How would you find a new source of unique callsigns for your 10m log? Operate in a dx contest.
The ARRL 10m contest ran for two days of the weekend of 12/13 December 2015. I decided I would operate on 10m on one of the contest days.
I arranged to operate from Spring Hill, using the IC703 at 10 watts. I had a 10m dipole and a quarter wave vertical that could be used for this event. To give me another antenna option I decided to cut and tune a diamond quad for 10m.
The quad is a full wave loop, closed at the end opposite the feedpoint. The conventional square quad is fed in the centre of one of the horizontal legs, usually the lower end. By rotating it 45 degrees you have a diamond quad, a square with one apex closest to the ground. This format has a great advantage for a backpack station, as it can be made using wire, with the outer corners held in place with guys. The wire antenna is simply attached to a stock standard squid pole (aka telescopic fibreglass fishing pole). I used a 7m pole and located the feedpoint about 1.8m above ground.
I used insulators made from chopping board plastic. One was required for each corner of the loop, the top and bottom being used to attach the loop to the pole and the two lateral corners being points where the guys were attached. I was unsure whether guying those points would maintain the loop in the right shape but it did seem to be ok. If the insulators slipped along the wire, the guy would have to be attached to the pole at the top of the loop.
Matching the quad
The feed impedance of a loop is in the region of 100-120 ohms depending in the height above ground. To feed this antenna with a 50 ohm line a transformer is required. I decided to use a quarter wave of 75 ohm feedline, using the impedance transforming behaviour of quarter wave feedlines.
The transformer action is given by the formula ZL/Z0 = Z0/Zi
or Zi/Z0 = Z0/ZL
where Zi is the input impedance of the quarter wave feedline, ZL is the load impedance and Z0 is the impedance of the quarter wave transformer/ feedline.
For a quarter wavelength of coaxial cable the length required is the free space quarter wave adjusted for the velocity factor of the cable used. Most solid dielectric coaxial cable has a velocity factor of .66 and the cable I used was of that type.
The “free space” length of the quarter wave transformer was 300/28.4/4 = 2.64m approx. This length needs to be adjusted to account for the velocity factor, so our final length is 2.64 x 0.66 = 1.74m.
I had a “video cable” of almost that length so I set up the antenna with the 75 ohm section connected to the feedpoint, then connected a short 50 ohm (RG58) extension to the radio. The antenna displayed a reasonable SWR of about 1.2 on 29.4 MHz so I had to add some wire to the loop. the difference in a full wavelength at 29.4MHz and 28.4 MHZ was about 400 mm so I added that length to the loop. The SWR then was optimum at 28.4 and acceptable (1.5) at 28.0 to 28.8 MHz.
After testing and adjusting the antenna at home, I rolled up the wire and its guy ropes, ready for deployment on the hill.
How did it work?
Quite well. There was a very strong sporadic E propagation in the first few hours of operation from the hill, giving excellent reports from VK5 and VK4. This was very encouraging and I made steady progress in my log, handing out contest numbers to those who wanted them and giving others the SOTA summit code.
I later set up my standard linked dipole and was able to compare the quad loop with the dipole. In some directions the dipole received and transmitted stronger signals than the quad, consistent with the orientation of both antennas.
I did find that during the afternoon, signals from Japan were more consistent on the quad than on the dipole.
As for my unique callsigns score, I did make enough contacts to add 28 new uniques. I had hoped for more, but conditions were just not good enough for contacts into the USA and that was a factor. Still, the antenna experiment was fun and worth doing.