QRM
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7, Ry
Q
l”‘ -'l
BSulletin issued by the Northern PO Box 1010
Branches of the Wireless lastitute Launceston
of Australia, Tas. Division. Tas. 7250
July 1974
Vol.ly No L
Registered at GPO Hobart
for transmission by post Price 20¢
as a periodicsal. Catsgory B.
This month I am not going to have a say, other than to ask you
to thinlk about our hobby and the W.I.A. for a moment.
Is there anything you can contribute other than holding a
callsign and operating on FM from the car.
There are many jobs in the branches that need someone to take
an interest in them, for instance, raising money for the
clubroom accounts, broadcasts, social functions, articles for
QRM (HI), YRCS can always use more maii power, and last but not
least Intruder Watch, its no use complaining about interflerence
from non-amateur type signals, do something about it.
Log thelr frequency, date , time, type of signal, type of
messages and if possible record them, then report them to
Intruder watch (71i2).
The South and North West will have their annual general branch
meeting in Augusi, attend this time and be prepared to accept
rather than decline a nomination.
7% John VK7
MESTINGS
July 2nd North Jest Branch, Lakins Hall, Ulverstons.
July 12th Northern Branch, AEB rooms, High st.
Lecture by Joe VK72GJ, FM azlignment.
July 13th Council meeting at the QTH of VKYZIRY,
120 po.me.
YRCS
July 13 and 27th room 38, Kings Mcadows High School.
QeRelMe July 1974, Page 2
Northern notes by Reg VK7KK.
While winter brings a slow down in many activities there
still seems to be plenty of activity in the branch.
On the financiel front it's been agreed to bulld up the
interest of the clubhouse funds by placing $1200 in an
investment account. )
Members are still buying plenty, treasurer Kevin had 250
worth of accounts to present for payment.
Gordon 7GW got something for his investment when he won a
cheese in the raffle at the meeting, (the only brand MADE
backwards), be prepared to be "fleeced" in similar ways at
future meetings.
"Talks man" Wally 72WX has & good line up for meetings from
here to January.
July - Qur man in AWA, Joe 74GJd will talk on the correct
alignment procedure of FM equipment.
August - To be announced
September- TV translator networks, by R.Greenwood 70V (PMG)
October - EBquipment display
November
- Diamond tool life, by R.Kerrison (Repco)
December - Barbeque
All it needs now is your attendance. Judging by the interest
shown it is likely that a party to the hotel Tasmania cabaret
will be arranged for the near future, stay tuned.
Robin 7RH advises that he is keen to help C.W. aspirants, if
you are seriously interested in learning morse give Robin
8 ring on LL4232l.
It's good to see volunteers coming forward to assist with
broadcasts. Different voices, noise and hum types is a change,
and creates a talking point.
Harvey 24YZ is now a Launcestonian fleeing from warmer lands,
he operates on che. 48 and 4O but hasn't a VK7 call yet.
There is no truth in the rumour that Steve 70A has taken
shares in Ansett to get some of his own back.
It would seem to be connected somehow with the frequent trips
to VK3 and back of him and Norma VK3AYL.
Norm 7NR has been promoted to the rank of "Captain" since the
purchase of a boat. If you're looking for a name Norm how
about "IHIF' (I hope it floats). We hope to work Captain
Richardson maritime mobile soon. How about the crew Norm ?
all females ?
00000000000000000000000
VHF notes by Joe VK7ZGJ.
As I mentioned last month, things happen on 6 metres even in
winter.
On the 2nd of July the band opened around 1250 local time to
VK2 and later to VK5, in all 14 stations were worked, over a
period of 3 hours. lost signals were around 33 with little
QSB. The following week provided no activity, the only signals
heard were the VK2 and VK- beacons on meteor scatter.
It is rather dissapointing that only the regular operators, 7JdV,
7ZAH and myself are there to take advantage of the act1v1ty.
