Slimjim 70cm

This week's interest has been branching out into more 2m and 70cm work.  During a rain afternoon, I decided to recycle a lot of old bits of copper pipe into a weather-resistant 70cm slimjim.

The only problem with a 70cm slimjim is that the 90 degree elbows for standard plumbing are large in comparison to the antenna's dimensions.  However, the gap between elements is not important, so I used them anyway, with the shortest possible piece of copper pipe to complete the bend.

Slimjim for 70cm.

Anyhow, the upshot is that, thanks to this great site, I was able to just apply the calculated measurements to yield a perfectly-matched 70cm slimjim in no time at all.  The gap between elements is about 50mm, showing that this distance, unlike all the others, really isn't critical at all - the calculated value was only 16mm!

The fitting of end caps to the pipes presents two problems.  First, the gap is only 6mm, and end caps add a couple of mm each.  This makes the measurements all that much more difficult, and so it's best - as I did - to simply fill them with silicone sealant or similarly easy-to-work mastic.  Second, fitting end caps seals the pipework, so with the application of heat, you end up getting air expansion and bubbling of the solder.  You can drill a small hole in the bottom of the antenna, but it's easier just not to fit the end caps at all!

On just under 2 Watts, and so far only on a low test stand, the antenna allows the local 70cm repeater to be worked reliably, even though it is somewhat hidden behind a nearby low hill. I was amazed to find later that 5W is reliably opening both of the 70cm repeaters so far tested on the Isle of Man - that's 61 miles - and that I could hold a perfectly good QSO as well (thanks to the repeater keeper, GD4HOZ (/M), for that one!)

Now it's a case of figuring out where I get £1000 to finally put together a decent satellite-working VHF/UHF station; so far, I've had to go outside with beams for this - very successful, but rather uncomfortable!