.
From this he concludes that you need a better ground radial system for
a half wave antenna than for a quarter wave antenna. He make the
statement that you need radials at least 2 wavelengths long before the
half wave antenna will show any gain over a quarter wave antenna. -
(Just an interesting tid-bit)
Jerry, K4SAV
Jim Smith wrote:
>I want to end-feed a half wave vertical so I can get a nice low angle of
>radiation. The base of the antenna would be about 0.05 to 0.10
>wavelengths above ground. It looks like the antenna impedance will be
>around 4,000 ohms. I would prefer not to use tuned circuits or stubs as
>I would like the device to work over a 2:1 frequency range. (No, I'm
>not expecting the antenna to be a half wave over a 2:1 frequency range!!
>I would have 2 different antennas to choose from.) Power level is 200W.
>
>I was thinking that an auto-transformer with a 9:1 turns ratio would do
>the job and provide a reasonably low SWR for 50 ohm coax. I figure on
>making the primary winding reactance about 500 Ohms at the lowest
>frequency of interest and adding 8 times as many turns for the
>secondary, presumably all on a suitable toroid. The wire insulation
>would have to be suitable to withstand several hundred volts.
>
>The auto-transformer would also nicely solve the static drain problem.
>
>Am I nuts or is this actually feasible?
>
>If it isn't, my next step would be to look into stubs.
>
>73 de Jim Smith VE7FO
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
>questions and ask for Sherman, W2FLA.
>
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>
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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