
90-11Grounding and Shielding
Slots act like small antennas. Because they are antennas, the longer they are, the better they radiate.
Wh
ile
a ha
lf
wa
velength
is ve
ry
e
cient,
as a ru
le
of th
umb,
we li
ke
to li
mit
sl
ots
to
1
wavelength or
le
ss
at th
e
hi
ghest
fr
equency
of co
ncern
. Fo
r
10
0
MH
z,
th
is
is 15 cm (a
bout
6 in.); at 30
0
MH
z
(E
SD
fr
equencies),
th
is
dr
ops
to 5 cm (a
bout
2 in.), an
d
at 10
00
MH
z,
it is on
ly
1.5 cm (
2
in.). And even these
di
mensions
ma
y
be too la
rge,
as th
ey
on
ly
as
sure
20 dB (1
0-fold)
of sh
ielding
th
rough
th
e
sl
ot
. Cl
early,
ev
en
s
mall
s
lots
a
nd
o
ther
o
penings
m
ean
b
ig
s
hielding
p
roblems
a
t
h
igh
f
requencies
.
e ot
her
wa
y