G8MNY TECHNI 30.08.19 06:15l 97 Lines 4491 Bytes #180 (0) @ WW
BID : 19458_GB7CIP
Subj: Simple 1/4 Wave 2m Mobile Aerial
Path: OK0NBR OK2PEN N3HYM N2NOV GB7CIP
Sent: 190830/0904Z @:GB7CIP.#32.GBR.EURO #:19458 [Caterham Surrey GBR]

From g8mny%gb7cip.#32.gbr.euro@n2nov.ampr.org Fri Aug 30 05:05:35 2019 Received: from n2nov.ampr.org by n2nov.ampr.org ([JNOS]) with SMTP
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Message-Id: 19458_GB7CIP@GB7CIP.bbs
From: g8mny@gb7cip.#32.gbr.euro
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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
\ ────┐/
├┴┬────┐ Paint the wire if you want no rusting!
.----┴-|_____\______________________________________________________.-. Solder/'----┬-|~~~~~/~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~'-'
Pin ├┬┴────┘ \ Fill whole plug After tuning
────┘ 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
__┌┴┴┐__ Gutter Mount 239 Socket
239 _..--~~~~ Apex ~~~~--.._┌┴┴┐ drilled in roof! 239
window / of roof \┘_│ --┴┴------...__ Tail gate
┴┴─│ \/┴┴ Mount
│ \

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 75Ω!

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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