After walkies on Friday morning I met Tom for a radio demonstration. He wanted to see my RaDAR (Rapid Deployment Amateur Radio) setup and see my Elecraft KX3 in action.
Tom is interested in portable operations and that is about all I do. Lately I setup my little radio, the Elecraft KX1 (a four-band CW-only transceiver), with a random wire antenna to a 6m mast. I use the counterpoise wires from the Elecraft AX1/AXE portable antenna as the radiator (40m counterpoise) and counterpoise (20m counterpoise) to form the random wire antenna.
So I used this approach with the KX3 setup. When I checked into the 40m Noon Net (7.2835MHz), I noticed the KX3 threw a “High Current” warning and rolled the power back to 5w. That sent me on a hunting expedition later in the day.
I think the 40m counterpoise from the AXE is too close to 10m long, which is one-quarter wavelength at 40m and a half-wavelength at 20m. That’s not going to work because a half wavelength will be resonant and the impedance will be too high at the end of the wire.
I remembered seeing some work on random wire antennas (which really are not so random in length) so I retraced my path through the Internet. Mike, AB3AP (https://udel.edu/~mm/ham/randomWire/), wrote a small C program and a Matlab/Octave script to calculate the wire lengths NOT near a half wavelength.
I tried to get the Octave code working once before, but failed. This morning was too smoky to be outdoors, so I worked on it some more. I was able to get the code working and calculated the wire lengths that will work for my application. I decided on a 17ft counterpoise and a 42ft radiating element. That should work for 10m–40m on the ham bands.
With the target length of wire, I gathered up my materials (wire, open terminals, shrink tube, mast ring, and link) and tools (diagonal cutters, pliers, wire stripper, crimper, soldering gun and solder) and laid my 100ft tape measure on the floor of the garage. The counterpoise (17ft long) was easy and readily completed.
The radiating element required a little more engineering. I wanted to affix a tab that will fit on the top section of my telescoping masts. I also wanted to affix a link and leave a leader so I can add another 42ft of wire if I want an 80m antenna.
I decided to use a larkshead knot with shrink tube to make the antenna-to-mast connection and also a larkshead knot and shrink tube to fasten the tag to the link. If I add another section of wire for 80m, then I can use the link for strain relieve and an Anderson Powerpole connector or a pair of alligator clips to connect the two segments. I think the 80m add-on radiator will be something I deploy occasionally so I want the ability to attach and detach it when needed.
After a couple of hours of work, I had the antenna assembled and ready to test. It was too hot and smoky this afternoon, so I will probably test it in the morning after we walk.
It was a fun little project to build. I have a couple more of these planned, including a lightweight end-fed halfwave antenna. I have the matching transformer for that antenna. I just have to work out how I want to engineer the remaining parts.