Explanation:
(i)
An analog signal has some feature that is a continuous function of the program material being transmitted. It may take on any of a range of values.
In radio, amplitude, frequency, or phase are features of the radio signal commonly modulated when transmitting an analog signal.
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(ii)
A digital signal has some feature that is a discrete function of the program material being transmitted. It may take on any of a specific set of values.
A digital radio signal typically uses some combination of amplitude, frequency, and phase to represent a discrete state being transmitted. Only specific values of these are used by the transmitter.
(The receiver generally has to deal with corruption of the signal by the transmission channel, so the received set of values may not correlate well with those intended at the transmitter. This is why error-correcting codes are often used with digital signal transmission.)
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Additional comment
Analog radio signals are generally transmitted using a carrier wave at a specific frequency (or in some specific frequency range, in the case of frequency modulation). Processing techniques may be used at the transmitter to limit the bandwidth used in the transmission. One commonly-used technique is to limit the transmitted spectrum to one side of the carrier wave frequency. (This is called "single side-band" radio.)
Digital signals offer the opportunity to transmit different bits of the information at different frequencies. Sometimes, "frequency hopping" is used, so that the transmitted carrier wave jumps around from one frequency to another in a predetermined pattern. The effect of this is to spread out the spectrum of the transmitted signal so that average power in any frequency band is significantly reduced. In fact, it may be indistinguishable from background noise, except by a receiver that tunes to the carrier wave in the predetermined frequency pattern.