Vlth the MUF above 100Mhz on July 14, Northern VK 5 wera
working into VK2,3,5 and 7, I managed to work 4-VK4's zaround
tnv Rockhampton area. Cont. Page 5
GeR.M. July 1374, Page 3
North West notes by Max VK7MX
At times I have written some controversial stuff and look out
I am at it agdln.
This time it is the Sunday morning broadcast callback. From
time to time we hear some long drawn out replies which coupled
with a big callback becomes someshat boring and results in
some not waiting to report in.
A 30 minute broadcast plus a 50 minute callback is eqguivalent
to a one hour ¢u0 and this is rarely done.
Personally I prefer a short concise broadcast followed by a
brief callback and afterwards the band is wide open for rag
chews etce To sum up, we want to hear the news, the broadcast
officer wants to know if he has been copied, and this is the
object of the exercise.
The mob generally are not interested in the fact that my baby
has cut a couple of teeth (heaven forbid) or that my SwR is
10 to 1. If sSandy wants to know I can tell him later.
Maybe I am getting old and crochity which is possible, as I
passed retiring age during the month. But I can't do much
about that either.
Now to the news. Our meeting was held on 4th June at the
usual place, right at the finish of a thunderstorm.-
Maybe this kept a few away as the attendance was only fair.
Routine business was completed in good time with nothing very
important coming up. At the conclusion of the meeting we were
treated to a most interesting talk by Les 7LH, on the worxkings
of diesel-electric locomotives.
We were taken on an imaginary journey which at times was so
realistic that we could hear the wheels clattering over the
joints. It was surprising to find how little we know about
what goes on around us everyday.
Thanks Les, a fine eflffort.
e were pleased to see Terry 7IT and Ken 7AI in our midst
again. Apparently sen is stili heavely involved in 412 o'clocek
high activities but we still have to work him aero mobile.
Terry displayed a very neatly constructed vick smith 30 watt
s50l1d state 2 metre final, 1t certaiuly looked the part, hope
it works.
1t is mooted that our branch may hold a litltle personal dinner
at annual meeting tilme, more Later.
En route to the meetiiyg we worked Charlie 7Cr on 2 metres.
Good show and we hope t¢ heur more of you old timer.
£hil 3APO has again left the Bass Trader for some leave and
his next assignment is unknown at this stage.
Harvey 24YZ passed through our area recently complete with
Siamese cat which modulated well from the car. Bad luck he
arrived at the end of the trout season.
Grahem 74A¢ has changed his place of work and has now joined
the ranks of Norm and Joe. Very interesting he reports and we
are hopefull he will stir up some zBranch interest in 7AR
On 9th Jdune Tony 7AX did the Divisional Broadcast from his
QTH at Ulverstone. Secms that it was a pretty good effort.
He certainly set a high standard for those who have to follow
on. 1t is many years since the broadcast came from the horth
vest. 01d timers may remember that it was done once following
a dinner from my QTH, but that is historye.
#hat a lot of junk I hear some say. well in that case get the
pen out and make a journalistic offering. Show how it should
be done. I am sure the ~ditor will be happy to include it
ln QRM- C-U L. 7(\
‘;{_-RoM. July 19?“-’ Bage L‘-
_ADIES PAGE
Lucy's (XYL 7kA) article in the May issue has prompted me to
pluck up some coursge and relate to you all the intercom setup
at our place. One of its greatest adavantages is 1its convenience.
The intercom 1is mounted on a wail in the diningroosm.
I just push a button, say my piece, let it go, and the Ou (in
the garage) replies to my plea of 'come and get dinner'" or
"telephone for you'" or '"someone at the door for you" (his
popularity rating has by now, inflated his ego to its outer
limits!). The intercom proves very handy indeed in rainy
weather (all is vanity - it prevents me from getting wet
running from the back door to the garage,) but mainly it saves
time.
Another little innovation has been the installation of a
microphone.in the hallway connected in the garage, to the
above mentioned intercom system. The OM selects either the
microphone in the hallway or the intercom in the dining room,
as the spirit moves him. The reason for the hallway microphone
is so that, if I happen to be out at night, the OM becomes a
sort of "invisible babysitter", and is abie to hear the children
should they decide they are not so sleepy or that all is not well.
