A Dual Band
(2/70cm) Antenna on the Cheap!
This is not a construction or "how to" article but it will give you some ideas on how to build a 2 meter/440 band antenna from that junk pile of aluminum and scrap antennas and assorted old antenna hardware you may have laying around. You could call this a "proof of concept" design more or less. Your "milage" will vary as they say.
I had an old worn out Hy-Gain V2-R that I had retired that still had some good hardware and tubing so I got the idea to make a dual band 2/440 vertical from some of its parts and one or two other antennas that were also residing in the scrap pile.
The Hy-Gain V2-R originally was a mono band 2 meter antenna with some gain, but it had seen better days so rather than donate it to the trash men, I decided to make use of it and revive it as a dual bander with "slight" modifications.
The V2-R was raised from the dead (the junk pile) and I set out to revive parts of it for a better use.
So here we go with a simple explanation of what I wanted to do with it.
My brain storm was to visualize
that the antenna would be in 2 basic parts of aluminum
tubing separated by a core of phenolic (an insulator) between
the vertical and the radials.
The original V2-R had 2 sets of radials, one above the other, complete with radial brackets. It was designed to be a 5/8 wave collinear vertical for 2 meters with about 3dbd gain but was useless to me now due to age, etc except for a few mechanical parts and some aluminum tubing.
My idea was to fabricate a dual bander using only one "1/4 wave" vertical radiatior that would "work" both bands plus use the radial brackets from the old V2-R antenna.
The top section or vertical radiator is something less then 19.5 inches....probablycloser to 18 inches. This length will depend on the diameter of the vertical element you use. Start long and trim as needed.The bottom section can be a random length upon which is mounted the radial brackets and long enough to mount on a support pole or it could be the support pole. Your choice.
At the top of the bottom section and at the end where the phenolic is placed as an insulator is the bracket with the radials . There is about a 1/2 inch separation between the vertical radiator and the radial assembly. In simple terms, this means that the vertical radiator is insulated from the rest of the antenna. Refer to the drawing below.
The radials are cut to 1/4 wave plus 10%. Coax...center conductor attached to the top vertical radiator at the bottom of it...shield of coax to radial section. Seal the ends of the coax. You may use your "ham engineering" to fabricate a S0-239/PL-239 method rather than attaching the coax directly to the antenna..this gets more complicated though.
So there you have it......A "SIMPLE" 1/4 wave antenna on 2 and ??? on 440.
This antenna is actually nothing more than a 2 meter vertical that can be used on its 3rd harmonic which will be somewhere in or near the 70cm band so swr will not be "perfect" on 70cm.
You will have to strike a happy medium if you desire to tune the antenna for lowest swr on both bands. It is suggested that you should tune it for best swr on 2 meters and let the swr on 70cm lie where it may unless it is over 2:1. However, with some time and effort, you may be able to get the swr low on both 2 meters and the 70cm band. Some experimentation will be needed with the length of the radiator and the radials and their angle. Of course it goes without saying that you will need a good dual band swr meter to do the tuning.
The angle of the radials relative to the support pole can also be used as an swr tunng aid and both the length of the radiator and the angle of the radials will interact.
Refer to the simple drawing below for a basic design.
A = Vertical radiator
Notes from N4UJW....Many thanks go to Larry, KK5ID, for his idea. Hopefully this will give you some ideas or cause you to go to your "junk" pile and start thinking about how to utilize your "retired" antennas and mechanical parts for use in building other antennas from them!
"One man's junk is another man's treasure!"