...net net....if you want to lower your primary radiation angle going to
1/2 wave vertical isn't going to get you there. The work effort has to
be spent on the radial system but in reality if you aren't on a salt
marsh or ocean front, etc. there isn't much you can do to get a
significant lowering of the radiation angle. John Devoldere's book goes
into this seldom heard wisdom in great detail.
Good Luck,
dave
wa3gin
K4SAV wrote:
>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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>>TowerTalk@contesting.com
>>http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>
>>
>>
>>
>
>_______________________________________________
>
>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.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
>
_______________________________________________
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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