Although not directly applicable today, these charts from a 1952 issue of
Radio & Television News magazine showing signal voltage levels versus distance
from broadcast television transmitter locations provide a general sense of how attenuation
varies as a function of distance. Both low (channels 2−6 at 54−88 MHz) and high
(channels 7−13 at 174−216 MHz) VHF bands are included for a couple
different standard power levels. A Transvision Model FSM−1 field strength meter
(see example at left) was used to construct these charts. While the
Friis equation for signal free space attenuation
can be used to predict levels, actual physical measurements are often more useful
in real-world scenarios where landscape and manmade obstructions can affect
Another article in this issue entitled "Map Your
Fringe Area Signal Level" is useful here.
Television Signal Strength Calculation Charts for Proper Antenna Selection
Voltage Ratio -dB Chart
Signal Level Decline Charts
These signal level charts show the decline of signal level in db and in absolute
microvolts for typical high- and low-frequency band television stations measured
as a function of distance from the stations. It should be noted that the absolute
values of the signal in microvolts is a function of transmitter power, transmitter
antenna height and gain, frequency, terrain, and receiver antenna height and gain.
However, the signal decline in db is essentially constant for the two low-band stations
and for the two high-band stations and therefore have universal application.
In the preparation of the charts, the zero db reference point was chosen for
a signal level of 10,000 microvolts. The absolute microvolt readings were based
on an antenna height of 30 feet, with a standard folded dipole for each specific
channel measured. The transmission line used was 300- ohm twin-lead, 30 feet long.
A Transvision Model FSM-1 A field strength meter, accurately calibrated, was used
for the measurements, together with a Cornell-Dubilier Model 6R5 power converter
(to convert the 6 volts d.c. from an automobile battery to 117 volts a.c.), and
If another type of antenna is used, the absolute readings given on the charts
should be increased by a factor determined by the gain of the antenna. For example,
if the antenna used has a gain of 3 db on Channel 5, the microvolt scale should
be increased by a factor of 1.41 for all distances.
Since, as a rough approximation, signal voltage varies directly with receiving
antenna height (in the clear) doubling the antenna height would double the signal
all along the microvolt scale.
The charts permit you to predict average signal levels at any distance from a
station if a reading can be taken at some point within the range indicated using
the same standards of measurement. The absolute readings indicated in the charts
were recorded during midday hours which represent average propagation conditions.
In general, it can be said that these measurements were taken under average-to-poor
propagation conditions in the atmosphere. Night reception conditions are better
- particularly beyond the distance represented at 1000 microvolts. The dotted curve
on Chart #4 shows the influence of atmospheric conditions on fringe signal levels.
This curve is the result of measurements made just a few hours past midday, and
shows the start of a signal level rise that extends into the evening hours. The
percentage of increase is greater with distance. Installations should be planned
to handle weak signals adequately for performance reliability. Thus, it is advisable
to present the average-to-poor conditions.
Charts #1 to #4 are signal decline charts. Chart #5 shows the relation between
db and signal voltages. For example, if between 24 and 32 miles on one of the decline
charts the signal drops -6 db, it means that the voltage at 32 miles is one-half
the voltage at 24 miles.
Typical Chart Applications
How to predict signal levels in your area using the charts.
1. Take a sample measurement of the field strength of your local television
station at a prescribed distance from the transmitter. Choose a position of average
terrain height in your district and use the standards suggested.
2. locate this point on the curve (intersection of distance range calibration
with the curve) representing the channel nearest in frequency to the one with which
you are concerned. This locates a single point of measurement on the curve.
3. Determine the db differential for that point between your measured value
and the chart value. Look up the voltage ratio for this differential.
4. Microvolts at other distances can be found by multiplying the microvolt
reading on the chart for those distances by the microvolt ratio as determined in
5. Another method is to find the distance at which 10,000 microvolts is
delivered by the station. Locate this point on the graph representing the channel
nearest in frequency to the one with which you are concerned.
6. Draw a new curve on the graph that is parallel to the curve already
on the chart, starting from the 10,000 microvolt reference point of Step 5. With
this method, the microvolt scale on the right hand side of the chart can be used,
eliminating Step 4.
How to select the proper gain antenna system for a low signal area installation.
1. Using the chart you have corrected for your specific channel and area
condition, look up the microvolt level indicated for the distance at which the installation
is to be made.
2. Find on the chart the db difference between this level and the minimum
microvolt sensitivity of the television receiver to be installed. This db differential
is the db gain required of the antenna, or of an antenna, mast, and booster combination.
3. To raise the gain of the signal at the receiver, either a gain antenna
can be used alone, if its gain is sufficient, or used with a booster. The use of
a mast, of course, will increase the microvolt level at the receiver. If a receiving
antenna is raised to sixty feet, the signal will be approximately twice that shown
on the chart for any specified distance (since these measurements were taken with
on antenna thirty feet high).
Posted October 19, 2021