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Dual-Band
Loop of nylon line
(to hang antenna)
VHF/UHF
Twinlead
J-Pole Antenna
Standard Flat
300 ohm Twinlead
By James A. Williams
W
Cut out wire
and insulation
1/4" notch
54inches
ant to get mo re range ou t of that
handhcld transce iver that just
can · t quite make that di sta nt repeater all your friends brag abo ut? Or how
about an emergency ante nna that yo u can
eas ily put up and use whe n a tornado o r o ther
natural d isaster takes down your ma in ante n na, o r lightning cau ses you to have to
15.25" inches
d isconnect that beam you have 60 fee t in the
air'l Or do you li ve in an apartment or deed
restricted subdi vision that will not le t you
have outside a nte nnas'>
If you have 30 minutes to Cut out small notch ,
spare. you can bui ld a Dual - Band remove only insulation
Cut out small notch,
VHF/U HF J- Pole ante nn a that DO NOT NI CK WIRES
remove only insulation
is g uaranteed to get you on the Solder coax center conductor
DO NOT NICK WIRES
air q uickly, outperform the rubSolder coax braid (shield)
berduck antenna that came with yourhandhe ld.
a nd solve all the above problems and then
.25" inches
some.
It will serve not only as an emergency
antenna, but as a portable antenna system for
anyone needing an easy- to-set-up antenna.
Twist wires together and solder
such as trave lers who want to monitor o r
ope rate from the ir hote l room. The low profi le
For thi s artic le the we wil l concentrate on
antenna can be set up and take n down qui ckly.
le ng th radi ator and a 1/4 wave le ng th matchand it stores in the space of an eyeg lass case.
the most popul ar bands in thi s country: the 2
ing stub that operates like an end-fed halfwavc
It can be hung in a corne r of a room. attic or me ter band ( 144 M Hz to 148 MHz) and the 70
antenna. If built to exact specs listed. there is
other o ut of the way locatio n for a mon: c m band (440 M Hz to450 M Hz). I have found
no tuning needed for thi s antenna. Just build it
permane nt insta llati on, or can be put up and the ante nna to have a voltage standing wave a nd USC it.
taken down when needed.
ratio (VSWR) of le ss than I :5: I across the
The Dual -Band Twinlead J-Polc ante nna
The J- Polc antenna has been used by ham e ntire 2- me ter band, and less than I: 7: I across
is made from standard fla t 300 o hm twin lead
radio operators around the world for years . the 70-c m band. As a scanner antenna (non - TV ante nna wire. avai lable from most hardIt ·s one of the first antennas that most new transmit ) it is usable from 30 MHz thro ugh the ware and electronic sto res. Most of us o ldtimcrs
ha rm hear about and wi sh to acquire. Se vera l g igahe rtz bands.
wi ll re member this wire as the twinlead used
I will not get into the theory or math in- to connect a TV to the roof-mounted antenna
commercial antenna manufac turers make and
sell this ty pe of antenna in the price range of vo lved in the construc ti on of thi s antenna. that almost all homes in th is cou ntry d isfrom $ 15 to $25 dollars each ; you can make it othe r than to state that it is a vertically polar- played before the advent of cab le TV. You can
for $3 or $4 dollars.
ized ante nna with two c leme nts: a 3/4 wave- use an o ld pi ece of twinlcad you o r a friend
J_
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20
MONITORING TIMES
April 1998
may have lying around or huy a new pi ece 6
foot long.
You will need the following:
6 feet o f nat 300 ohm twi n lead
6 feet or more of RG-58 coax
PL-259 or BNC connector (your choice)
Soldering Iron and Solder
Wire Cutters, strippers or knife
Plasti c electrical tape or heat shrink tubing
I Building the Antenna
Begin hy carefully removing 0.75 inch of
insulation from one end of the twin lead. being
carefu l that you also do not cut the wires. Once
the wire has been stripped. tw ist the pair of
w ires together and solder. Thi s is the bottom
of the antenna.
Cut the twinlead to 54 inches exactly from
top to bottom. Doub le check the length: it
needs to bt: exact.
Measure 1.25 inches up from the bollom of
the antenna and mark the in~ulation. From th is
mark. carefull y remove the insulation for .25
i nchcs above and below the mark. being carefu l not to nick the wire. Repeat on the other
side of the twin lead.
M easure exactly 15.25 inches from the one
o f the notches you have just cut and remove a
.25 inch notch from thi s point: remove both
the insulation and the wire. Thi s is the ground
side ( 1/4 wave stub). The other uncut side is
the radiator side (3/4 wave) of the antenna.
Using a piece of RG -58 coax at least 6 feet
long. careful ly remove .75 inches of insulation from the coax to expose the braid. Separate the braid from the center conductor and
twist together. Remove the insulation from
the center conductor. Connect the braid from
the coax to the ground side (stub) of the
twin lead and the center conductor to the long
side (rad iator) of the twinlcad. Try to connect
the coax so that it lays in the center o f the
twi nlead. Solder the coax braid and center
conductor to the twinlead.
Use elec trical tape or heat shrink tubing to
secure the coax to the twinlcad. This will help
take the stress off the solder points and make
the j oint waterproof. A lso tape or use heat
shri nk tubing at the notch you cu t out to make
the ground side (stub) of the antenna.
Punch a hole at the top of the an tenna at the
center and ti e a loop of nylon line ( fishing) or
another nonconducting l ine to hang the antenna from the ceiling, tree limb or other
locati on. Install your choice of connector (PL259. BNC. etc.) to the coax and use. You w ill
find that the antenna will have a gain of 3 to 4
dB.
When using the antenna try to keep it away
from metallic objects such as electrical w iring
and plumbing. It is possible to detune the
antenna i f it is close to such objects. For a
permanent outdoor installation. mount the JPole inside a piece of PVC pipe capped at both
ends. Drill a hole in the bottom cap for the
coax to pass th rough and seal around the coax
with a good waterproo f scaler. M ount on top
of your mast or tower.
By bui lding several of these low cost. easy
to construct antennas you can keep one handy
at all times-such as in your car glove compartment. briefcase or back pocket- and enjoy the improved performance of this antenna
any time.
Listening Post
State-of-the-art scanning software.
Features:
Standard Flat
300 ohn Twinlead
Solder coax
center conductor- - to wire
-
Solder coax braid
(shield) to wire
/
Tape or Heat Shrink Tubing
Coax to Twinlead. Try to
center coax on twinlead
as shown.
•
•
•
•
Easy to use Explorer-type interface
Full featured scheduler
Advanced reporting engine
Audio Processing Plug-In support
for ACARS, inversion , etc.
• Digital audio logging to database
• Full 32-bit multitasking for high
performance scanning and
reporting
Scanner Support:
Opto OS456, 535, ICOM 7000, ICOM
7100, ICOM 8500, ICOM 9000, AOR
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Includes frequency database
• FCC database
• Aviation database
• Fed frequencies
• Cut 'n paste or drag & drop into LP
• Sort, search and proximity search
All for $99.95 on CD
$79.95 on diskettes w/o database
For more info, see www.lpcom.com
Coax
LP Communications, Inc.
51 14 Balcones Woods Dr. Ste. 307305, Austin, TX 78759
Phone(512)260-3478
April 1998
MONITORING TIMES
21