Question T3C07 in the Technician Class question pool reads:
What band is best suited for communicating via meteor scatter?
- 33 centimeters
- 6 meters
- 2 meters
- 70 centimeters
The answer, of course, is B. 6 meters.
I make a big deal out of this question when I teach my one-day Tech classes because I think meteor scatter is probably one of the coolest things that Techs can do. Have I ever done it? Well, no, at least not yet.
Two months ago, I came across this video:
I also had a bunch of 75Ω coax that was installed back in 1985 when the house was built to distribute cable TV to the living room, bedroom, etc. I’ve never had cable TV in the nearly 25 years I’ve lived here, so I just cut a length from one of the cable runs. So, not only did the antenna look easy to build, I wouldn’t have to spend a dime on it.
Today, I finally got around to putting it all together. As you can see from the photo below, I used the military surplus fiberglass mast to get it up off the ground.
The photo below shows how I connect the loop to the matching stub using a terminal strip.
Troubleshooting the antenna
When I first connected my antenna analyzer to the antenna, I was a little dismayed that the SWR was 14:1! I hadn’t expected it to be perfect, but I did expect it to be a lot lower than that.
The first thing I did was make sure that the wires were connected to the terminal strips properly. When that proved not to be the problem, I thought that maybe it was too close to my other antennas, so I moved it to the other side of the yard. That didn’t change a thing, either.
My next guess was that perhaps the coax was bad somehow, so I disconnected the matching stub and measured the SWR again. At 50.1 MHz, the SWR was now a much better 1.76:1. Since the feed point impedance of just the loop is supposed to be about 100Ω, I knew the problem was the matching stub.
In another video, I’d seen the coax used for the matching stub coiled around a PVC pipe. The theory behind this is that not only the stub matching impedances, but also acting as a choke. I didn’t have any PVC pipe big enough, so I simply coiled the 75Ω coax and taped it together. WRONG! When I uncoiled the matching stub, the SWR came down dramatically, measuring about 1.1:1 at 52 MHz. Down at the bottom of the band, where I planned on playing, it was a bit higher, but it was still less than 1.5:1, and that is easily handled by my IC-7610’s antenna tuner.
Of course, now that I have the antenna, the band is just dead. I’ve been monitoring for the last hour or so, with nary a signal received. That’s just how it goes with 6 meters, I guess.
Meteor scatter here I come
I think that to be more successful on 6 meters, I’ll need to make some improvements to the antenna. For example, I’ll have to figure out a way to make the connections more robust and maybe add a little wire to the loop to bring the resonant frequency down a little. I probably should get the antenna up a little higher, too.
Building this antenna has been fun, and even though I still haven’t made any contacts, I’m looking forward to operating 6 meters in the future. Maybe I’ll even get lucky and make some meteor scatter contacts. Then, when I cover that question in class, I can say that I’ve operated meteor scatter and it’s pretty cool.
I think you’ve probably got common mode current on your 75 ohm matching section (and likely on the 50 ohm coax below it too), accounting for that matching section’s touchiness. Don’t make the matching section out of coax. Make a balanced matching section using a pair of large diameter wires spaced close enough to have a 75 ohm impedance. Don’t bunch this matching section up. Then below that install a good 1:1 current balun. I think I’d try a toroid made out of type 61 material, not type 43. Don’t put too many turns on it; you don’t want it to go resonant at a lower frequency than 50 MHz and therefore have capacitive reactance in the common mode current path on 6 m. Making it go resonant in or near the 6 m band would be fine though. Use a LOT fewer turns than you would on a balun meant for use on say 20 m.
Yes, but isn’t that why one would coil the coax, to form a common-mode choke?
The so-called “ugly balun”? Disconnect everything from both sides of your ugly balun and grid-dip it. Likely it will go parallel resonant way lower than 50 MHz and have a modest amount of capacitive reactance on 6 m. Parallel resonant circuits go capacitive above their resonant frequencies. In any event ugly baluns typically have pretty low impedance not too far below and above their resonant frequencies; chokes made by winding sensible numbers of turns on appropriate ferrite toroids generally do much better over wider frequency ranges because of significant resistive component due to the ferrite core. So if you’re going to use an ugly balun, best to get it resonant right where you need it to do some good. Ugly baluns may be cheap but they’re not all that cheerful.
I did some meteorscatter in the past with MSK144 back in 2019. Even did some on 10m. You, and others, might find it useful to read some tips.
https://pe4bas.blogspot.com/2019/08/meteorscatter-fun.html. I had a lot of fun experimenting with it. I might get into it again around 11 august. Propagation overall on 6m is not as we expected from the top of the solar cycle…meteorscatter does only need a good reflection of a meteor trail. Good luck trying. 73, Bas