Saturday, October 17, 2015

The quasi-Random Wire Antenna

These things are about the worst imaginable antenna that even begins to make sense, are big goofy fun, and work surprisingly well.  Here's the basic idea:

Just that easy.  Now, you will need a tuner to match the transceiver's expected 50 Ohms, and a very good ground to give this one-legged wonder something to "push" against, but beyond that it's stupid simple and free-as-in-beer cheap.

Now the question of "how long?" naturally arrises, and that's why this post refers to quasi-random.  (Not pseudo-random, that's something else entirely.)  I mean, if we're worried about the length at all, then it's not really random anymore.  But some lengths are known to be, if not exactly unworkable, really freekin' difficult.  Specifically these are at integer multiples of half-wavelengths of any frequency(ies) one might wish to use.  Electrically speaking, these are like trying to open a door by pushing on the hinge side.  So... just go through all the ham bands and eliminate these and you end up with a set of magic lengths that ought to generally work, if the minor deities of RF smile and a squirrel doesn't crap upon your handiwork.  Sounds like a lot of work, but fortunately for us all a really smart guy at the University of Delaware wrote a program to handle the grunt work, and he posted the results on the web right here.  In case that page ever goes away, here's the main result:
As usual, click to embiggen.  Lengths that are covered by the colored blocks are not so good.  Here is a color code to the bands:

A 136' hunk of wire works really well all the way down to 160m, but a 36' wire will give you everything you really need for 40m and above – and that's where the real action is anyway.  If you've got to do 80m NVIS as well, 71' will get it done.

Last Winter, I managed to work all over the Southeast and even out to California on 160m with one of these on only 100 Watts.  I started out with 36', but kept patching on more wire to add lower bands and didn't quit until I got to the bottom.  Kind of a sick looking antenna, all twisted together and wound around the yard.  Sure, it's wasn't the most efficient thing, but when a band opening occurs, usually almost anything that even sort of works is enough.  This evening before dark, I broke down and cut a nice piece of 12ga to 136'.  We'll see how it does in the next week or so.

If you're a new ham operator, you can learn an awful lot very quickly playing with one of these.  And if you're an experienced operator who, say, wants to have some kind of antenna to leave up at the hunting shack, beach cottage, or some convenient, RF-quiet spot in the woods, a random wire fills the bill.  Just remember, it will need a good ground.  RF burns are no fun.

No comments:

Post a Comment