KL7GQ Portable Dipole
Ok, in my time, I've done a lot of portable HF work. My original portable dipole of choice was a wire dipole wrapped around a coffee can with insulators at the 20 and 40 meter lengths; each of the band insulators had a jumper for switching to the next band.
If you suddenly thought 'Iditarod Antenna', you were right; that was the one originally made for service on the Iditarod trail, and has seen service supporting the emergency communications effort when the Princendam burned in the Gulf of Alaska.
Main problem, of course, is that the darn thing is 120 feet long, and sometimes it's hard to find a way to string it up. Another problem I had was one time when the only way I could get it strung turned out to have a large part of it paralleling a metal fence. Needless to say, the SWR was through the roof!
So here's 'Portable Dipole' version 2. It's
made out of two mobile whips on a mast clamp, with a HyGain 1:1
balun in the center.
The mast clamp is a Hamsick Rotatable Dipole Mount (Cat #901). This unit only has only one isolated SO239 whip mount, and the other whip is just bolted right to the clamp. I wanted to put a balun in the center for better performance, so I drilled the other hole out to 1/2 inch and installed a second Firestick Mount Assembly (Cat #275), then made the connections to the balun with a couple of PL259s with a heavy conductor soldered to the center conductor and filled the shell inside with epoxy. All connectors are sealed with hot glue for resistance to the elements.
Shown above is the mast clamp and balun and the resonators off to the side. I chose Hustler for the swappable resonators (the 80 meter resonators are shown here); a wee bit on the spendy side, but good quality stuff to work with. The only downside with these is that they are a bit heavy, but the upside is that they come apart to store is a very small space compared to the Hamsticks.
If I redo this, I think I will try to come up with a heftier mast clamp that can be put together without tools (big beefy thumbscrews or something). I think I would put the dipoles closer to the U-Clamp for less bending force on it, and perhaps the balun directly below the dipoles.
First time out, I tried this with a Radio Shack TV mast tripod and a couple chunks of TV mast. It performed as well as the full length dipole, but was narrower in bandwidth. I tuned the resonators with my MFJ antenna analyzer (part of the emergency antenna kit) to the 80 meter frequency used by the Alaska ARES, and we typically don't go very far off that so the bandwidth is not a problem.
Ah, but that darn TV mast tripod was a pain to
set up. Then along came the The MFJ-1919 Heavy Duty Antenna
Tripod (I got it from AES).
This thing is
pretty slick! It's small enough to toss into the back seat, but
deploys to a decent altitude. Only problem I had with it, was
once I pulled the telescoping pole all the way out, and the
little plastic wedge inside fell off the end of the tightening
screw, and I had to hunt for it in the snow! I think it needs the
pole marked somehow so that you can tell when you've pulled it
out far enough.
So here it is sorta deployed (I didn't mark the pole
yet, and was afraid to pull it all the way out). My 'emergency
generator' is right behind me (the car).
Want to change the band? Just loosen the hand clamp, lower it down, change the resonators, and crank it back up!
Piece of Cake!
The performance is truly amazing, and was right up there with a full-length dipole.
I tested this with both the long and the short Hustler mobile masts, and there is no perceptible difference in performance or SWR bandwidth, so I went with the shorties.
But then again, I still have the wire dipole...
