What causes erroneous NDB bearing indications, and why is the ADF considered unreliable at night?

1-1-2. Nondirectional Radio Beacon (NDB)
a. A low or medium frequency radio beacon transmits nondirectional signals whereby the pilot of an aircraft
properly equipped can determine bearings and "home" on the station. These facilities normally operate in a
frequency band of 190 to 535 kilohertz (kHz), according to ICAO Annex 10 the frequency range for NDBs is
between 190 and 1750 kHz, and transmit a continuous carrier with either 400 or 1020 hertz (Hz) modulation.
All radio beacons except the compass locators transmit a continuous three-letter identification in code except
during voice transmissions.
b. When a radio beacon is used in conjunction with the Instrument Landing System markers, it is called a
Compass Locator.
c. V oice transmissions are made on radio beacons unless the letter "W" (without voice) is included in the class
designator (HW).
d. Radio beacons are subject to disturbances that ma y result in erroneous bearing information. Such
disturbances result from such factors as lightning, precipitation static, etc. At night, radio beacons are vulnerable
to interference from distant stations. Nearly all disturbances which affect the Automatic Direction Finder (ADF)
bearing also affect the facility's identification. Noisy identification usually occurs when the ADF needle is
erratic. V oice, music or erroneous identification may be heard when a steady false bearing is being displayed.
Since ADF receivers do not have a "flag" to warn the pilot when erroneous bearing information is being
displayed, the pilot should continuously monitor the NDB's identification.