T EVIS i - AmericanRadioHistory.Com

Fig. 1.
(A) Right. a corn.
merciai mast -mounted
TV booster. (B) Below,
matching power supply.
Fig. 2. A singlechannel
TV booster.
By DOUGLAS H. CARPENTER
IHE notable improvements in TV
receiver performance in the past
few years are a real tribute to the
engineers responsible for the advancement
of high frequency circuitry. The
newer television sets exhibit better
contrast, resolution, sensitivity, and
most important, a greatly improved
signal -to-noise ratio directly attributable
to marked changes in tuner design.
The introduction of efficient high
gain tubes, such as the 6BQ7 and
6BK7, has resulted in excellent low
cost "front end" amplifiers, formerly
the stumbling block to extended TV
coverage. Hand in hand with receiver
improvement has grown the problem
of providing acceptable preamplification
or signal "boost" in areas remote
from the TV transmitter.
The basic problem of so- called
"fringe" reception is to obtain enough
signal from the antenna system to
override the inherent losses in the
transmission line, and permit the
booster -set combination to be "driven"
hard enough to provide a satisfactory
picture. In other words, the signal -tonoise
ratio at the receiving system
input has to be of a value sufficient
to allow proper synchronization of the
receiver sweep circuits, with enough
overriding signal amplification to take
advantage of weak signals. The enemy
of such a desirable state of affairs is,
of course, cumulative noise.
The first element to be considered
in successful fringe reception is the
television antenna itself. Stacked
yagi types provide consistent results.
A seldom considered advantage of the
yagi antenna is its restricted bandwidth.
This response "slices" off, or
greatly attenuates noise at the channel
edges. The sharp horizontal and
vertical directivity patterns of such
an antenna system permit only that
segment of noise picked up in these
sharp patterns to be delivered to the
receiving system. A broadband system
on the other hand, may have excellent
gain, but will amplify all noise at
70
An analysis of modern booster circuits and the factors
affecting optimum performance at very high frequencies
adjacent frequencies. When extended
TV reception is desired, cumulative
noise becomes the most formidable
problem.
Noise, or TV "snow," may be listed
categorically. The first offender (which
at this moment we can do nothing
about) is thermal noise due to the
movement of electrons in any conductor.
A second type is random noise
(static, ignition, etc.) which is distributed
across all of the v.h.f. -band
frequencies, and extends to a slightly
lesser degree through the u.h.f. spectrum.
Two additional types, over
which the engineer has some control,
are "shot" effect and partition noise.
"Shot" effect is due to the irregular
arrival of electrons at the plate of an
amplifying tube. Partition noise is
caused by the unequal division (vs
time) of electrons between the screen
grid and plate of a pentode amplifier.
To eliminate partition noise, it is
only necessary to restrict design to
triode amplifiers. A pentode amplifier
has inherently high gain and at first
it might be thought that this would
be ideal for fringe amplification. The
high noise factor of pentodes, however,
limits their usefulness to i.f.
rather than r.f. applications. In deference
to manufacturers still making
this type of booster, it might be said
that unusual although rather unpredictable
results are possible with such
a circuit. The semiregenerative characteristic
of a pentode tube at these
comparatively high frequencies injects
positive feedback that, in turn, raises
the "Q" of the input circuit and restricts
bandwidth faster than it increases
noise. A marked improvement
in picture reproduction may be
realized at the expense of reduced
audio carrier level. This is a simple
method of obtaining sharp selectivity
with poor resolution. Parallel amplifiers
for sound and picture with positive
feedback is certainly a feasible
idea, but the complexity of common
termination through selective filtering
has not as yet been solved.
To further stress the importance
of signal -to- noise, it might be in order
to consider the entire approach to pre -
amplification in the present commercial
booster market. The first amplifiers
available to the TV public were
of the capacitively -tuned type and
employed pentode tubes. As a rule,
these amplifiers covered either the
high or low television channels, and
provided signal improvement generally
because early television receivers
themselves had rather low sensitivity.
Noise as such was no better (or generally
poorer) in receiver front ends
than that generated in these early
boosters.
Gradually, receiver front ends obtained
higher gain from using improved
pentodes such as the 6CB6
with lowered noise factor. RCA led
the way to improved fringe reception
with the still famous 630TS chassis
incorporating a matched (300-ohm)
input with a push -pull triode circuit.
This arrangement gave sufficient amplification
to deliver signal with minimum
noise to succeeding stages. At
this point, the early variable gain,
variable bandwidth boosters began to
disappear from the commercial market,
and circuits of the balanced triode
push -pull type came into popular
acceptance. The Wallman cascode circuit
consisting of a grounded- cathode,
grounded -grid amplifier was introduced
at the same time but was not
generally used in receiver design due
to unavailability of highly efficient
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