Fixing joints, whether coaxial cable connectors or plug-in connections, is not a new thing. There are whole product families of contact cleaners to fight this issue. It's just the first time I've ever watched one go from working to failed in real time.
Two years ago this past week, I had started trying to think of ways of getting my lowest frequency antenna to work on the last band lower in frequency that hams are allocated. It's called 160 meters after its approximate wavelength and its frequency limits are 1.8 to 2.0 MHz. For the newbies, ham radio has a schizophrenic (but useful!) tradition of referring to their bands by a wavelength in meters instead of frequency. Any frequency can also be specified by a wavelength; 160 meters exactly is 1.875 MHz My lowest frequency antenna, a Cushcraft MA8040V, is an electrically short antenna for 3.5 - 4.0 MHz, 80 meters and 7.0 - 7.3 MHz, 40 meters. My antenna is slightly shorter than 1/8 wave on 80 so around 1/16 wave on 160.
The problem with electrically short antennas is that they tend to be low impedance and act like capacitors. It's a law of RF design that optimum power transfer occurs when the source and load are the same impedance and one way of doing that is an external circuit that tunes the antenna to make it match 50 ohms. I think it's fair to say there are no radios on the market today that are designed for something other than 50 ohms resistive.
Impedances can be thought of as series resistor/capacitor (RC) or resistor/inductor (RL) circuits. Transmitters are designed for 50 ohms resistive. My antenna was 2 ohms resistive at one frequency; at others it was close to 2 ohms in series with a capacitor while at one spot above the 160 band it looked like it was in series with an inductor. In Smith Chart* format, it looked like this (red trace along the left circumference and table of values below the chart):
Matching an impedance ratio of 2 ohms to 50 (25x) brings trades and limits that are hard to deal with (have I ever mentioned that physics is a bitch?). I decided to find an approach that would get me on the band, even if it wouldn't be broad enough to include the whole band. That circuit is in yesterday's post.
Over the course of a few days, I got an impedance transforming circuit to work but never really tried it out. There's a handful of reasons, but in the last several months I've looked at this circuit and wondered if it was actually useful. It's possible to transform the antenna into a reasonable load but still have crappy results due to other reasons (too much loss in the cable to the antenna, or problems with radiation angle from the antenna, for example).
When I installed the mandatory Windows 10 "upgrade" on the ham shack computer last week, it refused to allow the cable I used for the USB to serial port connection on my antenna analyzer. After fighting that for a day, I surrendered and ordered a new USB to serial port cable. That came Friday. When I got it to work with the analyzer, I retook the same measurements I had taken on the antenna two years ago and got a very similar plot. Not identical, but “within experimental error.” The circuit sweep looked like this (just focus on the red curve - where it's lowest is the best).
As Friday afternoon turned into Friday night, I put my matching circuit inline with the radio, and found it didn't work. I hooked up the analyzer again and the antenna as viewed through the circuit looked different. “That’s weird.” Then I tried the antenna on bands that I always use it on without this little circuit, 80, 40 and 30 meters, and some of those wouldn't work. I thought it would be best to try experimenting on the antenna during the day yesterday. Because these bands are busiest at night, I figured if I worked on it during the day I wouldn't interfere with other stations.
Yesterday afternoon, I swept the antenna again and this time it looked even more different. In place of the three curves above were three lines sloping a bit downward to the right, but very flat. The unusual appearance extended over the entire 1.7 to 10.5 MHz spectrum I tested. That would be very hard to do deliberately. More experimentation followed, and I eventually tested my two other antennas. Those worked fine, which tells me whatever it was that went wrong, it was just this antenna.
Eventually, I sat down on a work stool at the 80/40 antenna and took the connection between my buried cable and the antenna apart, finding it was horribly corroded. I cleaned it up with sandpaper, files, and a spray on contact cleaner, eventually getting it to look more normal. Then I decided the prudent thing to do would be to replace the connectors and treat them as a new antenna. When I retested the antenna, the new plot laid almost perfectly on top of the one with the formerly corroded connectors.
I tried the antenna out last night around 9PM on 160m and found it seemed to work well. There was a contest going on, which leads to lots of people calling CQ (contact with anyone) and lots of chances to see where you contact. I worked a handful of states, from Arkansas up to New York, and two Caribbean islands: Aruba and Grand Cayman. I didn't spend much time testing it out on the air, just about an hour.
* Looking back at the blog, I see I've never done a "meet Mr. Smith Chart" tutorial post to link to here, so I'm not quite sure where a good one lies. Spread Spectrum Scene has a page with a lot of links that you might find useful. One of the most useful freeware programs I've ever come across is SimSmith by Ward, AE6TY, where you can not only analyze and simulate circuits on the Smith chart but can do things like filter simulations.
PS - I know if I called this post “Putting the Cushcraft MA8040V on 160 Meters,” I'd get a whole different bunch of readers. Maybe five or ten.