Radar System Engineering

R-F
CHAPTER 11
COMPONENTS
BY A. E. WHITFORD
11.1. The R-f Transmission Problem.—In the block diagram of a
basic radar system shown in Fig. 14, the parts shown as heavy double
lines transmit the radio-frequency (r-f) energy from the magnetron to the
antenna, and carry the faint echo signals into the T-branch where the
receiving apparatus is located. For the types of radar treated in this
book this function is performed by coaxial lines and waveguides. A
considerable body of theory and a new set of techniques have grown up
around this class of transmission circuits. The essential new feature is,
of course, that the wavelength is of the same order as the physical size of
the circuit elements; the length of the line may be many wavelengths.
Although space permits only a limited treatment here,l sufficient introduction
will be given to show the general approach, and to make understandable
some of the reasons for current practices in microwave radar.
Standing Waves.—When a voltage is suddenly applied to the input
terminals of a long and uniform transmission line, ‘the current which flows
in the initial interval, before reflections from the far end arrive to confuse
the situation, is determined by the property of the line known as its
“characteristic impedance. ” The characteristic impedance, ZO, is a
function of the geometry of the conductors and insulators of the line, and
for good conductors and low-loss dielectrics is almost purely resistive.
For a concentric line, neglecting losses,
where k is the dielectric constant of the material in the annular space
between the conductors, r, is the inner radius of the outer conductor, and
TI is the outside
radius of the inner conductor.
A uniform line terminated at any point in its characteristic impedance
behaves as if the line were infinitely long; there is no reflection. Power
introduced at the input terminals disappears into the termination with
small losses in the line. However, any discontinuous change along the
1 Xfl-... -... --2.2 .-... —.-. --- L.f... –.l :- _. L-- L__,.. _.
.)1”, .L:. --:- .-.. -;. 11..
c CALC,,Utl, l,,edLII,e,, b Uii,, lJ~,UU,, U Ill“1,,,~~ ““”KSUlLILIS SSlltX) ~h~CL-l?llly
(1)
vol. 9; (z) Microwave hpk%rs,
Micrwave Transmission ~01. 14;
Circuits,
(3) Wavegwid. Handbook, Vol. 10; (4) Principles of Microwave Circuik?, Vol. 8.
391