Where the "phase" of the current amplitude varies with time. One complete cycle occupies 360⁰ irrespective of amplitude (visualize a circle). The number of these cycles-per-second is the frequency of the signal.

For mathematical reasons this is referred to as a sine wave. A signal may commence at 0⁰ then go to its most positive value at 90⁰ then recede back to zero value at 180⁰and continue to its most negative value at 270⁰ and then turn back to zero again at 360⁰. This is then one complete cycle.

Perhaps the most common frequency around a home is our power mains. In Australia the frequency used for power mains is 50 cycles per second or now referred to as 50 Hz. The abbreviation is an acknowledgement to Heinrich Hertz. In the U.S.A. and other parts of the world the mains frequency is 60 Hz.

With a 50 Hz mains frequency one cycle occupies 1 / 50th of a second or 20 milli-seconds.

Therefore the signal is most positive after 5 milliseconds, back to zero after another 5 milliseconds, down to its most negative after the next 5 milliseconds and finally back to zero after a final 5 milliseconds. This whole cycle occupies 20 milliseconds or 20 mS and repeats 50 times a second.

With a 60 Hz mains frequency of course one cycle occupies 1 / 60th of a second or 16.67 milli-seconds.

A.C. at audio frequencies extends from 20 Hz to about 20,000 Hz or 20 Khz. Depending upon your age you will not actually hear it beyond 15 Khz and older people are unable to hear much beyond 10 Khz. Animals can hear much higher frequencies. The audio A.C. frequencies are referred to as A.F.

Signals beyond those above are referred to as radio frequencies ( R.F. ) and generally cover the spectrum:

L.F. - 30 Khz to 300 Khz although there are signals transmitted well below this region principally the OMEGA navigation network.

M.F. - 300 Khz to 3 Mhz which mainly includes the A.M. radio band of about 530 Khz to 1650 Khz (varies between countries).

H.F. - 3 Mhz to 30 Mhz and comprises amateur radio, short wave broadcasters among a host of others. Largely becoming superseded by satellite transmissions.

V.H.F. - 30 Mhz to 300 Mhz occupied by traditional T.V. stations, some amateur bands, commercial two way radio, maritime and aircraft bands as well as the F.M. radio band of 88 - 108 Mhz.

U.H.F. - 300 Mhz to 3 Ghz this band is occupied by U.H.F. T.V., some radar installations, mobile phones, two way radios and a heap of other stuff.

Beyond 3 Ghz is virtually satellite transmissions.

It is interesting to note by way of numerical comparison that firstly, each band is 10 times the previous band. Secondly the L.F. band spanning 30 to 300 Khz could be duplicated 10,000 times over in the space occupied by the U.H.F. band.

Also at the bottom end of 30 Khz the signal cycle repeats 30,000 times a second. At the top of the U.H.F. band the signal cycle repeats 3,000,000,000 times a second (mind boggling?).