Sunday, 6 July 2014

'Jaybeam' Slim JIM

Many of us began our ham radio career as the then Class B licensees, confined to the frequencies above 50MHz, and for many of us, our first antenna will have been the ubiquitous 'Slim-Jim'. Some of us, those with money, might have bought a commercial antenna, but those like me, impoverished school kids with an agonising paper round, spent our birthday money on a radio, and built an antenna. My first radio was a Kenpro KT-22 2m Handheld. This was actually stolen off my belt in Doncaster market, but one day i'll get another for nostalgia!

To go with this, I built my first slim jim. This was made from mains wiring stapled to the cardboard backs of several A4 notepads. Incredibly crude, but it worked, stood against my bedroom wall. I only abandoned it after it folded over whilst on air on night, across my bare arm!

Most people these days build them from a length of ladder line, which is what ive done here. 300 ohm window line. I used dimensions from G0KYA's blog, where he discusses a problem found when converting from 300 ohm ribbon to window line, with the velocity factor. To prove a point, I started with the original measurements, then swapped to his new set of 53" overall length, 16" for the matching section.

The insulation on this stuff is quite thick, and takes some baring back. Once about 1/4" is bared at one end, its tinned, folded over and soldered together. This is done at both ends of the section of line.

The bottom few inches are also bared back to provide a location for the feedpoint. This is found experimentally, and is usually the most tedious part of the build - this one being no exception! A sharp knife is needed to expose the wires.

The gap cut into one side, which separates the 1/4 wave matching stub from the 1/2 wave folded radiator, usually makes the whole thing go floppy, here ive reinforced it by taping a wooden coffee stirrer splint over it! This is a 1" gap cut 16" from the bottom.

I used an offcut of RG-58 with an N-type plug on it for the feeder, about 2" long. After a lot of experimentation, I found that 2" from the bottom gave me an acceptable match, 1.6:1, when the antenna was enclosed in its fibreglass tube.

Now, a word about enclosing these antennas. Im using the remains of an old commercial antenna, but even so, putting the antenna into the tube drastically detunes it! So much so that the resonant frequency of my antenna is about 138MHz. Attempts to compensate for this by cutting the antenna for 145MHz resonance when in the tube failed, im not at all sure why, but I reverted to the original dimensions and found a workable feedpoint.

 The antenna is now in its fibreglass tube, with the metal mounting hardware attached. This is being reattached using epoxy filler. Once cured, four bolts will firmly attach the mounting, and the bottom of the tube will have some sealant pumped in.

I also found time to get around to sectioning the 4" Heliflex feeder offcuts. The outer copper on this stuff is so thick that in the end, it was removed with an angle grinder!

 I now have the two sections with connectors sectioned, and apart from a bit more filing to remove some swarf, they are ready for classroom use.

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