Antennas

Antennas

The antenna decides what the radio can receive. A dongle with a good antenna in a good place outperforms the best dongle with a poor one, and no setting makes up for a signal that does not arrive.

Which antenna

Band	Frequencies	Wavelength	A half-wave dipole is about
FM	87.5 to 108 MHz (76 to 95 MHz in Japan)	about 3 m	1.5 m across
DAB Band III	174 to 240 MHz	about 1.45 m	0.7 m across

• The small telescopic or magnetic-base antenna that comes with a cheap dongle is cut for television frequencies. It is too short for FM and marginal for DAB; expect strong local stations only.
• A dipole kit (two telescopic elements on a centre piece, as sold with the RTL-SDR Blog dongles) can be set to length for either band and is a good general choice indoors. Set each element to a quarter wavelength: about 75 cm for the middle of the FM band, about 35 cm for DAB.
• DAB antennas made for cars (glass-mount or magnetic) work well for DAB. An active one, with an amplifier at the antenna, needs power: through the cable from a dongle with a bias tee, or from its own supply. Avoid ones that need 12 V if you have no way of providing it.
• A roof or loft antenna for FM or DAB, with a proper coaxial cable, is the best there is. An existing FM or Band III installation can be used directly with an adapter.
• Polarisation matters. DAB is transmitted vertically polarised; hold or mount a DAB dipole upright. FM is horizontal, vertical or mixed depending on the country.

For building an antenna yourself, with lengths worked out for both bands, see the Antenna Design Guide.

Where to put it

• High and near a window, on the side of the building that faces the transmitter. Height matters more than anything else indoors.
• Away from the player and other electronics. Computers, power supplies, LED lamps and network equipment radiate noise across both bands. A USB extension cable for the dongle and a metre or two of antenna cable help.
• Try several places. A move of half a metre can change DAB reception from none to perfect, because indoor signals are full of reflections.

The Antenna Positioning Tools in the Station Manager show what the antenna delivers while you move it.

Connectors

Dongle	Socket	Typical adapter
RTL-SDR Blog, Nooelec SMArt	SMA female	SMA male plug on the antenna cable; F or IEC (TV) to SMA adapters for house antennas
Cheap generic sticks	MCX	MCX to SMA or MCX to IEC pigtail

How good is good enough

• FM degrades gradually: a weak station is noisy but audible. RDS (station name, text) needs clearly better reception than listening does.
• DAB works or does not: above a threshold the sound is perfect, just below it there are gaps and burbling, and below that the station is not found at all. A station that plays with interruptions needs a better antenna position, not a setting.

The reception indicator on the player (five dots) and the levels in the Station Manager's station lists show where a station stands.