915 MHz antennas for MeshCore: what to buy for a pocket, a window and a roof

Most MeshCore boards ship with a short stubby antenna that is tuned somewhere in the general neighborhood of 915 MHz and is there so the radio has a load on it. Replacing it is the first thing to do, before a second radio, before a repeater, before anything.

Here is what I put on each kind of radio.

In your pocket: a tuned 17 cm whip

A quarter-wave whip for 915 MHz is about 8 cm; a half-wave is about 16 cm. The 17 cm whip in the list is a flexible half-wave-ish antenna with an SMA male connector, tuned for the U.S. band. On a T-Deck, a Heltec board in a case, or a Wio Tracker, the difference from the stock stub is obvious the first time you check RSSI on a message from the same repeater.

Keep it vertical when you can. A radio lying flat on a car seat with the antenna horizontal is throwing half its signal into the floor mat.

In a window or on a balcony: the ALFA 5 dBi

The ALFA AOA-915-5ACM is a short outdoor antenna with an N-male connector, sold for 802.11ah and LoRa, which is the same band. It is about the size of a highlighter, weatherproof, and it does not look like a ham radio installation, which matters if you rent. It is what I use on indoor room servers near a window and on the box of a small repeater where a 26-inch fiberglass stick would be in the way.

You will need an N-female to SMA-male adapter to connect it to a board.

On a roof or mast: 5.8 dBi fiberglass

The fiberglass omni is the standard fixed-node antenna for a reason: it is sealed, it survives weather, and it has a proper N connector. The listing in the box comes with a mounting bracket and a length of cable; check the cable’s connectors before you order, because you may still need an adapter at the board end.

Two things about the gain number. First, “5.8 dBi” on a fiberglass omni is achieved by squashing the pattern flat, so the antenna hears well out to the horizon and poorly up or down. Mount it vertical and level. Second, a longer, higher-gain stick is not automatically better; on a hill, a flatter pattern can shoot right over the town below you. The 5.8 dBi size is the safe choice for most rooftops.

The mistakes that waste the money

Buying by gain. Height and a clear line of sight beat a few decibels of antenna gain every time. A 3 dBi antenna on the roof outperforms a 10 dBi antenna in the attic.

RP-SMA. Wi-Fi antennas use reverse-polarity SMA, which looks the same and does not mate. Every MeshCore board I own is standard SMA. If an antenna listing says RP-SMA, it is for a router.

Wrong band. 868 MHz antennas are everywhere because Europe uses that band. They will “work” at 915 with worse match and shorter range. Buy 902 to 928 MHz, U.S., 915.

Long cheap coax. RG174 and RG58 lose a lot at 915 MHz. If the antenna is more than a few feet from the board, use LMR-400 or shorten the run. Better yet, put the board in a box at the antenna.

Transmitting with no antenna. Powering a board with nothing on the SMA connector can damage the radio, especially on the higher-power boards. Antenna on first, always.

Trusting the number on the listing. Amazon antenna listings claim whatever they want. Judge an antenna by the RSSI and SNR you see from a known repeater before and after you swap it, not by the dBi in the title.

What to check after you swap

Open the MeshCore app, find a repeater you can hear, and note the RSSI and SNR on a few messages or a trace. Change the antenna, same spot, same orientation, same repeater, and compare. If the numbers did not move, the antenna was not the limit; the location was. Move the radio before you buy another antenna.