However, the microphone is so sensitive that it does not permit
one to emit certain obscenities which before would have been
entirely in private. .
It needs to be remembered now that Big Brother is listening
(perhaps). Now everything I say about the OM at night, in the
Wprivacy" of home, is complementary.
Vote 1 for free speech!
73s, Diane XYL 74BY
0000000000000000000
wWwhinge to the kditor.
Dear 3ir
The new Ladies page in your magazine is really a great
idea, but the name of the contributors just has to change.
Take June for example, "VK7ZAH's XYL Judith" Now really.
Makes it sound like HIS pet dog or something, and why X ?
YF is much nicer. sShades of male chauvenism! This sort of
thing is just not on for today's liberated and free woman.
After all, the only reason I read your magazine is because
it's Q.R.'"Mary"!
Lots of love Ms.Greer
As the writer used a pseudonym, I imagine she had a fair idea
what kind of reception 1t was going to get. Well you got it,
but I can't print it. For the benefit of those who may agree,
all articles are signed as received by the Editor.
As Tar as your example goes, no doubt you will abhor at the
thought that some Ladies (not liberated women) are proud to
be regarded as their husbands PET.
As for liberated women, most of them look better with their
bra on.
The Editor
QeRoNs July 1974, Page 5
DX notes by Den VA/LK.
This last month has produced results between really good to
very poor. June 1st and <Znd was a little unusual as there was
a short path opening to Vgy area at U4uU4, and the same days,
a very good opening to purope a little later on.
FR7Z4U is on Juan de Nova for 3 weeks as from June 10, not yet
heard. FR74Q on suropa (ccunts the same) as trom June 20th,
length ol stay not known.
Kfo is sayd to be activated by the n.California DX club, and
is expected to be 1in operation as from approximately June 25th
depending on weather, and should be tiicre for 5 dayse.
Now for a round up of DA heard.
80 metres, w's, AV54C, LU, VR4BS.
4O metres, 17504, W's, U7004, v's,AE1J, VR3aG, JA's, 12002
Ua and JA.
20 metres, 01/02004, -M2Dg, YV4iOH, YBY, 9Y4, VR1, TIZ, W's
02/0300Z4, VP2DH, VP2HHK (gsl via W@MHK)
04/0000%, FR?aU M1D, G3BVK/mm (near V@33R, short path)
UO/O?OOL, N Sy Vb S fiu’l, G‘L‘, bfid 5U7’ &DB,
HH: (gsl via wA<dDT).
U7/06004, JD1, Ok3, KV5aB, M2, JA.
12002 Wy G4, Jny, UKo, and some nights....NOTHIKG.
15004 G35, 04, YO, UK2, GI3, I3, OHZ, DMZ, OK1.
18002 Sk, URZ, UA3, Gj, Ok, Dn, OH, UKY9, mll.
21/24004, 9Y¥L4's, BR1, 6Y5, 9k2,YV1, G6, KG6, Jh,
UAas, Dd, LU3, and KCOJC who is none other
than VK7CL kerve, who wishes to be
remembered to ail his rfriends in VK7.
15 metres, KHO, Ja, JID1 Marcus Isl.
conditions have been very patchy, one day
good DX zand tne next day ut the sans time completely unproductive
Thats the way the band is.
The A4ll Asian Contest sas held on the 15/16th of June and the
band conditions ware not pood. But it was surprisisg the number
of JA's that were active shere as normally we hear only a Lew.
There must be somethi.ig in thsse coutzsts, ~part from da's the
only other asian stations heard were, UL7 (o) Ua%, Hl, 9V1
and Uade.
75 Den VK7DK
0000
VOVOOCUUVO0CC
VHY notes _continusd from fuge
Next day whilz talking to 7.4.H, Lindsey L4IN troke in and 5/5
slignel repoeris were exchangea., Vil ras also worked about
an hour lester.
