OBSERVATIONS FROM SCOTLAND                            28 May 2008                                GM1SXX

Fun with Slinkies.

Now that New Year festivities are over, some of you may be looking for a nice simple quick project to get you back into action.  For those like me who live in homes with 'postage-stamp' sized back gardens, why not try some 'slinky' aerials.  The slinky was devised by a US naval engineer who discovered that a lightweight steel coil spring can 'walk' down stairs.  How, err... 'exciting'.   I guess some people are easily amused. I've never managed to get excited about a spring that can walk down stairs but I can offer some other suggestions for using slinky springs in a constructive manner.  They are cheap and they can be used as radio aerials. 

Oh, and don't accidentally buy a load of the plastic ones.  They don't work! (LA2QAA)

A metal slinky is made from a coiled flat metal tape. It's about 2.75inches diameter and around 67 feet long if uncoiled. At half a pound in weight, it's easily suspended on a cord catenary line.   Although only about three inches high when 'closed', a slinky can stretch to 15 feet in length without sustaining damage.

You'll probably already know that when you coil up a radiator, it needs to be almost twice as long as a plain wire to resonate at the same frequency.  A single slinky is near to a quarterwave on 40M and two joined together are close to a quarterwave on 80M, really an axial mode helical.

Putting it together.

The SLINKY has a small metal clip on each end to confine the loose end of the spring. I prised these off and tossed them in the bin. I tinned the last quarter turn of both ends of two slinkies by jamming a bit of cardboard into the first turn to avoid soldering any more metal than I wanted.  To one free end of each was soldered a wire loop then the two slinkies were then soldered together with a quarter turn of overlap. Use a wooden spring type washing peg for this, both to prise the turns apart and fix the two ends in the correct position while you solder.  The end result is a double length slinky with a pair of loops.

Slinkies seem to be made from a ferrous material but I accidentally left a couple out in the Scottish winter WX for a couple of months and they only had a whitish oxide deposit on them. Rather strange that.  When new, they take solder readily which suggests that they are probably plated. For permanent outdoor use, it's probably a good idea to spray paint them to avoid excessive corrosion. Remember... 'skin effect',...RF only travels in the thin outermost layer of a conductor.

I used an ordinary small ceramic 'egg' insulator at the house end to which I tied the catenary line. I stuffed one free end of the slinky through the same hole as the catenary and bent it over as a fixing point. The catenary line was dropped through the centre of the spring and the spring stretched out about 30-feet with the catenary attached to a pole at the far end of the garden. I soldered a flying lead and croc-clip to the free end of the aerial so I could adjust the active length. Because my catenary runs through another ceramic 'egg' at the far end, it's really easy to raise and lower the spring for tuning.

A single slinky stretched out has a natural resonance around the 40M band.  Two 'connected in series' will allow 80M operation.  A simple experiment using a pair of Slinkies joined together and stretched out to on a 'catenary rope' allowed operation on 80M without the need for an antenna matcher or 'tuner' as some people (especially manufacturers) mistakenly describe them.  To fine-tune, simply clip the adjacent turns together to short them out. In my first experiment, I used spring type clothes pegs nicked from the washing basket.  Almost any type of insulating line will do but I suspect that cord (I used braided Kevlar) is better than slippery types of line because it gives some friction against the turns of the spring so it doesn't slide around too much.  I fed it against the station ground.

You can probably imagine my surprise when after hauling up my pair of slinkies, I got a very acceptable SWR of 1.5:1 on the 80M band and lots of stations booming in.  This 'aerial' is only 30 feet long. If your thing is 40M, a single Slinky will suffice.  OK, it's not the best radio aerial in the world but as I see it, it  has two very useful advantages over some things I've tried.

1) It's dirt cheap

2) It's small enough to allow 80M working from a postage stamp.

OK, it's not as good as a halfwave dipole on 80 but it does fit in a small garden.

73 Al.

GM1SXX

Back to OBSERVATIONS