"Random Wire" Antennas
Perhaps the most misleading name in Ham Radio, "Random Wire" antennas are anything but random. They are specifically chosen lengths of wire that are intentionally not resonant on the Amateur bands. This is so that they can be operate on many/all of the Amateur bands with the help of a tuner, a 9:1 UnUn and a few 1:1 chokes for good measure.
There were some very bright Hams (VE3EED, AB3AP who have done all the hard work in calculating the desirable antenna lengths. Check out those two links for more in depth information). The key was understanding that the antenna cannot be resonant on any half wave of any Amateur frequency. That said, here are recommended random wire antenna lengths in feet:
This is a pretty simple antenna to make. Cut your wire to any of these approved lengths and hook it up to a 9:1 UnUn. For a ground plane/counterpoise you need a 17 foot length of wire on the other side of the UnUn (laying on the earth). After some testing, if using a 29', it is best to use a 6'6 length of counterpoise. Ideally, a 1:1 choke should be installed at the rig/tuner to nuke common mode current and another one connected between the UnUn and the coax at the antenna and counterpoise wires to maintain the intended radiation pattern. You should be able to tune 10m-80m no problem, all with manageable SWR. It might be more than an internal rig tuner can contend with so count on using a dedicated tuner.
There are commercially available 9:1 UnUns from various companies. However, making one is a straight forward exercise. See the EARC Build Guide for more info:
As a general rule, the longer you can make the Random Wire, the better it will perform on the lower bands. This of course depends on your operating location. If you are limited on space, a 29' vertical might work. If you've got more room and can use some of the longer lengths, all the better.
A nifty portable hack for longer lengths is to use a steel tape measure. These are relatively cheap and compact. The 100' tape length can get you up to the 84' recommended length or the 107' if you add a ~7' of extra wire. Alligator clips on a short piece of wire make it pretty easy to connect the UnUn to the tape measure. You can sand off the paint/coating from the tape at the desired length so the alligator clip makes a good contact, but after some testing, this isn't necessary.
This makes a simple and slick antenna for QTH or portable operating!
Thanks for checking out this build!
VE5REV 73!