J-Pole VHF Antenna
For 2M (144-146MHz) I’ve experimented in constructing a “J-Pole antenna“. Essentially, this is no more than a length of copper pipe folded back on itself with the feeder connected across the fold, a short distance away.
My experimental J-Pole was constructed from a length of copper tube, the very same tube that is used for car brake pipes, as I happened to have a reel of this pipe laying around.
To obtain the measurements I used M0UKD’s calculator. Adding an extra 10-15mm to the overall length given in the calculator (it’s easier to trim a bit off, than add a bit on) to allow for error and/or adjustments required.
The tube was then simply folded to the given dimensions and the feedline temporarily held in position with insulation tape on an old offcut of 42mm plastic waste pipe, and perched precariously on the wooden garden bench at a height of no more than 3 feet from the ground.
Once the copper tube was trimmed to the measurements given by the calculator (it was extremely accurate & I didn’t need the extra few mm that I had allowed for error/adjustment) and the feedline position finalised, the antenna gave a good SWR over a surprisingly wide bandwidth of approx 10MHz!!
After studying the photo, purists may argue that the only true match is at a frequency of just slightly higher than 145MHz when the resistance becomes exactly 50 ohms.
I have to say that I’m impressed with the results, I can open repeaters that I couldn’t even hear before.
My next stage with the J-Pole is to construct a more permanent version using 15mm copper pipe and solder joints, then mounting it higher up on a suitable fibreglass or plastic pole.