Monday 16th was most fruitfull, the first contsct was at 1439
43tk LAHT at Gympiz, 2 total of 5-Vil's were woerked as viell
38 Barry 24AY who was there unitill 10650.
At 1715 the VAE beacon was 2 5 points avove the noilse.
Little has happened since then. rsevin hias been hesrinz the
VK6 beacon on weteor scatber over the last secka
Tunable 2 metre activity is almost non-cxistent, Graemme 7iGG
has been trying Lo get niS LoB operaling bat so far no success.
Other stations worgaed oan 6 metres this month include.
Grasme 746G, dalcolan ?ug and Jully 7o do.wn ths bottom end,
and Allen 70H, Brilan 7RI and lan 72148 on 52.525.
g
7;; Joe VlxM{,LGej
GeR.M. July 197.4,Page 6
THI: VIDICON TUBE.
Adapted from R.C.A. application notes on 7038 Vidicon
by rrank Richards VK7iFR.
GRIDN®
4 FOCUSING COiL
HORIZONTAL AND
VERTICAL DEFLECTING
CoiLs
GLASS GRID N°2
FACEPLATE
GRID Ne I
CATHODE
GRID N°3
TARGET CONMNECTION
TARGET.
Fla.1. TyeicAL VibDiConN.
CRIOS 3r4
=inov BLANKING
e R M. July 1974, Page 7
THE VIDICON TUEBE.
Unlike normal tubes the vidicon is a sensitive device that
rezcts to electrons and light like nothing else in the
amateur arena.
The target arez consists of a photo conductive layer which
by its name conducts when light illuminates the face of the
tube. This layer is not as a photo diode as & whole but as
if the area was a mass of individual photc diodes each
insulated from one another,
The electron beam scans these diodes and the light level
allows different values of current to flow causing voltage
changes at the target terminal, these voltsges are amplified
and become the non composite video signal.
There are different types of vidicons, separate mesh,
integral mesh and the later silicon type .which is sensitive
to very low light levels.
Connection to the target is best made by a bronze sprlng
bearing on the target ring at the tube face (usually
integral with yoke).
Because of the inherent high impedance of the device
capacity is important and the smaller and shorter this
connection the better.
The focus coil is energised so that north pole is at the
tube face.
DANGHER
Underscanning will cause permanent damage to tube.
Scan failure will cause permanent damage to tube.
Strong light (sunlight, flash etc. through the lens will
also cause permanent damage to the tube.
These are called burn marks and are a permanent reminder
of the fatal mistakes one makes.,
Sequence of adjustment of the camera, assuming that all
systems are functioning normelly, see Fig. 2
1) Adjust for maximum horizontsl and vertical scan.
2) Adjust grid 1 for maximum negative (minimum beam current)
3) Adjust target for minimum voltage.
L) Place mask 3" X 3" centred on tube face with lens set
on highest F stop ie. minimum light and switch on.
5) Decrease grid 1 -ve till highlights Jjust show.
6) Adjust grid 3&4 in conjunction with lens stop and
optically focus for best picture.
7) Adjust scan to mask area or slightly larger.
8) Adjust alignment so that picture centre does not vary
as the voltage on grid 3&4 is altered.
9) Open lens and adjust aparture for good white level of
~the scene to be shot and adjust target volts for the
type of use.
For, live pickup, low illumination 60 = 100 volts
live pickup, good illumination 30 - 50 volts
back light, slides etc. with high illumination
levels will need lower voltages still.
Target current is usually measured but as it is about
»004 to .2 micro amps a trip to one of the local studios
will be needed.
Best of luck and remember you see the results on the monitor
so look forward to seeing all those vidicons from the
monitor end.
JeReM. July 1374, Page 8
Using the dual gate MOSFET. by Russell VK74AC
The dual gate MOSFLT is the solid state equivalent of the
pentode vacuum tube, 1t can be used as a direct replacement
in most applications, the only major difference being the
voltage rating.
when used in R.F. 1.F. acmplifiers gatc 2 is used for cegeCey
this poses a problem since the gain of the device is
directly proportional to the voltage on this gate, this
requires the gate to be up to 4 volts posgitive in respect
to source for maXimum gain.
