The Trail Friendly Dipole
Special to The ARS Sojourner
Most wire antennas are prone to turning into a rat's nest. The solution is to find a method where things won't get tangled. I started to brainstorm over a trail friendly dipole that's cheap, lightweight, compact when stored, easy to set-up and take down, tangle resistant and performs well.
One of my other hobbies is flying large stunt kites. The kites have 2 to 4 strings up to 150 feet long stored on a winding card. This allows them to be wound in a figure-8 pattern. It's wound in one direction then the opposite for every turn; hence the figure-8. This prevents the string from being twisted in one direction and causing unhappiness and frustration when it's pulled off the winder.
What if instead of kite strings, they were the elements of a dipole? What if the winding plate doubled as a center insulator? Hmm The goal is to create a method where a center fed dipole can be wound and compactly stored on a winding card without putting twist in the dipole legs when it's wound up or dispersed. The winding card will also serve as the center insulator and be the support point for the center of the dipole.
Here's what I came up with:
Kite string winding cards are sometimes expensive, hard to find and may not be the right size. The winder/center insulator for a 15-meter antenna will, of course, be much smaller than one for the 80-meter band. It should be made from semi-flexible plastic material, rigid enough not to buckle when winding the antenna, but flexible enough so it doesn't crack. You can use stiff plastic garden edging material, which is easily cut with a pair of shears. Make a hole in one end of the center insulator/winder for hanging the antenna at its center. This hole must be placed far off center because placing it in the center of the winder/insulator will often cause the winder/insulator to bend about the midpoint when the antenna is installed, which could result in damage to the card. For larger antennas, double-up or use thicker material. Make the end insulators from the same material and cut them small and streamlined since they will be wound with the antenna wire and support strings on the winder/insulator.
The coaxial feed point can be made using a chassis mount BNC connector (RS # 278-105). Use a large flat washer to prevent it from pulling through the plastic. Mount this in the middle and toward the non-winding (straight) side of the insulator/winder.
The best wire I've found for the job is small (20 gauge or so) insulated speaker wire (RS 278-1388). It's flexible and durable. Don't use wire that's too small or it won't hold up to the constant winding and unwinding in the field. When cutting the (joined speaker wire) elements to length, leave the two strands together and cut it for a º wavelength. Pull the wires apart and you'll have two exact lengths that make a &Mac189; wavelength. Be sure to cut a little longer than necessary to allow tying the insulators and tuning the antenna. Don't strip the insulation except where it's soldered to the BNC.
Run the antenna element wire through the plastic a few times to prevent stress to the solder joints of the BNC. Use small zip ties to hold the elements in place as they go from the BNC to the edges of the insulator/winder.
Next, attach one end of a length of double-sided hook-and-loop material (Velcro() to the non-winding (straight) edge of the insulator/winder. It should be long enough to wrap all the way around and secure the antenna when stored. This serves a double purpose. When the antenna is wound up, the strap keeps it secure. When the antenna is deployed, it provides an anchor for strain relief where the coax attaches to the BNC connector.
It's important to keep both sides of the dipole and support strings the same length because they will be wound together. 50 feet of nylon string for each leg of the antenna should be plenty. Be sure to melt the ends of the string so they don't unravel. Hint: nylon string can be cut with a hot soldering iron.
When attaching the antenna elements and string to the insulators, tie them, don't solder. You still have to tune the antenna and if you use a good knot, they will never come loose anyway.
Now it's time to test and cut the antenna to the operating frequency. Test and tune the antenna at a height and geometry for which it is most likely to be used in the field. It's easiest to use an antenna analyzer such as the MFJ-259.
To deploy the antenna:
Undo the hook-and-loop strap, find the string ends, keep them together and attach them to something or have someone hold it. Hold the hook-and-loop strap in one hand and start walking. Unwind the wire and string from the winder/insulator with your other hand. Once it's out, grab a wire in each hand and spread them apart as you walk back toward the string end. Once it's all separated, attach the coax to the BNC. For strain relief, attach the hook and loop strap around the coaxial choke/balun made from coiling a couple turns of coax into a 6î circle that has been zip-tied. Make sure there's slack in the coax between the choke and BNC. Be very careful there are no power lines in the area I've seen them in the backcountry and national parks. Some of them are set pretty low and blend in very well. Find a set of good throwing rocks, tie them to the string ends and sling them into the surrounding trees. Since rocks sometimes get stuck in trees (once I pulled a dead tree down while trying to hang food away from the bears), you might want to try something else a little more streamlined. Lead tire balancing weights come in all sizes and you can get them dirt cheap or free from most tire places. Pound the rim tab down into the weight to keep it from catching on things. This of course adds weight that you'll have to schlep into the wilderness. Every ounce you carry reduces the enjoyment factor; it adds up fast.
Putting the antenna away:
After pulling it down, disconnect the coax and hook-and-loop strap, take the winder in one hand and both wires together in the other hand and start winding in a figure-8 pattern. Wind around one end, diagonally across the long, cutout edge of the winder/insulator (opposite the BNC and hook and loop strap), around the other end, and so on. Keep doing this until completely wound. Wind it snug but not tight. You should see a figure-8 pattern when looking at the long (winding) edge of the insulator/winder with the antenna and string covering most of that edge. Secure with the hook-and-loop strap and it's ready for your next outing.
Once you buy a spool of speaker wire, string and plastic, you'll have enough material to build several antennas, maybe one for each band for which you have a radio. With a little practice, this antenna is a good performer, it's light and will take very little room in your backpack.
Good luck, and 72.
Marco Wikstrom, W7WIK
Ogden, Utah