Some Thoughts on Portable Antennas
There are many different approaches to portable operation. The backpacker with a four-pound station and a dub that sets up 90-foot towers for Field Day are both portable, though at opposite ends of the spectrum. Whether I am backpacking or car-camping, I want antennas that are simple and easy to put up, so I can spend more time operating. I hope the ideas I present here will be useful, no matter what your style of portable operation happens to be.
Two General Approaches
There are two general approaches to portable antennas: tune the antennas ahead of time and hope they still work, or put up whatever you can and use a tuner. Both have advantages and disadvantages. The tuner approach is especially good for multi-band operation on a single wire. If you use a half-wave wire on the lowest frequency (about 130 feet for 80 meters and up), it will keep the ground losses low and a simple L network tuner will tune it. Also, it gets the radiating portion up in the clear instead of down at the rig. A radial or two (length not critical) should be a sufficient ground system, or clip to a metal pack frame. An L network might be a toroid coil and a trimmer capacitor mounted on a piece of circuit board. It doesn’t have to be elaborate.
I have used several random wires with mixed results. Once, I made a grappling hook out of a coat hanger and tossed it out of my hotel window onto the rain gutter of the building across the alley. Often, it is convenient to feed the antenna some distance from the rig. For this I have used TV twinlead connected as a Zepp feed (only one side connected to one end of the wire), with a balanced matchbox. It isn’t ideal, but it can make contacts.
One of my memorable longwires was on Prince of Wales, Alaska. For Field Day, I found a location within a mile or so of salt water to the southeast (stateside). The feedpoint went 50 feet up on a cedar tree right on the shore. Then, at low tide, I erected an 8-foot pole out on the tide flats and tied the far end of the wire to it. When the tide came back in I had a sloping longwire over salt water, which broke up pileups on KG4 and KZ5 with an Argonaut. I was having so much fun I forgot the tide, which came within inches of adding the operator to the ground plane!
Center Feed
Although a long wire does work, and gives gain at the higher frequencies, a center-fed antenna is probably better for all-round use. If you are using a tuner, the length doesn’t matter, though a minimum of 1 /3 wave on the lowest frequency will make the matching easier. Again, TV twinlead is a good choice. Ladderline or twinlead with holes will snag on branches.
Sometimes the wires were hard to tune up (partly due to my tuner), and finding the right settings took time and battery power whenever I changed bands. I wanted to eliminate both the tuner and the time and bother of tuning it. My first attempt at backpack operation used dipoles of magnet wire and speaker cable for the feedline, which made it quite light. I learned several lessons that trip:
1. Packing extra socks inside an HW-8 to save space shifts the frequency 40 kHz.
2. The 40 meter antenna would not load (quite frustrating when KC4AAA from the South Pole was booming in as strong as everyone else).
3. It is difficult to send slow CW while sitting on an ant nest.
4. Magnet wire is not rugged enough for frequent use.
Without Tuner
Faced with the prospect of a three-month trip with my Argonaut and a full backpack, I came up with a portable antenna kit that is light, rugged versatile, and doesn’t need a tuner. I have used it in seven states and four countries, from the North Atlantic to the Indian Ocean, and it has always worked. After 10 years, it is still going strong. Best of all, I can (and have) put it up in the dark in a storm. It fits in a 5″ x 7″ x 2″ pouch and weighs only 700 grams (1-1/2 pounds).
Details
Actually, there is nothing new about it. It contains wire dipoles for the 5 “old” HF bands, 80-10 meters along with a piece of coax with a center insulator, and enough rope to put the whole thing up. The secret, of course, is choosing the right materials to make it small, light and adaptable. Mine is designed for backpacking, so weight was the most important criterion, but, you can create your own to fit your style of operating. Let’s look at each item.
Wire:
I use #22 stranded insulated wire. Stranded makes it flexible and less subject to kinking. The insulation lets you run it over tree branches without worry, and seals the wires so they won’t corrode. Anything from #24 to #16 is fine, though #18 and bigger is a bit bulky for backpacking. Cut the wires to the usual formula: two lengths each of 8.25,11,16.5, and 33 feet. If adjacent bands are cut from different colors of wire, they won’t get mixed up as easily.
80 meters can be a problem. A single dipole won’t cover the whole band with a low SWR. This isn’t a problem if you operate over a small portion of the band, but I wanted it all. So, I cut the 80 meter wires to 55 feet each. For CW, I add the 15 meter wires on the end and, for phone, I add the 10 meter wires. If you cut them to 50 feet, you can add the 20 meter wires for low CW, 15 meters for the novice band and low phone, and the 10 meter wires for high phone. I wanted to have 20 and 80 up at the same time. Usually I leave the 15 meter wire on the 80 and use the 40 meter dipole on 15.
