The Slim Jim can be a great portable ‘roll up’ antenna, if made out of 300Ω or 450Ω ladder line / twin feeder. Add a loop of string to the top, and hang it on a tree branch, use it with your handheld transceiver, then roll it up and put it in your pocket when done! A Slim Jim for 2m (145MHz) will be 1.5 metres long, and 70cm (433MHz) will be 0.5 metres. Alternatively, for permanent installations, the copper tube or aluminium J-pole is a good choice. I have had good success with both, but regularly use the balanced feeder Slim Jim mounted on a 9m fibreglass pole, as can be seen in the photo at the bottom of the page.
It is recommended to use some sort of choke at the feedpoint. Around 6 close turns (for VHF) of the coaxial cable around a 40mm former (PVC pipe etc), or taped up and hung freely should be enough. I have also used a clip on ferrite for VHF. As with any balanced feed antenna, this will help prevent the braid of the coaxial cable from radiating, and becoming part of the antenna, and therefore affecting SWR and performance. The 6 turns is tested as adequate for VHF operation (145MHz) More may be needed lower down in frequency.
The spacing between elements, I have shown as 45mm on 2 metres. This is not critical. It will have some effect on where the 50Ω feed point is, but i’m sure you’ll find it! ‘D’ and ‘F’ work together in the calculations above. The critical lengths are B, C, and E then adjust the feedpoint to find a perfect match. Ignore B and E if building the "J pole".
*Velocity Factor: I have added the ability to select the velocity factor of your conductor. It is set by default to 0.96, which is for bare copper or bare aluminium. If you use balanced feeder such as 300Ω or 450Ω, adjust it to 0.91 (or set to the cable manufacturers specification if available).
50Ω feed point: The 50Ω feed point varies between the Slim Jim and the J-Pole. A Slim Jim has a higher feed impedance, due to it being a ‘folded dipole’, therefore it will be lower down. I have included a 50Ω feedpoint value for both the Slim Jim and the J-Pole, however, it really is a starting point and should be adjusted up and down until you get a 1.1 SWR with your antenna. You could even use a 4:1 coax balun and feed it higher up the matching section.
I made one for 4m (70MHz) which is 3 metres long. The quarter wave matching section can be made horizontal, with the half wave radiator section vertical, 90° to it if space is an issue, although this will affect radiation pattern slightly. Just remember the whole antenna needs to be in the clear, away from any objects, especially conductive objects!