Today’s reseach: helical and monopole antennas!
One of the things you can do with radios (and without needing a license) is receiving NOAA weather satelite images! Yes, that’s right, you can LISTEN TO SPACE MACHINES. Another way to look at it is free selfies!
Anyway I was reading this article (Helical Quad Antenna for Weather Satellites) which walks through the process of building a better antenna for receiving these satellites. These NOAA satellites transmit in the VHF (~140MHz / 2m) range so the antenna is tuned for that. From a quick skim through it seems a bit more involved than what I need in my exploratory phase but the author does mention that they started with “a 1/4 wave whip with a 4-wire ground plane” so I started looking into that.
Reading through that article led quite a few things to click in my head. One thing that I hadn’t quite grokked properly was the relation between the 2 connection points on radio connectors like SMA and the parts of antenna. A monopole antenna has one of the connectors attached to the thing that most people think of as the antenna. That is, the long wirey sticky-outy bit. The other connection is either attached to the ground of whatever the circuit is (as in a cellphone) OR earth ground via some spike or something OR to a “ground plane”. A ground plane can consist of some kind of solid metal or mesh sheet. It can also consist of a another series of wires, which is what the “4-wire ground plane” was referring to earlier.
Next up I was reading about Helical antennas. These ones look much goofier. For example:
Helical antennas have 2 modes. If the circumference of the helix is a lot less than the wavelenght you want to tune to, it acts just like a monopole (sort of). If the circumference is close to the wavelenth you can send/receive polarized waves. I don’t quite know exactly what that means, but I do know that satelittes transmit polarized signals and that you need a polarized antenna to pick them up properly.
Now, for my purposes I care less about getting a great signal and a lot more about just getting a signal. I think I will spend some time looking into building myself a 2m/VHF/140MHz monopole.