Coax:
I use 25 feet of RG-174. Yes, it’s subject to losses. I lose 1/4 of my power on 10 meters. But, it seems to be a good trade-off between loss, antenna height, and weight. On 40 and 80 where the loss is less, you can use a longer feedline. (Hint: carry an extra piece to splice in when needed.) In practice, 25 feet is usually sufficient. Although I have gotten antennas up much higher on occasion, experience shows that 20 feet is a typical antenna height. If weight is not a problem, try 50 feet of RQ-58 or RG-8X. You can also try zip cord, 72-ohm twinlead, or twisted pair, but do your experimenting BEFORE you go out in the field.
Center insulator:
This can be the weak link in the whole system. It needs to be strong and light while providing strain relief for all wires and the feedline, with some means for quick and reliable electrical connections. A small piece of plastic is fine. Drill a couple of 1/2″ holes for the wires and one for the coax. Secure the coax to the insulator, then make the electrical connections. I suggest bolts with wing-nuts mounted on the insulator. Alligator clips on the ends of the coax is another method.
Rope:
A roll of nylon twine from the hardware or grocery store is sufficient. Cut a dozen 10 to 50-foot lengths, and tie an overhand knot in each end so that it doesn’t unravel. Braided line is better if you can find some. It doesn’t snag as easily on rough bark. My favorite is crabpot line from a commercial fishing shop. The braided line used in line-throwing guns is great, though a little thin. It needs to be big enough to tie and untie easily. 150 to 300 pound test seems about right. Dacron lines, although larger than my ropes, are excellent. Rope is light. So, throw in a few extra pieces, just in case.
Tune Before you Go
ALWAYS tune your antennas BEFORE you take them in the field. The element lengths I have given are good starting points, and you may have to trim them a bit. If you don’t have a convenient tree, borrow one from a local park. Hang the feedpoint up about 20 feet, with the ends at 8 to 15 feet. Trim the wires for the best SWR. Make sure everything works. You want a package that is easy and quick to set up and operate.
Other Pre-Cuts
Other pre-cut antennas will work well, especially for single band operation. Folded dipoles, extended double Zepps, and HF J-poles can all be pre-tuned for the band of interest. With a trap dipole don’t try running the traps over a branch. The traps are too lumpy. To help avoid tangles, wind each piece (rope, wire, feedline) in a figure-8 across your palm between your thumb and little finger. Tie off the free end, or secure each hank with a rubber band or twist-tie.
Setting Up
Here is my technique. I find a likely tree where I can set up the radio. I tie the rope to a fist-size rock and toss it over a convenient branch (or the whole tree). THIS IS AN ART UNTO ITSELF, WHICH REQUIRES PRACTICE! I tie on the longest element I am going to use, usually 40 or 80 meters, and pull it up over the branch until there is a few feet of wire left on the ground. Using one leg of the antenna to hoist the feedpoint saves rope, and, it puts the wire as high as possible. Tie something to the bottom end to keep it from running away.
Now, I get out the coax and the rest of the wires. The wires for each band are tied through a hole in the insulator, leaving a 2′ to 4′ free end with 1/2′ of insulation removed. For each side, twist the bare ends together, wrap them around the bolt, and secure them with the thumb nut. The coax is permanently attached to the insulator. Each wire is unwound and laid out on the ground in roughly the direction I want it, with a piece of rope tied on the end. A weight on the end of the rope helps keep everything in its place as it goes up. I use the wire over the branch to pull up the feedpoint until it touches the branch or the coax won’t reach the rig. Tie this wire, and each of the others, as high in the air as possible. Plug in the coax and—you are on the air!
Don’t I have to re-tune the antennas each time I put them up? No, not at all. By tuning them initially at a typical low height, they have been remarkably tolerant of less than optimum installations. It has been installed anywhere from waist level to 40 feet high as a dipole inverted Vee, sloper or vertical with a single radial, and with one to five bands on the same feed point. I have hung it from trees, rocks, roofs, and flag poles.
Some of the arrangements were pretty radical. Once, I camped out along the coast of Western Australia on a rock ledge. With a three-foot branch to prop the feedpoint off the ground, I worked stateside on long path. QRP, of course. The only time I couldn’t get an antenna to work as planned was in Nova Scotia, where a 40 meter dipole hung on a fence wouldn’t load up. But, somehow, it worked on 20 instead.
Try Different Ideas
You can do other things with the wires, too. One of the 40 meter wires makes a full wave loop for 10 meters. (Actually, it’s a bit short: add one 18″ clip lead for the phone band, and a second for CW.) The two 80 meter wires and one of the 40 meter wires will make a full wave loop for 40 meters. When strung as a delta loop you can get the radiating portion much higher in the air than the feedline otherwise would allow.
Of course, if you take a matchbox, you can still put them together for a long wire or whatever else strikes your fancy. On a jagged peak in the Sierra Nevada mountains I found two crags with a 300-foot drop in between. I strung the 20 meter dipole between them and used all the rest of the elements to make a feedline to the tuner
So, there you have it. Simple to put up, easy to use and versatile. What more could you want??
Originally posted on the AntennaX Online Magazine by Dale Hunt, WB6BYU
Last Updated : 26th April 2024