Total posts : 45366
Definitely a cool looking little antenna there, Brian!
I’d been looking at some old style loops as part of a project I’d been considering of seeing if I can get some of the kids in the neighborhood into the idea of building crystal radios when I get an AM transmitter up. Yeah, I know, a crystal radio isn’t going to have the range of something like a car radio, but for kids (or maybe some adults) in the near neighborhood it may be workable.
I always found a certain fascination in recievers that take no battery or power supply. And there’s that “built it yourself” thing to it that makes it more interesting.
I did an antenna experiment that I gave to a neighbor/listener back a bit before xmas. I was over at his place briefly and he mentioned it was a pity he couldn’t get my station on his home entertainment system, only his boombox. And then with it next to the window with the antenna up. Checking his home entertainment unit, no antenna at all. Place to hook a 300 ohm for FM, but not even a random piece of wire in it. We were talking about antennas and he was thinking of getting one of the ones basically made of twinlead that you tack up on the wall like he’d had years ago.
I remembered I had some twinlead over at my place and that I’d recently seen a webpage about making a “slim jim” antenna for FM 2 meter out of twinlead. FM BCB is 3 meter, only a bit physically bigger. So I looked at the diagram with what fraction of a wavelength, hit an online wavelength calculator and then roughly cut a piece of twinlead to length and cut out the notch and bared and tinned the wire for the sliding connection to the feedline to tune it, did the rest of the little bit of soldering and took it over with the rest of the twinlead I had. We put it right in the corner of the room, where it wasn’t very noticeable and ran the feedline back behind a bookcase and a small table, under the windowsill and into the back of his reciever. Almost invisible unless you know right where to look.
Then I set his receiver to my xmitter frequency and played with sliding wires up and down till it was coming in fairly good. That antenna is *very* picky about where the wires connect I found, and a lot of moving it a tiny bit and moving back away and then moving it a little more moving away was necessary before we actually got pretty decent reception happening. Like moving the wires even a 16th of an inch can be the difference between “nothing” and “hey, it’s coming in pretty good now!”. I soldered the ground (shortest) side first, then played with the “hot” side a little more and found that moving the wire up the teeniest bit brought my signal in nice and solid. Soldered it there, move back, still very nice. A little black electrician’s tape to cover the bare bits and it was done.
Then he checked other stations as well and found that while he’d only gotten a couple of the strongest locals before, now he had *lots* of stations. All over the band, in fact. It seems to make a really nice little reciever antenna, though tuning it is a bit of a pain.
My logic was that it’s a variation on a 300 ohm twinlead folded dipole, which at least used to be a very common FM reciever antenna. I didn’t use a balun, because I really don’t know much about them, and the 300 ohm feedline (is it still a feedline on a reciever?) is a balanced line, so a “BAlanced/UNbalanced” transformer seemed unnecessary. Besides, never used them for twinlead dipoles on an FM reciever before. Also, sites I’d read say the “slim-jim” has some gain to it. Which was good on the reciever end in this situation, since my transmitter across the street wasn’t even audible at all on his home entertainment center. I wasn’t sure how well it’d work as a recieve antenna, but I had a few yards of twinlead and he was up for trying it, so it seemed a good experiment.
Antennas with gain for the *transmitter* are pretty useless if you’re trying to stay legal part15 FM. Even the crudest simple dipole with no matching beyond cutting the elements roughly to length is probably more than you need and could put you over field strength limits. But gain antennas on the receiver end can be *very* useful if somebody *should* be near enough to pick you up and would like to but just can’t or can pick you up but not very well. I’ve thought about building some of the various high gain directional antennas I’ve seen on ham sites, cut to the FM frequency of my xmitter and doing some experiments with a portable radio this summer to see just how far receiver antennas can extend the practical range for part15 FM.
Brian’s loop project got me thinking again, and next time I run across some twinlead that’s being thrown away or I see some going cheap, I may make up a couple slim-jims to keep onhand for any new neighbors or people in my “fringe” range.. (meaning everybody 2 or more houses away..LOL) Even if they still can’t pick up my station, a twinlead antenna is cheap enough to make to just let them keep as a “friendly neighbor” gesture.
Kudos to Brian, great project.