A way of achieving this is to float the diode detector up
to 6 volts above negative ground as per fig. 1, this can be
used to apply manual gain control by the use of a pot.
A method of using the 2nd [.F. amp. as an L.5.B. detector
is shown in riz.2, this circuit hss not been tried as yet
but has been included for the interest it may arouse since
the device in the role of mixer has no peer, it being a
true square law detector.
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For AM. am» AGC
{gofinik‘.c JUly 137“-, Page 9
Reprinted from "oUup' Vol. 2, No 5.
/40 LINEONS wike rarrell vizan
The 6/40 maxes an excellent linear amplifier for medium power
358 and the tube will stand up to intelligent (watch the screen
current, abuse guite well.
The simplest circuit is the 431 lincar. 2late volts can be
anything between ouU-8UU volts with 250 volts regulated on the
screens. Grid blas volts ay oe supyiled by a simple negative
supply, witi: poventivmeter aqjastmeunt, provideu no grid cuirrent
flows. Standing current should pbe about 3U-4u ma and efficilesncy
40=50 %e The maximuin safe power input should be 4180-200 watts
peak. oee [igs 1.
Provided goced circuit layout is used and grid and plate circuits
are well isolated, no instabiiity troubles should be experienced
as the 6/40 is self neutralis=su. If problsams are encountered
a damping resistor of 224 or so can be placed across the grids.
A point which should be apprecisted with linear amplifiers is
that the tank circuit should provide a z2o0od match to the dyneamic
plate impedance ol the tubes used at the particular plate
voltage and that the loading should be adjusted for good
linearity consistent ~ith power output, rzther than the other
way around. This rather invoived topic iz explained Tar better
by the RSGB handbtoox than by tne ARRL. hiic point is ususlly
the explanation for difficultiss experienced with obtzining
power output from linesr amplifiers. (unless your 6/40 is crooa)
oplit stator capacitors and centre tapped tang coils can be a
little unwizsldy on 6 metres. The two sections of a 5/40 can ke
peralleld snd a pi-coupler usced as a tank with great success,
however neutralisatlon then becomes necessary, although this
is oimply done. Harmonic output is very low.
A typical example is shown in fig. 2.
21,3/ dia. QQE06~40
12bds 12 bk
s
¢00-800v
250v. reg.
=100
GeRells July 1974, Tayge 1U
4t,10¢g
2" dia.
470p.3kv 15'ang
1eg.
Fig.:‘. 6 meires
~ber voltage
11 circuite
adjuasteo for minimuam
GRHUr L Lhiat wll cilreulits
NEXIMm. cmpacitor is madz £rom Coaxs,
itw valuu day hove 2 for bezb cesaitse
T
unce neutraliseo wiais g very .~table and g guite
pload luotuning So ogu ¢ 2082lbig.
up sad loaddng zdjustuents of bouh wipliflers are caslily
wade by wonitoring screen current. Rescizacs 1s indicatsu by
5
T o opeal in sceresin curreot snd lacressing the loading
f» {
1 r uge S oogurecat, thus the Loading is sebt at the
lox
@ ct
4
W
)
(s
(s
‘rect value when the screea current is peaking 8-10maA (Lfor
*j(.‘,
— A
4 / L Gy This 15 a vett 2 Jfiubod thian tun‘”" for maximum e
N r >4 - Ty ey =
i i 1ot wors sensitive Lndication of
Most of the good books will sara o aguinst runaning 6/40
iineare into gric current, ie keeviiy them in aB1; 28
distortion will regult iueto t& Vary ing load on the driver
1 tive bi sguiati us
ufu.‘u L: je ~ome the 5,/40 s'lll
12pplily anoroachilig 7JD
\,‘.)}1 C&‘S'—l" mor
1 b;’ h\;,/&lly
2 the grid cA‘rvgt
mll uul*.u el lO‘/\ value
;:Ll‘uu*\..
