Evening star., 1923-03-18
Hints on SelectiNity.


There are nearly 500 broadcast
ing stations in oneration in the
United states. In every LOCALITY "
is rossible to choose from several
stations Cn"rating at the same time.
In order to do this We should KNOW
some of the principles uhderlEins
Selrcrivity or tho power II rho:c".
which permits us to tune out IN" sta
lions operating on wave lengths
differing frcm the wave band of the
station we wish to hear
An electric current is caused 70
a"w in YOUR receiver 10y the waves
from the transmitting station. II
the receiver is tuned to the same
wave length as the tranSmitter TINS
current is of maximum strength anu
sounds received are. therefore. loud :
est Currents due to the waves OF
other frequency or wave lengths are
relatively much weaker. The reia-
tive strength Of the currents received
from the two stations OF equal power
and equal distances from the receiver:
when the receiver is tuned to one of
the transmitting stations. depends
m the difference in ways lengths.
tho design of the receiving instru-
ments and the receiving antenna.
BY properly proportioning the lndlc-
lance and the capacity of n circuit, the
ratio of the current is received on the
wave length TO which it is tuned. Inc
use of regeneration still further m
creases this ratio. The principles Just
stated govern the design of virtually
all receiving Gets If there is no
powerful station in the immediate
vicinity a properly designated single
circuit regeneratiVe receiver uSed with
an efficient aerial will select the de-
sired station and tune out all other
stations Two stations that are so
nearly on the same wave length that
a sustained note or howl is heard, can
not be separated by any known methods
OF tuning that will permit of the satis-
factory reception of music and speech.
If the tuning is made so sharp that one
Of the stations can be selected all notes
IN the music of a pitch equal TO or
higher than the note heard. due TO the
interfering waves. will bs tuned out.
Therefore il is evident that the only
remedy for this condition is an assign
nent of wave lengths that VIII preFehl
two or more stations from transmitting
at the same time on wave IenSthS
that will produce such interference
II there are two powerful broadcast
ing stations in the vicinity and neither
can be received without interference
from the other. it is advisable to use
A small antenna. Very often an indoor
antenna of small wire concealed by a
picture moulding will be entirely salts
factory and does not require lightning
protection. as il is indoors. II this very
small antenna is not sufficient an an
tenna may be used consisting of single
wire strung in the attic. Ii the small
antenna and a single circuit regenera-
tive receiver is used the senSitlvity OF
the system is in general only slightly
less than when using a large antenna.
and the strength of the Interference is
reduced in proportion to the respective,
effective heights of the antenna. When
using small antenna the adjustment
for tuning and regeneration must be
made more accurately in order to bring
in weak stations.
There is no doubt that the neare st
high powered broadcasting station
can render better service to you than
can the station at great distance.
because of fading and atmospheric
disturbance. If the reception OF very
weak stations is not attempted while
the more powerful or nearby stations
are operating the small antenna and
regenerative receiver VIII be found to
meet all requirements.
A convenient method of reducing
interference from one station is to
connect circuit tuned to the inter
fering station, between the antenna
and ground terminals of your re-
ceiver. This circuit will pass the
current due to the interfering station
to the ground. and if the circuit has
considerable inductance and small
capacity the desired signals are not
noticeably weakened. The operator
of the tuner is not complicated to
the extent that Il is when two cir
cults must be adjusted to tune to
a station. When the tuned bypass
circuit has been adjusted to elm
mate the interference, it requires no
further attention.
These coupled and bypass circuits
will assist in receiving distant sta
lions when the local stations are
operating: but the simplest arrange
ment and a very satisfactory one for
receiving one of the several local
hroadcasting stations or one of the
more powerful distant stations is the
small indoor antenna which mas been
mentioned.