Ken Hough's Website
|This section includes information about contruction and use of J Poles.|
In essence, J pole antennas comprise a half wave element mounted on top of a quarter wave matching stub, as indicated here on the left, alongside a conventional dipole antenna.|
The radiation pattern and sensitivity of a J pole are similar to that of a dipole.
By adjusting the position of the feeder tapping point, J poles can be matched to feeder cables of practically any impedance.
Common forms of construction are:
-- copper pipe
-- flexible 300 Ohm twin feeder cable
The Slim Jim antenna is somewhat similar to the J Pole
Tuning a J pole can be a little fiddly, because to attain optimum tuning, there are three adjustments to make:
-- the tapping position of the feeder onto the matching section
-- the length of the dipole section
-- the length of the matching section.
The use of copper pipe for construction can allow these adjustments to be made quite easily, as indicated on the left. Dimensions are given for operation on the 2m band.
Initially, the dipole section and the matching section should be made slightly longer than is recommended. After an initial adjustment of the feeder tapping position, the lengths of the dipole and matching section should be trimmed back to a little shorter than needed to attain minimum SWR. Then fit adjustable bolts (see details below) onto the end of the dipole and the end of the matching section. Proceed to make small adjustments to the position of the feeder tapping point and to the adjustable bolts until minimum SWR is obtained at the frequency required.
With a little care, SWR values of 1:1 can be achieved.
Finally, lock down the adjustable bolts and replace the hose clips with 16 SWG copper wire soldered to the copper pipe. Solder the feeder cable to the 16SWG wire connections and wrap the connections and the end of the feeder cable with self amalgamating tape. Overwrap with normal PVC adhesive tape.
Size of pipe v length:
Pipe diameter is not critical, but of course will effect the optimum dimensions of an antenna. The dimensions given above should be nearly correct for 15mm diameter pipe. Larger diameter pipe (18mm, 22mm, 28mm) may be used with slightly reduced lengths for the dipole and matching sections. My own J pole was made using 15mm pipe and performs well.
To minimise possibilities for corrosion, brass bolts and nuts should be used and these should be fitted into ordinary copper pipe end caps. Do this by first making a hole in the each cap big enough to accept a bolt. Then insert a nut inside each cap and solder the nuts inside the caps. Finally, slip the caps over the ends of the dipole and the matching section and solder in position. Screw a nut to each of the bolts and screw the bolts into the caps.
J poles can be made from 300 Ohm twin feeder, as indicated on the left. Again, dimensions are given for use on the 2m band. This can provide a convenient foldaway antenna for portable use.
I have built a J pole of 15mm copper pipe and one of 300 Ohm twin feeder. The copper pipe J pole is mounted above my roof and serves well as a robust omni-directional antenna, and as a reference antenna for judging performances of other 2m antennas.
The J pole that I made from 300 Ohm twin feeder can be easily rolled up to carry in a ruck-sac or even in a pocket -- a useful addition to my hand held rig.
For folks wishing to explore J poles further:
J Pole Double Header -- provides a brief history of J poles and a co-linear design.
A J pole for 70cm -- details for a 70cm J pole.
Design a J pole for other frequencies