and airport authorities. NDB radiators are vertically polarised. NDB antennas are usually too short for resonance at the frequency they operate – typically perhaps 20 metres length compared to a wavelength around 1000 m. Therefore, they require a suitable matching network that may consist of an inductor and a capacitor to "tune" the antenna. Vertical NDB antennas may also have a T-antenna, nicknamed a top hat, which is an umbrella-like structure designed to add loading at the end and improve its radiating efficiency. Usually a ground plane or counterpoise is connected underneath the antenna. Other information transmitted by an NDB Apart from Morse code identity of either 400 Hz or 1020 Hz, the NDB may broadcast: Automatic terminal information service (ATIS) Automatic weather information service (AWIS), or, in an emergency i.e. air-to-ground communication failure, an air traffic controller using a push-to-talk (PTT) function, may modulate the carrier with voice. The pilot uses their ADF receiver to hear instructions from the control tower. Automated weather observing system (AWOS) Automated surface observing system (ASOS) VOLMET (meteorological information for aircraft in flight) or meteorological information broadcast Transcribed weather broadcast (TWEB) PIP monitoring. If an NDB has a problem, e.g. lower than normal power output, failure of mains power or standby transmitter is in operation, the NDB may be programmed to transmit an extra 'PIP' (a Morse dot), to alert pilots and others that the beacon may be unreliable for navigation. Common adverse effects Navigation using an ADF to track NDBs is subject to several common effects: Night effect Radio waves reflected back by the ionosphere can cause signal strength fluctuations from the transmitter, especially just before sunrise and just after sunset. This is more common on frequencies above 350 kHz. Because the returning sky waves travel over a different path, they have a different phase from the ground wave. This has the effect of suppressing the aerial signal in a fairly random manner. The needle on the indicator will start wandering. The indication will be most erratic during twilight at dusk and dawn. Terrain effect High terrain like mountains and cliffs can reflect radio waves, giving erroneous readings. Magnetic deposits can also cause erroneous readings Thunderstorm effect Water droplets and ice crystals circulating within a storm cloud, generate wideband noise. This high power noise may affect the accuracy of the ADF bearing. Lightning, due to the high power output will cause the needle