The New York herald., 1922-03-26
LIGHTNING HAZARD
SMALL WITH RADIOS
Dangers Are Greatest When
Aerials Are High, Expert
ence Shows.

Radio telegraphy, being a relatively
new art, is greatly hampered by tradi-
tions handed down from a bygone age.
Not the least Of these, relic OF the old
linghning rod days of our forefathers. Is
the fear held by many as to an aerial
attracting lightning" Now, as a gen.
eral statement, there Is some truth in
this, but, like everything else, there is
no universal law to suit all cases.
Seventy-fve per cent. or more OF the
aerlals used by amateurs and novices to-
day in the United States are for receiv-
ing only. and the majority of these are
small, relatively low structures, running
from the top of the house to conven-
lent tree. Indoor loop antennae need no
lightning protection. OF the remaining
antennae used for transmitting as we'll
as receiving purposes large proportion
are Of relatively moderate height, say
fifty feet above-ground. but here and
there some ambitious youth reaches
heights OF 100 feet and In q few cases
even higher.
The chances of an areal serving pick
up dangerous lightning charges are
roughly in proportion to its height modi-
fed, OF course, by the presence OF other
grounded conductors in the vicinity. In
a general way the three cases Of (a)low
antennae. as 40 to so feet high: OBJ
moderately high antennae, such as 100
feet in height. and high antennae. $50
feet or more above the ground, will be
discussed here.
when 1t Can BE Protected.
Obviously lightning protection can be
applied to an antenna only when Il is
1.e IN aN Ill a.
for receiving. Any adequate lightning
protector attached TO a transmitting an
tenna would certainly consider the radio
energy TO be lightning as far as 1t could
distinguish, and rightly so, for every
amateur using a transmitter Is q minia-
ture Steinmetz with equally miniature
artificial lightning. Therefore any per-
manent attachment to an antenna FOR
protective purposes must be so attached
as to be disconnected (by send-receive
transfer switch. for instance), when
sending Is to be carried on.
Amateur lightning protection falls into
the following classes:
(a) BY an individual grounding
switch.
fb) BY an approved spark gap or low
resistance vacuum tube device between
antenna and ground.
Now, as stated before. method b can
be used only for a receiving antenna,
since the fire underwriters rules permit
the use of such a device only when the
aerial Is permanently connected to earth
at all times" For the amateur who re-
ceives only and who has only n low an
tenna, method b is ideal IL is even
better than grounding switch, because
the latter requires conscious action on
the part OF the operator. which is quite
/ 70. Ia
doors (ns II should be) and the weather
happens to be unpleasant. or II II Is
o'clock in the morning and there is Just
enough consciousness Nft in the oper-
ator to snap off the filament current.
For combined transmitting and rcceiv-
ing stations, an outdoor grounding
switch must be used. though, to give
protection to the receiver against
lightning. 1t is an excellent Idea to
shunt the primary circuit with n spark
gap or vacuum device permanently. In
addition to a grounding switch outside.
This switch should be double throw
with the antenna connected to the mid
die point. one side leading directly to
ground and the other side TO the trans
mitter.
Merely short circuiting the trans
mitter to ground is not protection
enough, as experience in commercial
and naval stations has shown. BY
little ingenuity. ropes can be attached
to the switch and brought through wall
bushings so that the entire switch can
he operated from within. But the in
stallation should be 30 that II Is easy
to see, from window near your set,
lust what the switch blades happen to
be doing at the time.
A stiil better scheme Ia to install a
three position rotary knife switch, the
antenna being attached to tho blade. and
the three contact points respectively to
he transmitter. to ground and to the
! receiver This. likewise, should
be installed outside the building,
and the shaft for rotating It can easily
he brought indoors through the wall.
little scheme 10 help one remember
- ground the antenna when closing the
station Is to place on the shaft. inside
the building, n commutator-switch for
cutting Off the filament current. Al
most always the last thing done before