The
indirect
method
(no
physical
connection) of measuring transmitted fre-
quency
is
usually
the safest
and
most
convenient. Using the
Telscopic
Antenna
(optional
accessory)
plugged
into
the
appropriate input
on the count-
er,
set the
transmitter to
its minimum input
power
and
initially measure
the sig-
nal some five to
seven metres from
the transmitter
aerial.
This
is usually
ad-
equate
for
H.F.
signals. ln tests,
y4
watt
attenuated output
from
a
C.B. trans-
ceiver
gave
stable
readings when
fed
to
a 1lz
metre
base
loaded
whip antenna
about 10 metres
from
the
frequency
counter. Where low
power
transmitters
with
inefficient
aerials
are
used
(as
with some hand-held transceivers) the
sep-
aration
may need
to be
reduced
but,
in
practice,
the aerial
rarely
needs to be
closer than
1
to 2 metres. Where
a
high-power
transceiver
is
being
tested
and
the
power
level cannot
conveniently
be reduced,
use
the Telescopic
Antenna
fully closed
or
a simple
pick-up
coil
of
2 or
3
turns, 2"
in diameter
and earthed
at
one end.
With
direct
connection
of
the transmitter
to the Frequency Counter,
it
is most
important not to exceed the maximum input
voltage rating.
For example, con-
sider a 5 watt transmitter on
the
amateur 2 metre band
radiating
at 145MHz.
The
maximum input
is
2V
rms
into
50Q(lnput
B)
representing
80mW.
There-
fore the
minimum
attenuation necessary is
10
log
(5/0.08)
dB
or 18dB. ln
prac-
tice, as the Meteor
series of Frequency
Counters
are
so sensitive,
30dB
would
be safer
and still allow
a
comfortable
input
level of
0.5V rms. Note the
necessity
of
using
an adequately rated
attenuator
(5W
in this case). Input
A
has
a
high
input impedance
and a
suitable
'through'
matching termination should
be
used
when making
connection with coaxial
cables.
Modulated Carriers:
F.M
-
Narrow band
F.M. as used with
many modern
communications
trans-
ceivers
should
present
no
problems
as the
counter
essentially
averages
over the
gate
period.
Wide
band
F.M.
is rarely encountered but
should
give
acceptable
results with longer
gate
times.
A.M.
-
lt
is
difficult to
measure
highly modulated
A.M.
signals
as,
at some
points,
the
carrier
reaches very
low
levels.
Further,
the mean
power
may
con-
siderably exceed the nominal
power,
so
great
care must be
taken
when
cal-
culating attenuation for
direct
coupling. lf
the
signal
can be
measured
when
free
from modulation,
the
results
are
always
more
predictable.
S.S.B.
-
Single
side band transmissions suppress the
carrier which therefore
cannot easily be measured. lf an
audio
signal
generator
is available, transmit
a
lkHz tone
and
note
the
measured frequency
indicated
on the
counter. Sub-
tract lkHz from this reading for true carrier frequency
with
upper
side band
transmission
{and
vice versa
for
lower side band).
C.W.
-
Measurement is
straightforward.
lf
using a
morse
sender,
the
key must
be kept
depressed.
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