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Field Signals

Field Strength Meter

Broadcast stations must know the strength of their signals; this parameter is known as Field Strength. An engineer can use several instruments to measure field strength.

Measuring field strength is more of an art than a scientific endeavor. There are no mathematical equations, for television, that can yeld accurate predictions. In theory, it would take several supercomputers to account for all of the reflections, fresnel zones, and atmospherics. In practice, about 5dB is all the accuracy one gets in measuring field strengths.

The FCC requires the signal strength of AM stations to be measured quarterly along radials in all directions from the base of the tower(s). The measuring points are well established, with the idea that the resulting signal strength values are repeatable at any one location. From the data of many locations, a broadcaster not only knows the strength of the transmitter, but also the radiation pattern.

I took it upon myself to apply the same concept, of that used in AM, to television; although, the FCC does not require monitoring points for television. Over the years, I have carefully chosen monitoring locations. I choose locations that were line-of-sight to the particular transmitter of interest. I chose sites that were on hills (if possible); I choose open areas, free of obstructions, such as trees, buildings, cars, or power lines. Over the years, it seems that as soon as I would find a new good spot, I would loose two others to housing projects, or restricted private property. But that was not my main problem...

I used a dipole on several poles of differing heighths. I would be lucky to get consistent readings withen 5dB. 5dB is a factor of 3 times! And, all the while, I knew the power of a transmitter withen 2%. Such a waist of accuracy!
I reasoned that the best location would be about a half a mile up in the air. I reasoned further: that the second best, would be on top of a high tower; I had to check it out. I stuck a dipole and several PVC sticks, into a backpack, and climbed to the top of a 300ft tower, 16 miles from a transmitter. At the top, I took out the dipole, and installed it on a 2ft pvc section. I quickly could see that I had a very strong signal, almost exactly what I was expecting from my calculations. But the signal was jumping all over the place - wildly swinging 20dB. I stuck on anther stick; And then another. No change! I was out 8 ft from the tower with my body leaning out, and arms outstretched as far as I could reach. I waived the dipole up, down, and all over. The problem was the metal of the tower. I could not get free of the tremendous amount of metal. The whole experiment was a dismal failure, and drew attention to a waving guy on a tower. I am lucky a van did not pull up and take me away.

I next decided to stay on the ground, but to figure out the mathematical equation for a virtual-hight above ground. I would not need to know permittivity constants, nor water and mineral content of the soil. I devised a new and consistent method to operate the poles. Now, I would assemble all the sections together at once (for a total of 18 ft). The limber 18 ft pvc pole was heavily bowed with the weight of the dipole at the end. I would brace one end of the pole against my foot, and lift the other end (with the dipole) one foot off the ground. (People, in passing cars, probably thought that I was fishing out in a dry field, and had just landed a "big one".) A one-foot-reading was then recorded. Next, lifting the pole a little, a reading was taken at two feet above ground. And then another at 5 ft, and another at 10ft. And finally, one at 18 ft, with the pole straight up overhead. At 18 ft the pole was no longer bowed, but now swayed, back and forth, high above my head. With this information, I calculated signal strength at any "virtual height". I had to, unfortunately, take several time consuming readings at each location. The technique was definitely helpful in the eliminating ground effects; But I still could see the effects of passing cars - hundreds of feet away - moving my needle back and forth. This persistent problem is known as "multipath", and is intolerable. Not the best solution. And I was getting sick of fishermen from the highway, stopping their cars, and asking me: "What the hell are you doing?"

One time, while climbing Mt Lassen with the family, I packed in a signal strength meter, a dipole, and several sticks of pvc. After explaining to my wife why I was doing such a stupid thing as carrying an antenna to the top of a mountain, I met a ranger. I had to abandon my first reason, and I had to think of a different reason for the ranger - that did not involve love, or mushy stuff. The real - "main" - reason is that I had no readings from the north side of the transmitter. What I go through...!

Field readings from equipment...

Another source of field readings are from static installations, such as translators. An engineer can tell if there has been a "change".

During the last years, I hit upon an idea that was the most succesful of all. I had been noticing that my high gain yagis, that had solid mounts and permanent locations, yielded consistent readings (withen 2dB). And this made sence! Because side effects, such as multipath, were disfavored by the antenna design: A yagi sees only a small area.

If you want to get good field readings of FM, VHF, or UHF, digital or analog, then buy a high gain test yagi that is light weight, but as long as you can handle. I use "channel-cuts", which are antennas built for only one frequency. I have several that I always carry in company trucks. I carry two that have exactly 10dBd gain; which is a simple matter to subtract 10dB to obtain the signal strength for a standard (isotropic-2.1dB) half-wave dipole.

