[OZAPRS] 2M 1/4 Wave antenna

Tony Hunt wavetel at internode.on.net
Mon Jan 15 18:51:17 EST 2007


Andrew.
Your actually better off with a 3/4 wave .. Try 145.5 cm above a ground
plane. Either flat or 45 degree downtilt might be worth a bit of
experimentation. Ive generally used a flat ground plane by using a mag
base
on the car roof but I do have a 3/4 wave with lower angle ground plane and
that seems to work too.. Not sure I would pannic about trying to match the
feed impeadance too much either.. Maybee setup a mag base or something and
try some comparisons..

The 3/4 wave has the uptilt and some gain and Ive found its pretty good as
a
145.825 downlink and Ive used a similar setup for UO-14  AO-51 UHF uplinks

Have a look at the bottom of this page of Bobs.
http://www.ew.usna.edu/~bruninga/astars.html


Tony Hunt VK5AH

> > ----- Original Message -----
> > From: "Andrew Rich" <vk4tec at tech-software.net>
> > > Searched for ages on google, looking for a 1/4 wave vhf calculation
for
> > > 145.825 MHz
> > > I remember that the 45 degree radials had to be a ratio longer or
shorter
> > > than the vertical element.
> > > 1/4 Wave antenna = 51.43 cm
> >
> > It's been years since I built a 1/4 wave antenna, and a mate has my
RSGB
> > antenna book at the moment, but I always used to take 5% off the
calculated
> > length of the antenna.. I think for a bit of impedance matching,
though
it's
> > been WAY too long to say for sure :) I do remember that the radials
were
> > exactly 1/4 wave, and by adjusting the down-angle on them, you could
tune
> > the 1/4 wave to more closely match your 50 ohm coax..? (nominal on a
1/4
> > antenna is 37 ohm or something iirc?)
> >
> > Google seems to actually be suprisingly thin on 1/4 wave antenna
info..
> > Really gotta get my RSGB back :)
>
> Yeah, I should look that up while I've got your RSGB book here
>
> An exact 1/4 wave matches 36 Ohms, exactly half a dipole, and if I
> remember correctly (it isn't in the RSGB handbook), freq / 72 (roughly
> 95% of a quaterwave) can be used without a large elaborate
> groundplane.
>
> But back to the handbook, it says to make a 50 Ohm match, to make the
> antenna length longer than usual, then get out the SWR meter, not very
> scientific for the RSGB handbook, but it does make sense in a way.
>
> > Cheers,
> >
> > Damien
> > --
> > Damien Gardner Jnr
> > VK2TDG/VK2DGJ. Dip EE. StudIEAust
> > Home: rendrag at rendrag.net -  http://www.rendrag.net/
> > Play: vk2tdg at pinegap.net  -  http://pinegap.net/
> > --
> > We rode on the winds of the rising storm,
> >   We ran to the sounds of thunder.
> > We danced among the lightning bolts,
> >   and tore the world asunder
> >
> >
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>
> -- 
> VK3TST
> -- "People aren't stupid. Usually." -- jd
>
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