This spring, I volunteered to help Colin KD8CCQ get on the air. He had an Icom IC-706 and an SG-239 antenna tuner, and was trying to figure out the best type of antenna to use with this combination. My solution was a doublet fed with ladder line, so one cool spring day, we strung one up and connected it to the SG-239.
Unfortunately, we did not get the results we were looking for. The tuner never did find a sweet spot, and we ran out of time that day to play with it more, and before we got back to it, Colin had decided to purchase an Alpha-Delta multiband dipole. We put that up, and it did work, so fortunately, Colin was able to get on the air.
After this experience, Colin thought that the SG-239 was defective, and was ready to send it back, but I wasn’t so sure about that. I convinced him to let me take it home with me so that I could play around with it. I wanted to hook up my dummy load to the antenna input. If it could tune a dummy load, that would, at least indicate that it was somewhat working.
Well, it sat on my shelf for a couple of months, but this weekend I pulled it down to run some tests. The first thing I did was to read the manual. (I know this is against FCC rules, but I had to chance it.)
It outlined a test procedure for the tuner that harkened back to the early days of ham radio–it suggested using a light bulb as a dummy load! Back in the days of tube gear, it was very common to use a light bulb as a dummy load. True, most light bulbs are not 50 ohms, and their resistance undoubtedly changes as the wire inside heats up, but most tube gear had matching circuits on their outputs that you tuned when you fired up the rig on a particular frequency, and they were beefy enough to tolerate more than a wimpy 1.5:1 SWR.
Besides, in this application, it’s probably a good thing that the load presented by the light bulb isn’t 50 ohms. Not being 50 ohms, will make the tuner actually do a little work. The 60W bulb I had measured 17.5 ohms, which should yield a 3:1 SWR.
Anyway, I first had to make some cables: a power cable for the tuner, a cable to connect the output on the antenna tuner to the light bulb, and a coaxial cable to connect the rig to the tuner.
After hooking it all up, I fired up the radio. The tuner clicked and whirred, and in a couple of seconds, had found a match. The tuner’s TUNED light came on, and the bulb glowed brightly. So, apparently the tuner works just fine on the bench with a dummy load attached.
For more information on the SG-239 antenna tuner, read N9ZRT’s All About The SG-239 Antenna Coupler: A Simple, Imperfect Tutorial