There are other advantages - of big long yagis - that I never imagined...

One is that you no longer need chosen locations that are extensively cleared. A high gain yagi can tolerate more buildings nearby, and can tolerate moving tree limbs to the sides.

Sun coming through the glass out in the field.
Field strength using a spectrum analyzer inside a truck.
There are benefits with access to power inverters and bigger instruments...

Another advantage is that you can now mount the antenna on a truck or van. And as a consequence, can use a heavy spectrum analyzer in the van to measure not only field strength, but also, a whole world of other things.

bigger instruments...
"Overkill" for Field strength, but worth it.

bigger instruments...
Now, dozens of more things can be measured.
In this view...
Field strength: (measured with attenuator, gain, and bw settings)
Signal to noise: 38dB
Vis/aural ratio: 9dB
RULERMAR.GIF, 1 kB

dBm:

dBm is a "power-level" - not field strength.
Expressed in dB, referenced to one milliwatt.
Usually defined as the power delivered to a 50 ohm circuit.
dBm is not recognized or sanctioned - as a proper nomenclature - by physics as outlined in the SI system. Unfortunately, the "dBm-slang" is used everyday in the world of broadcasting.
Although technical instruments measure voltage, a greater display range can be achieved by squaring and converting to power. The unit is found on all portable field strength meters.
The unit is also found on all spectrum analyzers.
The unit is also used extensively for RF levels in general.
The exponential term "dB" is a total exponent - no mantissa - and replaces the exponential terms of "micro, milli, kilo, ...", which, in science and scientific notation, are the only accepted terms. Despite the "superfluous coinage", the unit is vital to everyday business.

dBu, dBu/m

dBu is Electric Field intensity squared
Expressed in decibels, referenced to one microvolt/meter squared (dB�V/m)
If it were not for the "decibels" and squaring, dBu would be the "classical" field strength, as defined by all physics conventions. The usage of the "volt" and the "meter" define proper units in the SI system. The unit was developed around WW2 when slide rules were the only convienent calculator. In those days, people found it easer to add than to multiply; handycapped, they sacrificed simplicity for workability. To measure true field strength, one must know the wavelength and antenna effective area (gain). Manufactures have found that manufacturing circuits to differentiate frequency - even if a calibrated antenna is used - as too costly. In my 35 years in broadcasting, I have never seen a portable field strength meter that measures "field strength".
Fields can not be felt or measured directly; They are only derived by associated - and manifested - forces or voltage. Field strength - necessarily - is a calculated unit, howbeit profoundly fundamental.

dB�V, dBmV

dB�V and dBmV are "voltage-squared-levels" - not field strength
Expressed in decibels, referenced to one microvolt squared, or one millivolt squared (across 50 ohms).
The dBuV is exactly the same circuit as the dBm; but one references the wattage, and the other references the voltage-squared. As a consequence, the voltage-squared is a value 107 dB greater than the wattage. (if the wattage has a value of one (mW), then the voltage has a value of 223607 (uV): 107dB greater) Unfortunately, the unit is found on most portable signal strength meters. You can take the square root, or divide the dB by 2, to use the meter as an honest voltmeter. All meters on this planet, measure only voltage. This may be a cheap crystal-set with a capacitor, or an advanced spectrum analyser. "Volt-meters" are easy to manufacture; all you have to do is arbitrarily devise a "volt-unit" to be printed on its face. Meters, as well as the use of this unit, are employed out of convenience and practicality when measuring a "voltage" produced by an RF field. Beautiful science does not always win; In this case, it lost the battle to an insane practicality - gone-wild.

�V

uV
The literal voltage at the input of an RF instrument: A true unit; Not an "interpreted" measurement. Used on Service-Monitors, receiver instrumentation, and many unrelated instruments.
Absolutely a valid unit; accepted by all disciplines; direct, descriptive, and intuitive. And useful to calculate field-strength...

Field-strength - in it's native language - is simply the microvolts at the input of the receiver times an "antenna factor"; yielding uV/m. You do not need a sliderule! You do not need to resort to adding! Today, as a modern person, you are perfectly capable of multiplying these two numbers! This unit of uV can be the same as the unit of electrical field strength uV/m: A one meter long antenna, of unity gain, of one meter wavelength, with no losses, will be both the same value of voltage as field-strength.