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.
Pretty cool! I've never tried to do antenna stuff like this, but I understand what you're doing. I never tried it because back in Lost Angeleez my noise level was just too high to make it worthwhile. I had S7 noise on 40, and S9 on 75 and 160. Here I have S1~3 on 75, and a bit less on 160. And for the first time since leaving Illinois I've been able to clearly hear things on 75 and 160. Hence the 88' wire I put up for 75.ReplyDelete
The wire is a bit low for even 40 (~15' on the ends, ~25' in the center), so on 75 and 160 it's either a Worm Warmer or Cloud Burner.
But I can hear things now!
The part I can't explain, because I don't understand it myself, is why get on 160? I've never really been interested in the band. I've been in the CW contests a few times over the years, but haven't spent much time there. I bet I don't have 200 contacts on 160 (Hmmm - don't know how to check that!)Delete
OTOH, it sounded better last night than I can recall it ever sounding. My miniature loop that I did for 30 meters, about 1/4 the size of the real K9AY loops, actually did OK on 160, too.
I can't recall ever making a contact on 160. Back when I was a Ham the first time, it was a shared band, and loaded with LORAN racket.Delete
And while I had antennas capable of working on 75, I was into DX and WAS, so I never bothered to get on 75.
Fast forward 50 years, and I find 75 to be somewhat interesting, but still don't care about 160. One of the guys I knew way back when was Barry Boothe, W9UCW, who was quite the 160 antenna guru. He had a quarter-wave 160 vertical, with an acre or more of radials.
I can see the challenge of building an effective station, but it's something I'll probably never do, like operating on 137kHz.....
it's something I'll probably never do, like operating on 137kHz.....Delete
But operating on 137 will get the idea of quarter wave verticals out of your mind! (It's "only" 1708'!)
When I was first licensed and lived in South Florida, there was a guy I knew who made a deal with a local daytime-only AM broadcaster to use their towers at night for the contests. ISTRC they were a 5/8 wave vertical on the bottom of 160. He said he could work anyone but could hardly hear a thing over the noise.
Apropos to nothing, we used to piggyback on a Japanese AM station. 'We' being a few private USCG-documented sailboats cruising around the Pacific basin. We maintained our own radio net for weather, etc. I mostly monitored, transmitting lightly. Someone mentioned, Boy, if that AM station ever goes down for maintenance we're screwed. Well, one night it did and in the middle of a broadcast on our net. The sender got nailed by our own FCC.Delete
Spread Spectrum Scene... that name rings a bell! Like, from back in the mod-90s, in Silicon Valley. Rummage around a bit: yes, founded by Randy Roberts; that's the name I was trying to remember! The company I worked for back then brought him in as a consultant on a couple of projects.ReplyDelete
And these days it's run by someone in Lenoir City, TN, not so far from where I'm living now.
I got to Spread Spectrum Scene by way of Microwaves 101's Smith Chart site. Been to SS Scene in the past, when they seemed to be more of a online magazine.Delete
Smith charts are a hard topic to do any justice to because there's just so much that can be done with the chart. I've used them for 30 some years and don't claim to be expert. There's hardly a better way to display a lot of circuit information succinctly, though.
27 pages of Smith Chart tutorial, in quite some detail:ReplyDelete
A pair of youtube videos: How to measure complex impedance with MFJ-259B
Impedance Matching and the Smith Chart Tutorial
via HackADay on nomograms which include Smith Charts (brief reference only but I didn't check comment section)
That concludes everything I saved about Smith Charts aside from a 65KB PDF of a Smith Chart. I forget where I got it, it's just named smithcart.pdf.
Hopefully there is something of interest in there.
Cool! Thank you very much. I'm sure that readers will find that useful.Delete
Hey SiG, thanks for the post. I have been seeing the mentions of 160 in the mags but didn't think I'd go down there... but tonight I tuned around just to see if I could hear anyone. I've got an R8 vertical that is supposed to be good to 40, maybe 80.ReplyDelete
I could hear some guys having a rag chew on AM at 1.980 and some guys on LSB around 1.870. noise was buzzing static at S3, and signals were weak. The 1.870 guy was in Dallas, I'm in Houston. I had to tune the DSP noise killer to even hear them.
Neat though, I didn't think I'd even be able to hear anyone down there.
Could you explain piggyback? Surely you don't mean operating on their freq...ReplyDelete