A COFFEE-BREAK QUARTER WAVE
VERTICAL FOR 20 METRES:
Let’s take a break by building a very simple and yet extremely effective aerial for 20 metres. Fed with 50 ohm coax with no need for a matching unit, this aerial is unobtrusive, has a very low angle of radiation (good for DX) and, depending on the junk you have kicking about in the shed, might not cost you a penny. I’m talking about a quarter-wave, ground-mounted vertical for 20 metres. This aerial is simplicity itself. Believe me, I knocked up the aerial within an hour or so and the first station I worked was in Indonesia.
All you need is about ten feet of 22mm copper pipe and about eight feet of 15mm copper pipe. You’ll also need a length of wood, some kind of metal rod to stick in the ground, a self-tapping screw and enough coax to run from the aerial to your radio. Oh, and a PL259 plug and a soldering iron and solder…
Hammer the length of wood into the ground to act as a sort of fence post. We’ll call this lump of wood the insulated aerial mounting assembly because it sounds better than a lump of wood. Drill a small hole in the larger diameter copper pipe, about six inches from one end, and wind the self-tapping screw into it. Slide the smaller diameter pipe into the larger one (loosen the screw first) and tighten the screw to hold it in place. OK so far? Using pipe clips, string and tape, nails, or whatever, fix the larger pipe to your lump of wood so that the end is about six-inches above the ground.
You should now have some sort of ground-mounted whip aerial with a sliding up and down inner bit at the top for adjusting the SWR. To reiterate, you have the big pipe fixed to the wood and the small pipe sliding in and out of the top of the big pipe which can be fixed at any position by tightening the self-tapping screw. Bang an earth rod into the ground near to your sort of fence post lump of wood. Solder the inner of your coax to the base of the copper pipe and the braid to the top of your earth rod. If you haven’t got a blow torch to heat up the pipe and rod, then you’ll have to use screws.
Solder as many 16 ft lengths of any old wire as you can to the earth rod and fan them out across the garden. The more wires, the better. You can bury them, if you wish. Now go to the shack and plug the other end of the coax into your radio. It will help tremendously if you connect the PL259 plug to the end of the coax first. Using low power on 20 metres, somewhere in the middle of the band, check the SWR. Too high? Go and shorten or lengthen the smaller pipe by pulling it out or pushing it in to the larger pipe. Tighten the screw to hold it in place. As a starting point, the overall length of your aerial should be about 16 feet. Having run up and down the stairs twenty times and tripped over the cat ten times, you’ve finally found the sliding in and out position that gives you 1:1 SWR.
You now have a ground-mounted quarter-wave 20 metre vertical aerial. Cool, huh? In fact, you can use it on 17 metres by sliding the smaller pipe further into the larger pipe and shortening the whole thing.
CONSTRUCTIONAL NOTES 22mm PIPE FIXED TO WOOD:
The piece of wood I used to support the aerial was about three inches by one inch and about eighteen inches long. Rather than hammer it into the ground, I screwed it to the top of my substantial earth rod. I also coated the wood with old engine oil to protect it from the rain. With a gap of about two inches between the top of the earth rod and the base of the aerial, soldering the coax was easy. Don’t forget to seal the open end of the coax with something to prevent water getting in. It’s also a good idea to bury the coax. This soil acts as a kind of huge ferrite sleeve and prevents the braid radiating. I’ve pumped 100 watts into my vertical with no TVI or other interference problems.
I didn’t use any guy ropes to support the vertical aerial. Even in high winds, the rigidity of the copper pipe is enough to prevent the thing bending or blowing away. I’d like to improve the self-tapping screw arrangement as it’s a pain having to stand on a chair and loosen the screw to move the inner pipe up and down between the 20 and 17 metre bands. By the way, there’s no reason why this aerial shouldn’t work on 15 metres if you can shorten the thing enough.
If you can find a way of making the vertical aerial about 33 feet long, then you can use exactly the same design for 40 metres. Obviously, the problem with the added height will be the wind. Aluminium tubing with guy ropes? 33 feet is quite a length. Be careful or you’ll have the whole thing down through the neighbour’s greenhouse!
This is my improved aerial base. I managed to get hold of a beautiful insulator which I mounted on top of the buried earth rod. If the wind get too high, I can simply unscrew the copper pipe from the insulator.