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Unbalanced line
Feed line with one conductor at ground potential, such as coaxial cable.

Uncontrolled environment
Any area in which an RF signal may cause radiation exposure to people who may not be aware of the radiated electric and magnetic fields. The FCC generally considers members of the general public and an amateur's neighbors to be in an uncontrolled RF radiation exposure environment to determine the maximum permissible exposure levels.

Unidentified communications or signals
Signals or radio communications in which the transmitting station's call sign is not transmitted.

Upper sideband (USB)
The common single-sideband operating mode on the 20, 17, 15, 12 and 10-meter HF amateur bands, and all the VHF and UHF bands.

Variable capacitor
A capacitor that can have its value changed within a certain range.

Variable resistor
A resistor whose value can be adjusted over a certain range, without removing it from a circuit.

Variable-frequency oscillator (VFO)
An oscillator used in receivers and transmitters. The frequency is set by a tuned circuit using capacitors and inductors. The frequency can be changed by adjusting the components in the tuned circuit.

Vertical antenna
A common amateur antenna, often made of metal tubing. The radiating element is vertical. There are usually four or more radial elements parallel to or on the ground.

Visible horizon
The most distant point one can see by line of sight.

Voice
Any of the several methods used by amateurs to transmit speech.

Voice communications
Hams can use several voice modes, including FM and SSB.

Volt (V)
The basic unit of electrical pressure or EMF.

Voltage
The EMF or pressure that causes electrons to move through an electrical circuit.

Voltmeter
A test instrument used to measure voltage.