D.I.Y. J-Pole Antenna
The photographs above show a J-Pole antenna made for the 11 metre CB
radio band. It has a half wave radiator section, so it's a full size
aerial. Not only is it a very effective antenna, but being made of wire
is very light weight making it quite easy to fix in different positions.
If you have problems installing a permanent antenna, such as a silver
rod, then making a wire antenna that can be easily supported on a
lightweight push up telescopic fishing pole can make an ideal
The formulas to make a J-Pole antenna from 450 Ohm ladder line in this way are:
Length of 1/4 wave impedance matching section (450 ohm ladder line) Wavelength x 0.223
Length of 1/2 wave radiator (any reasonably strong PVC covered stranded wire) Wavelength x 0.471
The point at which the coax is connected to the 450 ohm ladder line will
be about 5 to 10% of the length of the ladder line section up from the
The wavelength at mid point of the CB radio band (27.500 MHz) is found
by the quick calculation 300 ÷ 27.5 = 10.909 metres
So, to make a practical antenna:
The 1/4 wave section of 450 ladder line will be 10.909 x 0.223 = 2.43 metres long
The 1/2 wave wire radiator will be worked out as 10.909 x .471 = 5.13 metres long
The connecting point of the coax will therefore be somewhere between 12
to 23 cm from the bottom of the 1/4 wave section. The optimal point has
to be found by some experimentation - as does the best length for the
The length of the wire radiator will be affected by surroundings. For
example I fixed the wire to a fishing pole. The proximity of the fishing
pole has the effect of electrically lengthening the wire; so using a
5.12 length of wire fixed to a pole I found that it resonated (as
expected) at a lower frequency (i.e. longer wavelength) outside the CB
band. It therefore had to be shortened until the point of
resonance (indicated by lowest SWR) was around 27.5 MHz. This should be
done in the antenna's expected final position since the J-Pole is quite
sensitive to its surroundings, so if these checks are done near the
ground, once it is raised into its final position the SWR will have
changed and the adjustments will have to be done again.
Don't expect the SWR reading to be especially low at this time; the low
point may only be, say, 2.0 at this stage. This indicates that the
connection point of the coaxial cable to the 450 ladder line needs to be
adjusted. The ladder line is used as an impedance transformer,
transforming the very high impedance (hundreds of ohms) of the half wave
wire radiator down to the 50 ohms required by the radio (rig) and the
coaxial feeder cable. This connection point therefore affects the
impedance of the antenna, the higher up the matching section it is the
higher the impedance will be, and visa versa.
Now, once the length of the wire radiator has been set, the connection
point can be moved up and down the ladder line until lowest SWR is
achieved. A few centimetres of the PVC insulation has to be carefully
scraped away from the copper conductor on each side of the ladder line
using a craft knife. I started at a mid point of 18 cm with a couple of
centimetres either side. The inner conductor of the coaxial cable is
quickly tack soldered on the side that is connected to the 1/2 wave wire
radiator. The coaxial cable's braid is quickly tack soldered to the
opposite side of the ladder line at this point, ensuring that both
points are equal distance from the bottom. At this point temporary croc
clips could be used, but I preferred a quick solder joint.
With radiator trimmed for resonance, and with my starting connection
point of 18 cm, the lowest SWR that could be achieved was only
1.7. The connection point was then moved down the ladder line
little by little; un-soldering and re-soldering the coax to the
ladder line; until a lower SWR of 1.2 was achieved, indicating
that the antenna was near the required 50 ohm impedance.
Once the ideal point is found the coaxial cable can be properly and permanently soldered to the ladder line.
The J-Pole took about 20 minutes to physically make out of the wire
components. However the testing and adjusting took a bit more time. I
used an antenna analyser which saved having to key the mike every time
when using a basic SWR meter, but even so, hoisting the fishing pole up
and down numerous times probably took another hour until I was satisfied
with the adjustments. It may take a little longer if using an SWR
The final dimensions were:
1/4 wave section = 2.43 metres
1/2 wave section = 4.85 metres (shorter than calculated due to being fixed to a fibreglass pole).
Connection point = 12.5 centimetres from the bottom.
The feed point of a J-Pole antenna made from Wireman 450 ohm ladder line.
For a CB antenna this feed point is 12.5 cm from the bottom of the ladder line section which
is on the right hand side in this photograph. The coaxial cable used in this case was Mil spec
RG58 c/u. But any good quality, low loss 50 ohm coaxial cable could be used. The wire
radiator section is connected to the same conductor of the ladder line as the coaxial cable's
centre conductor. For my antenna, fixed to a fibreglass fishing pole, the radiator wire was
4.85 metres in length.
The only disadvantage that I see with this antenna is that it has
somewhat limited bandwidth of about 500 kHz. That's no problem at all if
you only use either the UK band or
the EU band, or just the centre of activity around 27.5 MHz. However
it's bandwidth isn't really wide enough to cover all of the E.U. and
U.K. bands from bottom to top which would need about 1000 kHz of
bandwidth. However if you use an A.T.U. at the radio it's easy to use
across all CB channels.
Sealing and waterproofing
Once the antenna is complete and has been checked and tested all the
bare joints should be sealed against the weather with self amalgamating
tape or liquid electrical tape. The coax should also be secured against
the ladder line with a nylon cable tie as a strain relief to prevent the
soldered feed point joints from breaking.
All in all I think making this antenna was well worth the effort. Given
the considerations of time to adjust and somewhat limited bandwidth I
would highly recommend a J-Pole antenna for those who need a cheap,
effective and easily demountable and for those who just like to take
part in some Do It Yourself!
D.I.Y. 1/4 Wave Ground Plane Antenna
- Really easy to make out of some old scrap PVC covered wire or
loudspeaker wire that can be purchased from a 'pound shop'. Support it
on a plastic pipe or a 2 metre long fishing pole, that are often
available in 'pound shops' and you'll have an aerial as good as any
commercially bought 1/4 wave ground plane antenna but at a fraction of
The antenna consists of one vertical radiator wire connected to the
inner conductor of the coaxial cable (I used an electrical 'choc block'
connector, but you may devise something more substantial and ingenious)
and three or four ground plane wires connected to the outer braid of the
coaxial cable. Each wire is one quarter wavelength long. At 27.5 MHz
this will be a length of about 2.4 metres:
(300 ÷ 27.5 MHz = 10.090 m = 2.52 metres ÷ 4 = less 5% = 2.39 metres)
Once built the radiator wire, and perhaps the ground plane wires, will
need to be carefully trimmed until lowest SWR is achieved.
D.I.Y. 1/2 Wave Dipole Antenna
- Another very effective and easy to make antenna. Simply two quarter
wave lengths of wire (each about 2.39 metres long - see above
calculation), one connected to the inner conductor and the other to the
outer shield of the coaxial feeder cable (I used an electrical 'choc
block' connector, but you may devise something more substantial and
ingenious). At this point wind about 10 turns of the coaxial cable
around a small piece of plastic water pipe to create a simple choke
balun. Hang the completed aerial from a convenient high point as a
horizontal dipole, which will make a good DX SSB aerial, or fix it to a
10 metre telescopic fibreglass fishing pole with some tape or cable
ties to make an easy to erect vertical antenna, good for local contacts
on FM, and which will be easy to take down when not in use or use
Once built the two quarter wave wire elements will
need to be carefully trimmed until lowest SWR is achieved - take the
same amount of each leg, little by little, one centimetre at a time, so
that both legs remain the same length.