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VK2TV > HELP 05.04.07 10:56l 52 Lines 2020 Bytes #999 (0) @ WW
BID : 150061VK2TV
Read: GUEST
Subj: Re 1/2 wave End-Fed
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Sent: 070405/0839Z @:VK2TV.#MNC.NSW.AUS.OC #:7512 [Kempsey, QF68JX] $:150061VK2
From: VK2TV@VK2TV.#MNC.NSW.AUS.OC
To : HELP@WW
G4TZX wrote:-
> Hello All, Re My Bull, asking about 1/2 wave End fed aerials,
> ( PRACTICAL )
> It has been interesting to read all the replys that this raised, BUT!
> As far as I can read, they all quoted the theory of 1/2 wave E/F
> Not one has said, Yes I have tried them and this is
> How I matched it, A Web page gave links to various sites,
That's funny, I will swear I said this ....
2) A method I have used at 2m, and it's something I stole from a
dismantled commercial antenna, is to have a series coil to ground from the
bottom of the antenna. The coax then feeds via a suitable capacitor to a
tap on the coil.
Which;
a) says I have tried it and,
b) it describes how I matched it.
No, it doesn't give values since these would be useless given that my
antenna was at 2m and you're looking at 20m. The principal remains the
same. From memory, the coil was about 10mm diameter and about 6 turns of
approximately 18AWG wire spread over about 1.5 inches. The capacitor was a
4.7pf 500v disc ceramic.
Whilst we haven't seen inside the "matching unit" of the antenna you
described, given its small physical size, and applying an educated guess,
I'd say it's nothing more than a series coil, possibly wound on some form
of ferrite or iron powder core to reduce the number of turns required, and
some form of tapping of the coil for the coax feed point. Either that or
there is a second winding associated with the coil to provide an isolated
low-Z connection.
I mentioned a simple series coil being used with a 5/8 antenna to make it
up to 3/4 wavelength, to provide a low-Z feed. I can think of no reason
for this not also being applicable for a 1/2 wave antenna.
The series coil actually performs two functions:
a) impedance matching and,
b) being coiled up, its radiation is minimised.
The latter point is important because the current in that section is out
of phase with the current in the halfwave (radiating) section of the
antenna.
Cheers ... Ray
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