Radar System Engineering

670 MO VINGTARGET INDICATION [SEC. 16.18
The end cell shown in Fig. 16.10 is highly efficient as an acoustic
absorber but is useless outside of the laboratory because the unsupported
quartz plate can be cracked by standing the line on end. Several methods
of supporting the crystal from the rear while retaining the mercury as a
backing medium have been tried. A more promising approach to the
design of an absorbing end cell is the use of lead soldered to the quartz.
The lead provides a good acoustic match to mercury, while strongly
attenuating the supersonic energy that it receives.
The use of absorbing end cells is
preferable with mercury lines of
m [ ‘E;:;;’ delay less than about 500 met.
/
With longer lines, the use of reflecting
end cells may be better for two
reasons. In the first place, attenuation
always takes place within the
v’
@. . mercury. Thus, by the choice of
steel
ISctrode carrier frequency and tube diameter,
Ftc+. 16.34.—A reflecting end cell for a it is possible to reduce the amplitude
mercuryline.
of unwanted echoes to less than 1 per
cent of the desired signals. Second, the use of absorbing end cells costs
12 db (6 db per end). The construction of a reflecting cell is shown in
Fig. 16.34. The quartz is supported directly by the steel electrode.
The occluded air film provides the necessary acoustic mismatch.
Design Constants for Mercury Lines.—The delay time is presumed to be
given. From it, the lentih of mercurv column can be calculated according
to the formula
D = L(17.42 + 0.00522’), (18)
with an estimated probable error of 0.06 per cent between
10”C and 40”C,
where D is the delay in microseconds, L the length in inches, T the centigrade
temperature.
The two quantities to be chosen, along with the line configuration and
the end-cell type, are the carrier frequency and tube diameter. These
enter into the design mainly in connection with attenuation, bandwidth,
and demodulation,. The over-all delay-line attenuation can be divided
into two parts: that which occurs in the medium
itself, and that which has
to do with the efficiency of the quartz crystals in converting electrical
energy into acoustic energy.
Both carrier frequency and tube diameter affect the attenuation that
occurs within the delay medium. The free-space frictional attenuation
in liquids is proportional to the square of the frequency. In mercury this
attenuation is about 0.11 db/in. at 10 hlc/sec. The attenuation due
to wall effects is less clearly understood but is believed to vary inversely