Radar System Engineering

SEC. 11.11] DESIGN CONSZDERA TZONS FOR THE R-F HEAD 421
for confining the disturbances set up by the radar transmitter so that
they do not affect other equipment.
11.11. Design Considerations for the R-f Head.—The form which
the r-f head takes will depend on whether it operates on the ground, on a
ship, or in an airplane; on the degree of exposure to the elements; and on
the power and transmitting frequency of the magnetron. Pressurization
has advantages and disadvantages. Among the latter are inconvenience
resulting from inaccessibilityy of parts for adjustment and repair, and
difficult y of transferring heat through the pressure wall. The r-f head
of a small airborne set will undoubtedly be a completely pressurized unit.
The r-f head of a very large ground set might operate inside a protective
housing and not be pressurized at all. Between these two extreme types
are designs where part is pressurized antf part is not. No firm rules can
be laid down. The treatment here will outline the conditions that must
be met, and then give illustrative examples of two quite different designs.
Heat Removal. ‘—The maximum safe ambient temperature for most
of the r-f-head components—such as composition resistors, oiI-paper
condensers, and blower motors—is about 85”C. The temperature of the
air around the r-f head may get as high as 50”C in desert areas inside a
housing exposed to the sun’s rays. If the unit is not pressurized the permitted
dMerential of 35°C is easily met. The air from a simple blower,
properly channeled, will readily carry the heat released inside the enclosure
out into the surrounding air. Often the air from the magnetron
cooling blower can be so directed as to do what additional cooling is
needed. Subunits tightly enclosed for reasons of electrical shielding may,
however, need additional circulation.
Where the free flow of external air through the r-f head is blocked off,
as it must be by a pressure housing, the problem of transferring the heat
to the outside air can easily be a limiting factor in design. The difficulty
is not in getting the heat through the actual metal wall. A short computation
shows that a differential of a fraction of a degree is sufficient for
this. Almost all of the temperature drop occurs across the dead-air
films on the two sides of the wall. Natural convection results in a
transfer coefficient of only 0.006 to 0.010 watts/in2 per ‘C difference in
temperature between the air and the wall. If 30”C be taken as a safe
figure for rise of internal air over external air, and if natural convection
be assumed on both sides, then the average coefficient given above would
result in a maximum thermal load of 0.12 watt s/in2. Forced convection
from a gentle current of air along the surface raises the coefficient to 0.02
watt s/in2 per “C, but beyond this increased velocity results in onIy a slow
increase. A high-velocity flow of perhaps 50 ft/sec is necessary to
achieve a figure of 0.04 watts/in2 per “C.
I Amer.Sot. of Healing and Ventilating En~”neers Handbook,