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VK2TV > HELP 02.04.07 01:33l 78 Lines 3393 Bytes #999 (0) @ WW
BID : D10051VK2TV
Read: GUEST
Subj: Re: Info, 1/2 Wave End Fed
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<DK0WUE<I0TVL<F6CDD<F6BVP<VK2TV
Sent: 070401/2223Z @:VK2TV.#MNC.NSW.AUS.OC #:7149 [Kempsey, QF68JX] $:D10051VK2
From: VK2TV@VK2TV.#MNC.NSW.AUS.OC
To : HELP@WW
G4TZX wrote:-
> " Half Wave End Fed " antenna, In a copy of QST Feb 2007
> there is a reveiw of a 20 Mtr 1/2 wave End-Fed Dipole, (Par Electronics)
> searching through my many antenna books,
> I find Nothing on matching the High Z of a 1/2 wave to 50 Ohms!
> CB/10Mtrs have had end fed 1/2 wave Ant's for years, and
> I have seen 1/2 waves for 2Mtrs,
5/8 (especially) a 1/2 wave antennas have long been popular on 2m for
mobile use, and as base antennas for 27/28MHz, as indicated by Gordon.
In the case of the 5/8 wave antenna, the most common, and simplest, method
of matching is to have a series coil at the bottom, to make the antenna
"up to" 3/4 wavelength, electrically. This length having an impedance
close enough to 50 ohm to facilitate a good match to the coax. This method
is used commercially for VHF mobile antennas. A 5/8 (or 3/4) wavelength
antenna, being a low impedance feedpoint, requires some form of
groundplane such as a car body.
In the case of a 1/2 wavelength antenna I have seen a varirty of matching
methods:
1) The Ringo (TM)
The bottom of the antenna fits into an insulator that is mounted in the
metal mouting tube. This combination forms a coaxial capacitor. The "ring"
is a coil and, together with the capacitor, forms a parallel resonant
tuned circuit, one end of which is grounded. The other end, being Hi-Z
attaches to the Hi-Z end of the 1/2 wave dipole. Matching is achieved by
tapping the coax up the "ring" from the grounded end.
A discrete coil is used by some manufacturers in lieu of the ring.
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.
3) The antenna is fed with coaxial cable but a matching stub is attached a
particular distance away from the antenna feed point. This method has
appeared in some ARRL publications.
1/2 wavelength mobile antennas are common at VHF and UHF, and are sold as
GPI (Ground Plane Independant). They can be mounted on bullbars, mirror
mounts on trucks, etc, and return good performance.
FWIW, I once carried out an experiment, comparing 1/2 and 5/8 wavelength
antennas at 2m. Both antennas were mounted on the vehicle, the 5/8 being
attached to a base installed in the centre of the roof of the vehicle. The
1/2 wavelength was mounted at the end of a ladder rack. I used a coaxial
switch to perform A/B comparisons. The 1/2 wave GPI consistently returned
the best result. Not a very scientific method but a very practical one.
> The picture in QST just shows a Small moulded block with a SO239
> on the side and a terminal for the 33Ft wire,
To contain whatever matching network they have chosen to adopt.
> and it Does NOT need a network of ground radials.
That's because a 1/2 wavelength antenna is a complete antenna.
The reason for radials with a 1/4 wavelength antenna is that such an
antenna is only half an antenna. The groundplane, be it metal rod or wires
for an elevated antenna, or buried radials in the case of a ground-mounted
antenna, are there to supply the "missing half", or provide an image of
the vertical section. (Simplified answer).
The ARRL Antenna Handbook is a good starting place for more details.
> Gordon, G4TZX @ GB7COS
Cheers ... Ray
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