A 1itto g more or less
or the more constant the
N I'ay dLiele
ariving powvar lo
’
weReM. July 1974, Page 11
The bias supply voltage nuct be well regulated and it must
also have a low impedance output, although this is not as
important as the ariver problem.
However if the bias supply impedance is high, the bilas voltage
will riese with grid current, limiting the maximum plate current,
sort of simple aLc!
Fig, Z shows a typical linsar dM)lifibP circuit for 144Mhz.
The tank circuit consists of a 1/4 wave hairpin made of 1" wide
strip of 165 copper with the dimensions as shown.
The output coupling looy iu wm=de Tro 15¢ copper wire mounted
inside the tank loop. The gric coil is adjusted for resonoice
by expansion or coupresslon of @he turns spacing.
This cahl be safely doane with the plate wna screen voltages
removed, then acjust the spacing for aaximua grid cur“'rL
(heaters on).
Replace the plate and screen voltages and then set the standing
current =t Z5mA., Introduce sufficient carrier to increase the
plate current to 50mA or so =zad tune for maximum RF cut, the
output coupling loop should be coupled fairly tightly to the
plate tank and the acsoclated capacitor peaked for maximud
outyut,
. L "
<51, 1[2dia,
16 b8s
it
hias 80Cv
(adjust for 35mA g Y
A broza dip in plate current will be noticed at this stage.
Remecve drive Jn% check that the anplifier 1s stable by
monitoring plate current and swing th: plate tuning cap=zcitor
through its range, no ¢ suo in plate current should be
obsarved. Loy drive the slifier up to maximuam plate current,
this will depend on the actuzl plate voltege, for 800 volt
about 150mAa and for 600 volt about 200..4.
Peak input should not exceed 200 .atts or the 6/40 will not
last long. R/F power out sghould be respeszsked al meximun drive.
For maxiasmmn lluulAlu monitor the screen current and
regonate the tank ClPLuLt by adgjusting far o peak in the
screen carvent. The lozding QLJ\AN hs ricd to cobtain a
: i carrent of 8-30ma. If t‘v ‘;ve i varied from
zero to naximam 1t will be ootea > piate current
increases n pgrozoreiocn to Lhio driv notends to flatten
£ as
of bHhe shgoe ia driven intbo cloo
pE
QeRoM. July 1974, Page 12
Voice pewss should be adjusted to Jjust cause the plate to
reach the point sherc it begins to {lcattzen off.
At this point grid current will peaw at several milliamps.
It was previously aentioned that rectified grid current will
cause the bias to increase if a poorly regulated supply is
usew. I the bias supply is us=d for the linear and the
oJreceeding stages "ALC" action will take pl-uce ana will tend
to 1liiiv the maximuam plate current. IL this is the groblen
a low imvedance supply must be usel.
VR34ow has deviseld i simple arransemznt whichh overcomes the
problom. Fig. 4 shows the typical circuit.
The bius VULt*’b required to hold the lin=ar at its standing
currgat should be measured anud a zeper dlode string used to
regulate the bias at this value. Generaily 35-u40 volts is
/)/|'\11Pz(i
The 827 3 is an older version of the 5/i;0 sad will work well
as a ilnsar, but it will not accept as great a power input
as the 6/i40, apparently due to the maximum voltage rating
of the internal nsutralising capacitors.
A meationed initislly, = /qu sy be 6entl,/ thrb,ucu, a
Y e Vi
e, lu.;bb
e {JfA0 Ye s
1_./1 L 1\”-1‘\4’ ok e b';";'n- e :zfully kut ir you
i e gl Aw a ) 3 T o .
3G L.yal' Liade Lule 501 tura thie picr D plLGhe Y wnern JOL Cu-éf;n
toes . Y . VN ) . Ao + z iy .
nG Jqu L the soroarn coLrrernt
T
1J oLl 2o bl’l@ .4/«-,v uLJ..A.
Larvive Lhin trectment, the corecln QA of fine yire ancg
will zive off occluded gmseez if overloadzd it 411l melt 1if
QIOdbl overlcaded.
