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Have you noticed how heavily burdened
utility poles (formerly referred to as electric poles or telephone poles) are these
days? Many of the decades-old creosoted wooden poles originally were designed to
carry a single set of high voltage distribution lines (13.8 kV) and a multiconductor
telephone cable containing twisted pairs. Look around now and you will see at least
twice that number of cables, and often three times as many due to multiple coaxial
and fiber optic cables and needing to route extra AC power circuits (of larger gauge
to handle higher current) in increasingly crowded areas. Notice how many are leaning
over (particularly at corners) and/or are being supported with additional guy cables
and/or splints. Half a century ago when music, talk, and television was broadcast
over the air, the need to string wire all over the landscape was not necessary.
Some people complained about "ugly" TV and radio station broadcast antennas, but
their numbers paled compared to all the cellular network towers littering the landscape
these days, and the "carbon footprint" was way smaller. The "wireless revolution"
is anything but wireless, and it certainly is not eco-friendly.
Another article in this issue entitled "Television
Signal Strength Calculation Charts" is useful here.
Map Your Fringe Area Signal Level
 By Edward M. Noll and Matthew Mandl
Practical information on measuring and plotting signal levels to facilitate TV
installation work.
In a fringe or near-fringe area service organization, it is important for both
the sales and service departments to know the signal level distribution (for each
channel) throughout the district. Such information is of help in choosing a suitable
receiver, deciding whether or not a booster is necessary, and selecting the correct
antenna type for the location.
There has been a definite need for a practical and useful link between propagated
signal levels, service type field strength meters, and receiver performance. The
signal level "Data-Print" on the facing page, together with the measuring equipment
and measurement standards here introduced, represent a practical plan for predicting
and solving signal level problems in a given locality.
The charts can be used to predict local channel signal levels as a function of
distance. Antenna gain and height can be linked directly to the chart information,
thus assuming a more understandable significance. The method of making such measurements
is simple and permits the service organization to take sample measurements in its
own locality. The information thus obtained can be added to the "Data-Print" charts
to give more specific data concerning stations in its own area. The "Data-Print"
will then reflect the signal level variations due to terrain differences, climatic
conditions, etc. Using the charts, range maps can then be prepared showing the location
of specific signal level contours for each station. Finally, a sectionalized map
of a single community or district can be prepared with finely calibrated contours
and terrain height corrections. Such a map will enable a service organization to
select the proper antenna and a booster, if necessary, for any installation without
further measurement.
Fig. 1 - Signal range map for Channel 5 (N. Y.) and Channel 3
(Philadelphia). The receiving antenna for these measurements was a standard dipole,
cut for each channel. For Channel 5, the 500 μv. radius is 60 miles from the
transmitter: the 25 μv. radius is 104 miles. For Channel 3. the 500 μv. radius
is 38 miles: the 25 μv. circle is the signal level 81 miles from the transmitter
site. Copyright American Map Co., Inc., New York, No. 11343
Fig. 2 - A signal range chart for Channel 5 (N. Y.) and Channel
3 (Philadelphia) using a conical reflector receiving antenna to obtain the readings.
Channel 5, whose transmitter is at the Empire State Building, delivers a 25 μv.
signal over a radius of 112 miles and a 500 μv. signal at a radius of 64 1/2
miles. Channel 3 (which is transmitted from a suburban Philadelphia location) delivers
a 25 μv. signal at a radius of 90 miles and a 500 μv. signal at a distance
of 40 1/2 miles from the transmitter as shown by the concentric circles on the chart.
Fig. 3 - Signal range map for Channel 9 (N. Y.) and Channel 10
(Philadelphia). For Channel 9. the 500 μv. radius is 41 miles from the transmitter;
the 25 μv. radius is 76 miles. The 50 μv. radius for Channel 10 was determined
using dipole. conical reflector, and Yagi antennas. The circles show the effect
of using succeedingly higher gain antennas. The yagi antenna will supply the receiver
with a 50 μv. signal at 68 miles from the transmitter; the conical reflector
at 62 miles; and the standard dipole will provide this figure at a distance of 49
miles.
Field Strength Equipment
The equipment used to take the field strength measurements for this article consisted
of a field strength meter (the Transvision FSM-1A), a power converter, a "Variac,"
and a folded dipole, mounted on a thirty-foot mast, for each of the channels measured.
The power converter was a Cornell-Dubilier Model 6R5 which converts 6 volts d.c.
to 117 volts a.c. The power plug and cord were the type used with an auto trouble
light so the 6 volts for the converter could be obtained by plugging into the cigar
lighter receptacle of the car. The "Variac" was used to supply constant voltage
to the field strength meter for each measurement. Thirty feet of 300-ohm twin-lead
was used between the antenna and the meter. It was found advisable to keep the car
engine running whenever measurements were taken to keep the car battery charged.
Measurement sites must be chosen carefully. The antenna used for the measurements
must be erected in the clear and away from large metallic objects, power lines,
and telephone wires. In urban areas, lots, open fields, athletic fields, etc., are
likely locations for taking readings and erecting the mast (use three ten-foot bolted
or locked telescoping sections). Fringe area measurements can be conveniently made
along the flat stretches of a turnpike and at various distances around the outskirts
of small towns. Two ten-foot mast sections on a ten-foot rise or embankment will
give the required thirty-foot elevation.
It is important to realize that the more measurements made, the better the variables
will average out. This permits construction of a smooth average curve of signal
decline because of the many plot points.
Signal Range Maps
The signal decline charts on the "Data-Print" indicate the type of plot that
can be constructed for any television area. Measurements were taken and charts plotted
for Channels 3 (Philadelphia), 5 (New York), 9 (New York) and 10 (Philadelphia).
