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Ian Jackson[_2_] · August 8th 10, 09:17 AM posted to rec.radio.amateur.antenna

In message
,
writes
On Aug 7, 12:40*pm, Robert Keeler Robert.Keeler.
wrote:
I want to use an NMO whip for 460 UHF band. I know a 1/4 wave would be
about 5.8 inches. can I cut it to be a 1/2 wave or 5/8s wave? if so what
would the correct length be? this is a straight whip, no coil or spring.
Thanks!

--
Robert Keeler

Neither will work properly without matching. Without using a matching
device, you are limited to 1/4 wave, or 3/4 wave. And 3/4 wave is not
desirable as you start seeing higher angle lobes past .64 wave in
length. "5/8 wave"
If you have an NMO mount, and want to use a 5/8 wave, you need
one of the NMO mount 5/8 whips with the loading coil at the base.
A 1/2 wave will be high z, and require a matching device.
Without the proper matching, you are best off running the 1/4
wave whip.

A 5/8 wave gives you the highest gain at low angles (which is what you
want). The length of the antenna is a '1/2 wave and a bit more'. The
loading coil at the base is essentially the rest of the length required
to make it electrically look like a 3/4 wave, so that it will have a low
input impedance, and be a good match.

Over a perfect ground plane, the 1/4 wave will have an impedance of
around 37.5 ohms, and the 5/8 wave around 50 ohms, so the 5/8 is
potentially a better match for 50 ohm coax. Also, the part of the
antenna which does most of the radiating (1/4 wave from the tip) will be
some 6" higher in the sky and, at 460MHz, if the antenna is mounted low
down, every inch of height above the ground counts! Despite this, in
many situations, you may not find much practical difference between a
1/4 and 5/8 wave.
--
Ian

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