HELIX(1) A spirally wound transmission line used in a traveling-wave tube to delay the forward progress
of the input traveling wave . (2) A large coil of wire. It acts as a coil and is used with variable
inductors for impedance matching of high-power transmitters .
HELIX HOUSEA building at a transmitter site that contains antenna loading, coupling, and tuning
HENRY (H)The electromagnetic unit of inductance or mutual inductance. The inductance of a circuit is 1
henry when a current variation of 1 ampere per second induces 1 volt. In electronics, smaller units are
used, such as the millihenry (mH), which is one-thousandth of a henry (H), and the microhenry (
which is one-millionth of a henry .
HERTZ (Hz)A unit of frequency equal to one cycle per second .
HERTZ ANTENNAA half-wave antenna that is installed some distance above ground and positioned
either vertically or horizontally .
HETERODYNE DETECTIONThe use of an a.f. voltage to distinguish between available signals. The
incoming cw signal is mixed with locally generated oscillations to give an a.f. output .
HETERODYNING(1) The process of mixing two frequencies across a nonlinear impedance . (2)
The process of mixing the incoming signal with the local oscillator frequency. This produces the two
fundamentals and the sum and difference frequencies .
HEXADECIMALSame as SEXADECIMAL. A number system with a base of sixteen; also pertains to
conditions, choices, or selections that have sixteen possible values or states .
HEXADECIMAL SYSTEMPertaining to the number system with a radix of sixteen. It uses the ten
digits of the decimal system and the first six letters of the English alphabet .
H-FIELDAny space or region in which a magnetic force is exerted. The magnetic field may be produced
by a current-carrying coil or conductor, by a permanent magnet, or by the earth itself .
HIGH FREQUENCYThe band of frequencies from 3 megahertz to 30 megahertz .
HIGH-FREQUENCY COMPENSATIONSee PEAKING COIL .
HIGH-LEVEL MODULATIONModulation produced in the plate circuit of the last radio stage of the
HIGH-PASS FILTERA filter that passes a majority of the high frequencies on to the next circuit and
rejects, or attenuates, the lower frequencies. Also called a LOW-FREQUENCY DISCRIMINATOR
HITS PER SCANThe number of times an RF beam strikes a target per antenna revolution .
HOLE FLOWIn the valence band, a process of conduction in which electrons move into holes, thereby
creating other holes that appear to move toward a negative potential. (The movement of holes is
opposite the movement of electrons.) 
HORIZONTAL AXISOn a graph, the straight line axis that is plotted from left to right .
HORIZONTAL-DEFLECTION PLATESA pair of parallel electrodes that moves the electron beam
from side to side in a CRT .