The chart covers two channels in the low-frequency and two in the high-frequency
television bands. For those channels not specifically covered, use the chart for
the closest channel. For example, for the signal range of Channel 2, use the chart
given for Channel 3; for Channel 7 use the Channel 9 chart, etc. The strength of
the signal (in microvolts) can be obtained by taking a few sample measurements in
your area and using the signal decline charts to predict signal levels at any given
distance. For example, if the signal level obtained from your local Channel 5 station
is only 500 microvolts at fifty-three miles instead of the 1000 microvolts (refer
to Channel 5 signal decline chart), the entire microvolt scale is simply halved.
The microvolt scale is changed by what-ever ratio exists between actual and chart
readings for the range at which the sample measurement is made.
The signal range maps are constructed from the information given in the signal
decline charts. Each contour represents a certain signal strength for a particular
station at a given distance from the transmitter location, as obtained from the
chart for that station. The first signal range map (Fig. 1) depicts 500 and 25 microvolt
contours for New York's Channel 5 and Philadelphia's Channel 3. These signal levels
represent actual microvolts applied to the input of a receiver or to the 300-ohm
input of a service type field intensity meter that has been calibrated accurately.
It should be noted that similar microvolt contours for New York's Channel 5 and
Philadelphia's Channel 3 are at different distances from the station. This is a
result of different erp's, antenna heights, and terrain conditions. Thus, the factors
of erp and transmitting antenna height have appreciable significance in the fringe
range of a station.
An explanation of the 500 and 25 microvolt choice is instructive. For the average
television receiver manufactured during the last few years an input signal of 1000
microvolts (at the tuner input) was the level at which the receiver noise became
apparent by very close observation of the line structure of the picture adjusted
for normal contrast (first appearance of faint snow effect). When the signal level
is at 50 microvolts, the snow effect is severe and the picture is near a point at
which it cannot be considered tolerable for satisfactory viewing. With the new cascode
low-noise type of tuner, levels of 500 and 25 microvolts are a better approximation.
The maps show extended range possibilities of lower noise levels. It is true that
these noise conditions vary from receiver to receiver and from channel to channel.
Nevertheless, the values chosen represent an ultimate practical value.
In summary, such maps will tell you, as a function of channel (low or high band)
and erp of station, etc., the approximate microvolts of signal that can be expected
at the receiver input at a given distance from the station.
The logic behind the choice of a dipole reference is demonstrated in Fig. 2.
Normally, gain figures of various commercial-type receiving antennas are given (or
should be) with reference to the signal delivered by a standard dipole. For example,
if an antenna with a gain of 6 db was used instead of a reference dipole in the
case of New York's Channel 5, there would be twice as much voltage at the receiver
input for each of the contour distances from the station. Likewise, readings taken
from the microvolt scale on the signal level chart for Channel 5 on the "Data-Print"
must be doubled. This indicates that the actual contour values of Fig. 1 are further
separated from the station transmitters as a function of the gain of the antenna.
This range extension factor is demonstrated in Fig. 2 for a conventional conical-reflector
type of antenna with a gain of 3.5 db on Channel 5 and 2.5 db on Channel 3. Just
how much farther the contours fall can be calculated from the signal level charts
on the "Data-Print." Simply raise the curve by the db gain introduced by the antenna
and read the microvolt-distance figures directly off the chart for the new curve.
With these new figures, draw the new contour. The service organization can follow
this same procedure for whatever type antenna used.
For example, the 500 microvolt contour for Channel 3 is 38 miles out, using a
standard dipole with the measurement standards specified. This point on the chart
is at the -26 db level (chart 1). With a conical-reflector at this distance, the
signal level at the receiver would be 1.33 times 500 or 665 microvolts (2.5 db antenna
gain). The actual 500 microvolt level for the conical-reflector would be located
at a distance represented by a signal decline of 28.5 db (26 pi us 2.5). On the
chart, this level is located at the 40 1/2 mile point.
The New York's Channel 9 portion of Fig. 3 shows the 500 and 25 microvolt points
for this station. Notice that these contours are nearer to the station location
than they were for the low-band stations although transmitting powers are higher.
To demonstrate further the influence of antenna gain on microvolt contours, the
Channel 10 (Philadelphia) portion of Fig. 3 shows the 50 microvolt contours for
a dipole, a standard conical-reflector, and a Channel 10 yagi. Notice how the 50
microvolt contour can be extended further and further from the station with antenna
gain.
A number of important facts are revealed by the signal decline charts and range
maps.
The rate of signal decline is much faster than the practical rate at which gain
can be added to an antenna system. For example, using the Channel 3 signal decline
curve we note that the 1000 microvolt contour is at 31 1/2 miles . Just 26 miles
beyond this, or at a range of 57 miles, an antenna with a phenomenal gain of 20
db would be needed to bring the signal up to the 1000 microvolt level. The signal
decline averages one decibel per mile over most of the range.
We cannot expect too much from antenna systems in fringe reception. For example,
let us assume we have a good quality receiver that does not show any effect for
a 500 microvolt signal. Now, at a range of· 70 miles (Channel 3) we have a signal
of approximately 45 microvolts. This level produces a useful signal but one with
high snow content. Suppose, in our enthusiasm, we decide to double the antenna height
(up to 60 feet) and use a high-gain yagi (12 db). This more elaborate installation
raises our signal by perhaps 15 db (assuming a conservative 3 db line, installation,
and mismatch losses). This brings the signal level up to approximately 350 microvolts
which is a decided improvement but still not high enough to take the snow effect
out of the picture. This example also indicates why you should not expect too much
from the increasing antenna height-it is quite a jump from a 30 footer to a 60 footer.
The improvement is there and is worthwhile but don't expect miracles.
Posted October 18, 2021
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