J Pole antenna for 2m

The J Pole, or J Stick, antenna has a reputation for being easy to make. Some constructors say it also has a reputation for being difficult to tune and match. So long as you build carefully then during tuning and matching, only change one parameter at a time, it is a friendly and accommodating aerial.

Design notes
The aerial is made from round aluminium rod and uPVC rectangular cross-section plastic strip, both impulse purchases from B&Q. I knew that they would be useful one day and, as it turned out, the J Pole was an ideal use when that day came!

Most of what follows is observation and discussion: waffle, if you like. So, if all you want to do is make one, go straight to the Construction section. It's very easy! And quick.

The original measurements came (I think) from the ROOstick by G3ROO and then modified by trial and error. (I now have an antenna analyzer which has made later projects a lot easier!)

As Ian, G3ROO points out in his RadCom piece, the J Pole is an end fed half-wave vertical dipole. It is much easier to end-feed an aerial like this rather than feed it at its centre but, of course, the end has a high impedance. The J Pole gets its shape - as well as its name - from a quarter-wave transmission line transformer which allows us to connect a feed point at fifty ohms impedance.

J-pole antenna for 144MHz by Chris Pearson, M0JRQ

The radiating half wave element

Make a dipole from two quarter wave elements and test it

It can be quite useful - as well as being quite informative - to make the half-wave element in two equal lengths and test it out for tuning, impedance, SWR and suchlike as a proper dipole. Once the dipole of two quarter-wave elements is tuned, the two halves can be joined to create the half-wave radiating element of the J Pole.

Join the quarter waves to create a half-wave end-fed element

Impedance on the quarter-wave matching section: Click for a larger image

Transformer

Where the bottom of the transformer is connected to the half-wave radiating element is a dead short. So at that point the current is highest and the voltage is zero. The resistance here is zero. At the top of the quarter wave long transformer, the voltage will be at its peak and the current will be at zero. Here the impedance is at its highest (by Ohm's law R = V/I)

At a point in between zero (at the bottom) and practically infinity at the top, the impedance will be fifty ohms - We just have to find that point!

Construction
With the materials already bought (!) it was a question of making an aerial that was light, durable and short enough to ride up and down hills strapped to a rucsac.

The finished item was made in two sections. These are of similar length and can be carried strapped to a fishing pole, then hoisted aloft on the pole. The top section is made from rod that (just) slides inside the rod used for the bottom section. This actually made setting the radiator length very easy when the top section was cut over-length and a trombone action used to get best tuning.

Balun
I read about making a matching stub balun - which was more complicated than the aerial itself! - and tried (this was a bit of desperation, really!) using a home-wound 4:1 transformer which seemed to have little effect on performance (as measured point-to-point in QSOs) or SWR. The balun, according to the MFJ-269 is fairly flat right across the 2m band.
Measuring field strength along the coax (a simple coil around the coax feed, diode and an old mA meter) showed that a significant amount of RF was picked up around the feed.
I started with the choke about a metre from the bottom of the counterpoise, trying to keep it out of the way.
I looped one turn, then two turns, then three turns on about 100mm diameter and each turn reduced the field strength. Four turns made almost no difference; same with more turns up to 12.
I then rolled the loop up the coax towards the aerial itself. When the top of the choke loops reached about 50mm from the RF 0V point, pickup from the coax was at a minimum. This is either because it is the same distance from RF 0V as the 50 Ohm point or a complete coincidence! The result is a choke balun that can conveniently be taped/strapped to the counterpoise element at its optimum position.
Ready for use, the assembled antenna is a reasonable size and weight to handle, is attached to a fishing pole by three loops (loosely-fitted cable ties) The complete assembly weighs about the same as the big potted 4:1 balun I bought for an HF dipole!

References:
The VHF J Antenna
Ian Keyser, G3ROO
RadCom, September 1996
(Reproduced in The Antenna File, ISBN 1-872309-72-0)