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G8MNY > TECH 31.07.14 23:36l 88 Lines 4163 Bytes #999 (0) @ WW
BID : 6620_GB7CIP
Read: GUEST
Subj: Simple 1/4 Wave 2m Mobile Aerial
Path: IW8PGT<IZ3LSV<IV3SCP<SR1BSZ<GB7CIP
Sent: 140731/2224Z @:GB7CIP.#32.GBR.EURO #:6620 [Caterham Surrey GBR] $:6620_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Update Oct 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This is the simplest aerial always to have in the car as a spare. It works on
both 2m & 70cms. Although it is a 3/4 wave length on 70cms, 2 of those 1/4
waves are in opposite phase, so giving only the gain of a 1/4 wave. As it is
just a coat hanger it can be bent out of the way & straightened as needed.
CONSTRUCTION
It is made from, "Coat Hanger Wire" or similar fencing wire. soldered into a
standard PL259 plug. You will need a piece or wire at least 23" (58cm) long to
allow for trimming & the plug. (Tuned approx 20" (51cm) free from the plug)
Stop water ingress
& corrosion
with Grease
\ ÄÄÄÄ¿/
ÃÁÂÄÄÄÄ¿ Paint the wire if you want no rusting!
.----Á-|_____\______________________________________________________.-.
Solder/'----Â-|~~~~~/~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~'-'
Pin ÃÂÁÄÄÄÄÙ \ Fill whole plug After tuning
ÄÄÄÄÙ top with heat glue up, add heat
PL259 & cone top off. glue ball tip.
To solder to galvanised steel is straight forward with some solder fluxes, if
not with yours you will have file the wire to provide keys for the solder to
grip the wire. If using stainless steel wire rod, heavy cross keying with a
hacksaw will be needed to achieve any physical grip with normal solder.
MOUNTING
Before you attempt tuning up the mounting & location is often critical for a
good SWR.
239 Magmount 239
__ÚÁÁ¿__ Gutter Mount 239 Socket
239 _..--~~~~ Apex ~~~~--.._ÚÁÁ¿ drilled in roof! 239
window / of roof \Ù_³ --ÁÁ------...__ Tail gate
ÁÁij \/ÁÁ Mount
³ \
So choose the best option for you. Remember there are low garages/car parks to
enter, but also you gain about 6dB more Tx & Rx signal, for each doubling in
height!
Note that ideally the mount should make a good electrical connection to the car
body, but often the close fittings give high enough capacitance to work OK,
like in the magmount example. If a non metal body then use a 1/4 wave ground
plane in wire, or stick on conductive foil such as alarm tape.
Magmounts have thin rubber undersides to grip the body & they DO NOT SCRATCH
PAINTWORK, but dirt on the paint does, so make sure the rubber & bodywork are
clean first. Adding a plastic layer under the magmount often reduces the
grounding capacitance & permits mounting sliding!
TUNING UP
Start at the lowest frequency & measure the SWR on 2m, then trim a piece off
the aerial (say 1/2") remeasure. As log as the SWR is improving reduce the
length, & use smaller lengths to cut off. When SWR is OK then test on 70cms it
will be far more critical on that band. Test on the centre of 2m band & if SWR
is higher trim some more retest 70cms as well.
When done remove aerial & make a bobble of heat glue on the end & rotate aerial
to make into a ball over a heat source. Cool off in water. Paint up aerial.
BROADCAST BANDS
The same design works fine for Band 2 FM & Band 3 DAB. Except the length needs
to be...
BAND 2 BAND 3
Freq MHz 87.5 98 108 217.5 224 230
1/4 cm 86 76.5 69.5 34.5 33.5 32.5
The huge difference in frequency that the aerial has to work over especially on
Band 2, & the resultant poorly tuned aerial. This would never do for Tx, but on
Rx the slightly lower gain of an off tune aerial also attenuates the band noise
just as much as the signal, so there is often little difference in practice if
the Rx system is sensitive enough to be limited by the band noise. N.B. Rx are
75ê!
A home cut aerial may not look anything like as good as a shiny commercial
chrome one, but ususally works much better!
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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