Antennas and RF

The receiver’s code says little about antennas: this page is general RF guidance for the frequencies it uses, not tested recommendations. Results depend a lot on where you are. Reports of what works for you are welcome as issues.

The receiver listens on two bands:

Band	Frequencies	Used for
VHF	about 160 to 162.6 MHz	voice channels, defect detectors, the NOAA reference station
UHF	452.9375 and 457.9375 MHz	HOT and EOT

Antennas

Height beats everything. An antenna outside and above the roofline hears trains several times further than one indoors. EOT devices transmit from track level with little power, so every metre of height helps.
Vertical polarisation. Railroad radios use vertical antennas; mount yours vertically.
Two antennas are simplest: a VHF one for the voice dongle (the longer one) and a UHF one for the EOT and HOT dongles (the shorter one). A quarter-wave whip is about 46 cm at 160 MHz and about 16 cm at 457 MHz; it needs a ground plane (a metal roof, or radials).
One dual-band antenna also works: amateur 2 m / 70 cm dual-band verticals cover the area well enough, though they are tuned a little lower (146 and 440 MHz).
ITC (220 MHz) needs its own antenna: a quarter-wave whip is about 34 cm (the receiver suggests 33 cm). Amateur tri-band verticals for 2 m, 1.25 m and 70 cm (144, 222 and 440 MHz) cover voice, ITC and EOT from one antenna; the 1.25 m band sits just above ITC. A VHF/UHF diplexer’s low-band port may not pass 220 MHz well, so check its range before sharing one antenna with ITC.
The small whips that come with dongles work on a desk for testing, poorly in practice.

The receiver’s dongles are named for which antenna to connect: “the VHF antenna (the longer one, or the diplexer’s low-band port)” and “the UHF antenna (the shorter one, or the diplexer’s high-band port)”.

	Loss	Use it for
Diplexer (VHF/UHF)	well under 1 dB per port	one dual-band antenna feeding the VHF dongle and a UHF dongle. Check the low-band passband reaches 162.6 MHz: some amateur diplexers stop at 148 to 150 MHz and lose a few dB at railroad frequencies.
Passive splitter (2-way)	about 3.5 dB per port	two dongles on the same band, for example EOT and HOT from one UHF antenna. A 4-way splitter loses about 7 dB.

A splitter in front of a diplexer, or a long run after it, adds up: keep the chain short.

Coax

Loss grows with frequency and length, and it hurts most at UHF:

keep runs short;
use low-loss coax (LMR-240 or LMR-400 class, or RG-6 for short runs) rather than thin RG-58 or RG-174;
RTL-SDR dongles usually have an SMA (or MCX) connector, so plan the adapters.

Preamps (LNAs)

A low-noise amplifier at the antenna makes up for a long coax run. It can also overload the dongle near strong transmitters. The receiver does not switch dongles’ bias-tee power on, so power an LNA separately, or turn the dongle’s bias tee on with its own tools if it has one.

FM broadcast filters

Strong FM radio stations (88 to 108 MHz) can overload the VHF dongle, raising its noise and causing false openings. The receiver reports clipping: above about 0.1%, lower the gain or add an FM broadcast band-stop filter in front of the VHF dongle. Pagers and other strong transmitters near 150 to 160 MHz can do the same; a better antenna position often helps more than a filter.

Gain

Dongle	Default	Lower it when	Raise it when
VHF	20.7 dB	clipping is reported, or many channels open at once	the weather station’s signal is weak and the antenna is small
UHF (EOT, HOT)	40.2 dB	the noise floor jumps, or there are false bursts	bursts are heard but rarely decoded

Set gains per dongle in receiver.json (sdr.devices) or on the Mac status page.

USB

Power: dongles draw a few hundred milliamps each. Two or three on one computer often need a powered USB hub; “short reads” or dongles dropping off are the usual sign.
Noise: computers and USB 3 ports are noisy at radio frequencies. A short USB extension cable that moves the dongles away from the computer, and ferrite clips on cables, can lower the noise floor noticeably.
Serials: dongles from the same batch often share a serial number. The receiver can’t tell them apart; give each its own with rtl_eeprom -d <index> -s <serial>.