Radar System Engineering

34 TIIE RADAR EQUATION [Sm. 29
the neighborhood of the minimum. The signal threshold power plotted
in Fig. 2.5 is essentially the signal strength, relative to average noise
power, for\vhich anobserver \rouldidentify thesignal correctly nine out
of ten times. Foramore precise defir,itionof thee~perimental criterion
for threshold signal in these tests, the reader is referred to Vol. 24, Sec.
8.2. Our interest just now is in the position of the minimum of each
curve that is seen to lie near @ = 1.2/7. However, departure from this
+12
+ 10 I
+8
‘6
‘8
/ Y
/ L A“
[\
\
/’
‘ ‘% ~
-10
0.1 0.2 0.4 0.6 0.81.0 2 4 6810 20
I.fbandwithln Mc\sec
FIG. 25.-Signalt hreshold vs. i.f bandwidthforapulse durationof lpsec. The signal
power is measuredin unitsof the noisepower, .Y,, within a band 1 Me/see wide, In these
experimentsthe video bandwidth was 10 Mc/sPc,the signal presentationtime was 3 see,
andthe length of the pulseon the screenof the .L.scopewas 1,7mm.
value by a factor of 2 in either direction increases the minimum discernible
signal power by less than 1 db. The radar designer is inclined to
take advantage of this latitude to set @ somewhat greater than 1.2/7 as
this eases the requirement of accurate frequency control. .A vallle of CB
in the neighborhood of 2/r is typical of present practice; there is some
evidence also from controlled experiments on intensity-modulated indicators
(Vol. 24, Chap. 9) which favors this higher value.
The quantity S/kT@, which has been our primary concern in this
section, can now be rewritten by expressing @ in terms of T. If, for
simplicity, we require @ = 1/7 then S/kT@ becomes S7/k T. The prod-