The Blue K Antenna

May 2006

Here’s a portable antenna I used with my KX1on about 25 occasions during 2005, including a river trip, mountain top expeditions, a 75 mile backpack trip, and some overnight hikes. It is very light, and is inexpensive, durable and hassle free. It provided many great QSOs from the trail.

The antenna is simply a dipole fed with cheap TV twin lead. The tuner in the KX1 quickly loads it on 40, 30, and 20 meters. I used a 63 feet of wire, center-fed, but 50 feet works just as well. A Balun wasn't needed. A few tricks make this antenna trail friendly:

Antenna wire:

Stranded Teflon-coated 26 gauge mil-spec is light and strong, but it kinks and ties itself into knots. Reviewing “Sojourner” back issues, I decided to try W7WIK’s suggestion of rolling the antenna wire into a “figure eight” pattern for storage. This trick completely eliminates kinking and twisting. I experimented with many designs and found that a stiff piece of plastic shaped like the letter “K” works well. My “K”, made of 1/16th inch blue acrylic the size of my hand, holds 65 feet of wire.

Feedline:

Cheap 300 Ohm TV twin lead is lighter and less "lossy" than coax and easier to use than open wire lead. The length of the feedline is not critical (I used 20 feet). Twin lead is nearly perfect for temporary backpacking antennas except that it is fragile and it's flat shape makes it difficult to roll (and annoying to carry) in a rucksack. Here's a way to solve those problems:

Powdered "Gatorade" comes in small, whisper-light, orange plastic containers with a screw-on lid that can be used over and over. A length of cheap twin lead stuffed in one of these cute containers is great in a backpack.

The photo shows how I attached banana plugs to each end of the cheapest, lightest 300 ohm line I could find. I soldered stranded wires to the fragile conductors of the 300 ohm line, secured them with heat shrink tubing and punched a hole in the center of the feed line to fit over a fiber screw on the center insulator to act as a strain reliever. So far, it's held up great!

End insulators:

A loop formed at end of the antenna wire, twisted back on itself, soldered, and covered with heat-shrink tubing serves well and is as light as you can get.

Center insulator:

A “T” shaped piece of acrylic serves as the center insulator. The antenna wire is anchored to the insulator by forcing it through three small holes. Banana jacks and a wire tie attach a looped piece of plastic cut from a food container to the insulator. A fiber screw and nut on the center insulator provides strain relief for the feed line. String tied through the loop is used to hoist the antenna center, or I force the tip of my fishing pole "antenna mast" under the looped plastic between the two banana jacks. The looped plastic also serves to attach the center insulator to the "K" while I wind up the antenna and for storage.

Another version uses the "K" itself as insulator.

Installation:

I carry a tiny nylon bag with a drawstring in my "Gatorade" jar to use as a launcher. Placing a rock in the bag, I tie a line to the drawstring and throw the bag over a branch about 20 feet above my tent to hoist the middle of the antenna into the air.

When the center is secure, I stretch out the ends of the dipole and tie them as high into trees or bushes as I can reach because acute angles tends to cancel some of the radiated power of an inverted V. Securing the center of the antenna minimizes strain on the wire and insulators and allows the use of very light materials for construction and support.

String, fishing line or dental floss will hold up the antenna (I like carpenter’s line). A simple overhand knot is all that is needed to secure the antenna to twigs. I can generally have my station on the air within ten minutes.

Comment:

I would love to hear from folks who find ways to improve this antenna for backpacking purposes. Remember that the use of household plastics for antenna construction is for QRP power levels only.
* * * * * * * * * *
Dr. Bob Armstrong, N7XJ, an expert outdoorsman, homebrewer and QRP operator, is a contributing editor to The ARS Sojourner. He is a country doctor living in Manti, UT.



	The ‘Blue K’ Antenna: Teflon-coated Wire and TV Twin-lead Make a Great Portable Antenna
		By Dr. Bob Armstrong, N7XJ Contributing Editor, The ARS Sojourner
May 2006 Here’s a portable antenna I used with my KX1on about 25 occasions during 2005, including a river trip, mountain top expeditions, a 75 mile backpack trip, and some overnight hikes. It is very light, and is inexpensive, durable and hassle free. It provided many great QSOs from the trail. The antenna is simply a dipole fed with cheap TV twin lead. The tuner in the KX1 quickly loads it on 40, 30, and 20 meters. I used a 63 feet of wire, center-fed, but 50 feet works just as well. A Balun wasn't needed. A few tricks make this antenna trail friendly: Antenna wire: Stranded Teflon-coated 26 gauge mil-spec is light and strong, but it kinks and ties itself into knots. Reviewing “Sojourner” back issues, I decided to try W7WIK’s suggestion of rolling the antenna wire into a “figure eight” pattern for storage. This trick completely eliminates kinking and twisting. I experimented with many designs and found that a stiff piece of plastic shaped like the letter “K” works well. My “K”, made of 1/16th inch blue acrylic the size of my hand, holds 65 feet of wire. Feedline: Cheap 300 Ohm TV twin lead is lighter and less "lossy" than coax and easier to use than open wire lead. The length of the feedline is not critical (I used 20 feet). Twin lead is nearly perfect for temporary backpacking antennas except that it is fragile and it's flat shape makes it difficult to roll (and annoying to carry) in a rucksack. Here's a way to solve those problems: Powdered "Gatorade" comes in small, whisper-light, orange plastic containers with a screw-on lid that can be used over and over. A length of cheap twin lead stuffed in one of these cute containers is great in a backpack. The photo shows how I attached banana plugs to each end of the cheapest, lightest 300 ohm line I could find. I soldered stranded wires to the fragile conductors of the 300 ohm line, secured them with heat shrink tubing and punched a hole in the center of the feed line to fit over a fiber screw on the center insulator to act as a strain reliever. So far, it's held up great! End insulators: A loop formed at end of the antenna wire, twisted back on itself, soldered, and covered with heat-shrink tubing serves well and is as light as you can get. Center insulator: A “T” shaped piece of acrylic serves as the center insulator. The antenna wire is anchored to the insulator by forcing it through three small holes. Banana jacks and a wire tie attach a looped piece of plastic cut from a food container to the insulator. A fiber screw and nut on the center insulator provides strain relief for the feed line. String tied through the loop is used to hoist the antenna center, or I force the tip of my fishing pole "antenna mast" under the looped plastic between the two banana jacks. The looped plastic also serves to attach the center insulator to the "K" while I wind up the antenna and for storage. Another version uses the "K" itself as insulator. Installation: I carry a tiny nylon bag with a drawstring in my "Gatorade" jar to use as a launcher. Placing a rock in the bag, I tie a line to the drawstring and throw the bag over a branch about 20 feet above my tent to hoist the middle of the antenna into the air. When the center is secure, I stretch out the ends of the dipole and tie them as high into trees or bushes as I can reach because acute angles tends to cancel some of the radiated power of an inverted V. Securing the center of the antenna minimizes strain on the wire and insulators and allows the use of very light materials for construction and support. String, fishing line or dental floss will hold up the antenna (I like carpenter’s line). A simple overhand knot is all that is needed to secure the antenna to twigs. I can generally have my station on the air within ten minutes. Comment: I would love to hear from folks who find ways to improve this antenna for backpacking purposes. Remember that the use of household plastics for antenna construction is for QRP power levels only. * * * * * * * * * * Dr. Bob Armstrong, N7XJ, an expert outdoorsman, homebrewer and QRP operator, is a contributing editor to The ARS Sojourner. He is a country doctor living in Manti, UT.