Deleterous effects on o/40 were “escriced in 6U2, dJunz 4572,
-100v -100v
O
50K 6/40
grids
6/40
10k or grids
ielay contacts relay contacts
10k Rx
el
Rx
Fig.4. VK3ASQ 6/40 BIAS SYSTEMS
FOR SALe FOR oALl PUur vabe PUR SAbLs fux oalls BFUR SALE
Bendix rregquency meter in perfect condition, without
power supply. what ofler ¢
As.d.Manning VA7LR
or coutact VR7NX
B E SR AT R S e 3w 30 A0 AE B qt SR % R TR TRO
Bquipment stere, availtle now. 4000p# fee dthroughs (solder in
1 00 ot el f’tl ylytics 154,
\
t:];—)'-; J 1 t}li 2
47 2F clectrolytices 124
ZeR.l. July 1974, Page 13
_TEZ142"GROUND PLANS.
Feprinvea Irom the Jestlelkes Radlo Club Newsletlter, iay 1272
s.
e——— ee2 .ee et e eS e 8 ee 5 . eet.7 e i e. et ee et .i
This is an adaptiocn of ths _opulzr 4/4 ~ave Zrouand plane, but
it has several advant.oges. The main advantsge 1s that it
provides a dircect watch inte 75 obun coaxlial cable and by @eans
of a capacitor «t the base of the antenaca, the whole aerial
can be tuned ror the lowest owk. This capacltor saves trimaing
the length of the antenna to oblaln rezsocnance.
Here is Low 1t works. The n. fu;l 1/ ove ciround plene hias a
feed limpadunes 0f cpproximately 30 ohms. which is not a zood
mateh rfor 75 chil coaze
I{ the entzuna lenght is increased beyond o quarter .avelength
to a c:zrtain point,; the resibnlv component rises to /5 ohm
with a series inductive reactance. 1This inductive reactance
is easily tuned out with a series capacitor in the base of the
antenna, This is done so that rinal matching to the 75 ohm
coaxizl cable may be accomplished.
Now that the acsrial has been properiy
tunea, which operation is carried out
Lengrh = by observation of the SwR briage, it
L Gmvevear will be noticed that it is much less
4 ' dependent on a ground plane than is
x 125
= 21'8" the classical 1/4 wave type.
For 20 Mx 1t gppearo from experiments conducted
1135Y model, that three radials each
of a guarter savelength, are sufricient
to cnsure efficient results.
In cunstructing the antenna, aly
avallable materials may be pressed
into service. However, it will be
f'ound that aluminium tubing of some
convenlent diameter will be best ror
the vertical section. The radaials ars
- best made of wire and for optimum
results should be at right angles to
the vertical section.
The lengths given ror the unit in the
y > diagram are ror 20 metres. However it
G WAVE is quite possible that this aerial can
RaviaL pe used on 15 or 10 (with changed
; 16'7""Fond lengths) and there is no reason to
ANY LEHGTH suppose that it would not function
74 L CoAx efficiently on VHF. As will be seen
20 1x
FEEDER the lengths can be calculated very
roximately by using the conventional
1/5 wave formula for the vertical and
multiplying this by 1e<5e
The name, "1140 Groundplane", is derived from the electrical
length of this aerial. a quarter vave 1is 90 electrical degrees
so the 113% model must pbe 25 electrical degrees longer than a
quarter wave. 1t seems that this array may be a halfl size
version of the "five-eighth vertical' now popular among the
VHF men. In fact this aerial is, 1f you like it that way, a
"five-sixtecenth" vertical. It may be aaded that the aerial
described in this article has been exhaustively tested by the
club over a period of several months on 20 metres. It gives
superior results to the groundplane and has been responsible
for its owner getting among the DX on several occasions.
Its omnidirectional characteristics would make it an ideal SWL
aerial for either the beginner or the experienced amateur.
Jel. GUNN PTY. LTD.
50 Brisbane Street,Launceston.
Telephone 2 2251
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