Home radiosonde receive sites need a good antenna. Most hobbyists build their own. One of the simplest is a monopole antenna, which can be constructed from an an antenna connector and a few pieces of copper wire cut to the proper length. Bryan Klofas has a nice example of building a quarter-wave monopole antenna that Jon and I used as a basis for our own.

We iterated over a few designs and decided on one based on an SMA connector, with stiff Romex wire used as the elements, cut to the proper length, tuned, and then zip-tied into a 3D-printed frame for stability. Although it’s harder to solder onto an SMA connector than the big chassis-mount N connector Bryan used, the SMA connector is more convenient to attach to modern SDR dongles that typically also use SMA connectors.

Step 1: Buy an SMA connector and wire

We used a through-hole, right-angle SMA connector. These are very common and can be purchased at

• Amazon
• Digi-key
• Ali Express

Stiff wire works best. Jon had a reel of 14 AWG Romex cable (common for in-wall home wiring) so we used a few scraps from that.

Step 2: Cut wire slightly too long and bend to shape

This antenna calculator is useful. We targeted 404.6 MHz giving us a vertical element of about 18cm and ground radials of about 20cm each. We cut 3 lengths of wire: one for the vertical element, and two for each pair of ground radials. There’s about a 1cm gap in between the radials where they meet the connector, plus a few cm “to spare” for a total of about 44cm each, bent into a V shape as shown below.

It’s very important to start the elements a little bit too long so they can be trimmed during the tuning step. It’s a lot easier to make wire shorter than longer!

Step 3: Solder the wire to the connector

Solder the vertical element to the SMA connector’s center conductor. Solder each of the two ground-radial pairs to two of the through-hole contacts of the SMA connector.

Step 4: Tune

If you have a vector network analyzer, attach it to the antenna to check the tuning. By making the wires a little bit too long, it’ll be tuned for a frequency a little bit too low. Gradually trim pieces of the wire away with a pair of diagonal cutters until the antenna’s peak performance moves up to your desired target frequency.

Antenna analyzers:

• The classic NanoVNA broke new ground as one of the first hobbyist antenna analyzers. It works decently at 400MHz and only costs $60. It’s remarkable because a similar instrument a decade or two earlier would set you back thousands of dollars.

• Inspired by the NanoVNA but developed by different people, the NanoVNA v2 is a more accurate and reliable instrument and has a larger range of frequencies, but also costs a lot more, about $300.

Step 5: Add antenna frame

One disadvantage of a wire antenna is that it’s fragile. It easily bends out of shape, and the solder joint is a weak point. To solve this problem, Jon designed a 3D-printable plastic antenna frame. It has a small housing where the SMA connector sits and 5 arms with wire-sized channels to support the 5 antenna elements. The legs are at the proper angle for a monopole antenna. Each arm has a little notch for attaching a zip-tie to hold the copper wire into the arm’s channel.

The easiest way to print it is to split it in half just below the SMA holder, as seen below. If you use a Prusa printer, you can use the pre-split version in this PrusaSlicer project file.

Another variation is a simple windowsill mount. This one isn’t quite as sturdy because it doesn’t give the arms support, and isn’t for portable use. But, it’s worked well to keep an antenna permanently mounted on my window sill.