W5ALT Travel Antenna
W5ALT Travel Antenna
Since I do quite a bit of traveling at times and still like to operate my radio gear, I began taking my Yaesu FT-817 QRP transceiver along with me. It's nice, small and performs well on CW and PSK31, which are my favorite operating modes. Throw in a small CW paddle or take along my laptop computer, some spare batteries or a charger, and I'm ready to go. It all fits into a small case, no problem.The wires are connected directly to the back of the rig, so there's no transmission line at all.
Don't cut the wires. Just reel out the length you need and leave the rest inside the reels. It won't affect the performance enough to worry about.
The angle between the wires isn't very critical. Just adjust the shorter wire length and position a little to get a match.
The exact orientation of the wires isn't very critical, either. Droop one over a balcony, hang the end from the curtains, etc. The antenna is somewhat directional broadside to the plane of the wires.
The higher the better. Both modeling and experience indicates that close to the ground a lot of the signal gets lost in ground losses. Stay above about the 3rd floor of a hotel to get the best results.
This isn't a good choice for camping, unless you are close to salt water. Note that the antenna will tune at low heights, but it won't work very efficiently.
The distance from my fingers to my opposite shoulder is 1 meter. Knowing that saves having to carry something to measure the wires or having to mark them.
There is nothing special about the reel antennas. Any type of wire could just as well be used. I used the Radio Shack product simply because I had them and they saved me from having to carry a bunch of wire in my luggage through airport security, etc.
Oops, forgot the antenna. A roll of wire and a coax connector. Still OK. A tuner - uh-oh, we got a problem. The tuner is bigger than the rest of the gear combined! Now what?
While pondering the portable antenna issue and playing with some antenna modeling software, I stumbled on an interesting fact. Basically, if you take a 1/3 wavelength of wire and a 1/6 wavelength of wire mounted at an angle of 45 degrees to each other and feed them at the intersection, the thing will resonate and show an impedance of 50 ohms resistive. It's sort of a bent, off center fed doublet. Depending on how high it is and the quality of the ground, it can actually show some significant gain over a dipole.
So with that information, we can leave the tuner at home and all we need is 2 pieces of wire that are 1/3 and 1/6 wavelength long on the bands of interest. Since I operate QRP, wire size isn't much of a concern and the wire can be rolled up when traveling and not in use. Some additional playing with MultiNEC seemed to indicate that the idea should work. So I got a couple pieces of copper wire, measured them for the 15 meter band, put them on a coax connector and attached them to an MFJ-269 Antenna Analyzer. Sure enough, some minor adjustment of the lengths and angle gave a pure 50 ohm resistive impedance.
The next step, of course was to try it out on the air. I pulled out my FT-817 and set it up on the dining room table using batteries, similar to how I would operate from a hotel room. Once again, a slight adjustment and the little rig thought the SWR was 1:1. I gave a call to an OK station CQing on 15 meters, got a reply and got a report of 559. Not too bad with 2.5 watts from YV1-land. I tuned around a little and heard an HK0 DXpedition, called and got a standard 599 report. That's not too far away, but those 2 contacts showed that the antenna does actually work.
For portable use, I have for years used Radio Shack "reel" antennas for my SWL portable. These consist of a thin wire (must be around #20 guage stranded) that is reeled up inside a plastic case when not in use. They should contain about 10 meters of wire, enough to get me on 30 meters, my favorite band. (I checked some newer versions and they only had 23 feet of wire or about 6 meters, though.) I connected one to the center of a coax connector and put an alligator clip on the other. The alligator clip can easily be connected to the shield of the coax connector or can be clipped to my SWL portable antenna. The result is shown below.
Figure 1. My portable travel antenna.
So, now I have a portable antenna that can be wound up and stuffed in a shirt pocket or wherever else is convenient, works at least as well as a dipole, and doesn't require a tuner. Not too bad for an afternoon's playing around. But does it really work?
Shortly afterwards, I made a trip to Puerto La Cruz, YV6-land. I took my rig and left the tuner at home - that's confidence! I forgot the battery charger, so I only had about 3 hours operating off the batteries. I laid out the reels of wire in the hotel room on the 4th floor over looking the Carribean (a nice salt water ground plane helps). The short one was on the floor, the longer one ran up to the corner of the ceiling on a balcony. A slight adjustment of the lengths and the SWR was close to 1:1. The bands weren't very good in July, but I managed to work a handful of OM, OK, DL and UA stations on 15 and 20 meters using the antenna with 2.5 watts output. I'd say it works.
As a result of doing some additional modeling and experimenting, here are some additional guidelines for using the antenna: