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Television Signal Strength Calculation Charts
July 1952 Radio & Television News Article

July 1952 Radio & Television News
July 1952 Radio & Television News Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

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 measured levels.

Another article in this issue entitled "Map Your Fringe Area Signal Level" is useful here.

Television Signal Strength Calculation Charts for Proper Antenna Selection

Television Signal Strength Calculation Charts, July 1952 Radio & Television News - RF CafeVoltage 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 a Variac.

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 Step 3.

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

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