Thursday, June 23, 2011

Field Strength Calculations: Measurements

Continuing on with ground conductivity, the higher the conductivity the farther away the station's signal will be copyable. Study the accompanying graph showing the range of a 600 KHz signal at the 50KW power level at various ground conductivities. 50 kilowatts over good ground conductivity of 15 mS/m will produce a very copyable signal at a distance of 375 kilometers (233 miles). At an average to poor ground conductivity of 4 mS/m, coverage is reduced to under 200 kilometers. Excellent ground conductivity of 30 mS/m takes the same signal out to more than 500 kilometers! A purely over-seawater path (5000 mS/m) should result in a fair signal out to about 750 kilometers or some 465 miles.

Also study the graph of the transmitter with only 1KW output at 600 KHz. It gets out quite well considering its output is a mere one-fiftieth of its bigger brother! Its range is perhaps a third of the 50KW transmitter.

Take another look at the M3 map from the previous part in this series. The country is filled with pockets of different ground conductivities, some large in area, some small. The mid-section of the country has some of the best. A large portion of eastern Colorado and western Kansas are in the 15 mS/m range. Most of Kansas proper is an incredible 30 mS/m. Signals propagating eastward out of Colorado travel a long way. Northern Texas and Oklahoma also have great ground conductivity.

Frequency of operation makes an important difference in how far a signal travels. Propagation distance at the high end of the mediumwave band is less than half the distance of that at the low end for the same received signal strength. A study of the two accompanying graphs comparing the distance coverage of 600 KHz, 1100 KHz, and 1600 KHz signals clearly show this.

The common unit used in measuring received field strength is volts per meter, or usually, millivolts per meter "mV/m". This is also the FCC requirement. An odd term, millivolts (thousandths of a volt) per meter. We all know what voltage is, but per meter of what, exactly?

Volts per meter expresses the voltage that would be induced in a one meter long wire placed parallel to the lines of flux of the received signal (remember the electrical flux from a mediumwave tower is vertical, the magnetic horizontal). This induced voltage results from the movement of the flux across the wire.

It is important to note that the E (electrical) component of an electromagnetic field is measured in a single dimension. Why? The intensity-versus-distance relation is a straight inverse rule, not the inverse square law commonly used for calculating received power density. In the perfect environment of space, if you double the distance the signal has to travel, the received voltage (in volts/meter or fraction thereof) is halved. Ten times farther away results in 1/10 of the voltage. This is known as field strength. Power density, a different method of measuring received signal strength, follows the inverse square law. That is, the density is related to the inverse of the square of the distance.

In broadcast parlance, millivolts per meter is often referred to in a different context, that being "dBu", or more accurately, dBµV/m. This is decibels above or below 1 millionth of a volt per meter. It is a convenient way to represent field strength, as the decibel is simply a ratio of values. Be sure not to confuse this dBu (lowercase "u") with the Greek "mu" ("µ") on the new model DSP ultralight radios. They are different. These radios actually measure dBµV - voltage across antenna terminals at a certain impedance, not volts per meter.

The FCC offers a conversion calculator to convert from dBu to mV/m and back.

Or, you can figure it yourself by using the following formula:
  • dBu = 20 * Log(mV/m * 1000)
To reverse the computation, converting dBu back to mV/m:
  • mV/m = (10 ^ (dBu / 20)) / 1000
But let's put received millivolts per meter into practical application, something a little more understandable. Hatfield & Dawson, Consulting Electrical Engineers out of Seattle, WA have a wealth of interesting mediumwave engineering documents for perusal, floating about the web. This document seems to describe it best in terms the layman can understand.

From an engineering report by Hatfield & Dawson, 2002:

"MW radio signal strengths are measured in volts per meter (V/m). The FCC requires that MW radio stations provide a predicted 5 mV/m daytime signal and a 5 mV/m nighttime signal or a Nighttime Interference Free (NIF) signal, whichever is greater, over the city of license."

"MW radio receivers vary greatly in sensitivity and much has been written lately about the poor performance of MW receivers built today. The general practice for broadcaster use for coverage is 2 mV/m for coverage in vehicles, 5 mV/m to 25 mV/m for in home and 25 mV/m in downtown office buildings. On a Walkman-type portable radio, you may need as much as 5 millivolts (5 mV/m) of signal to have static-free reception. The reason for these recommended signal levels is to overcome the effects of interference. Sources of interference include fluorescent lights, computers, TVs, office equipment, overhead power lines, and other appliances operating near a radio that can overload the receiver. In a city core, stations generally need more signal than this because of heavy attenuation inside large steel structures like office buildings. In your home, depending on the location, type of radio, and the utilization of any external antennas, you can have good reception with signals between 1 mV/m and 25 mV/m." produces widely used antenna pattern plots, available to interested parties over the web. These depict the expected signal coverage of the mediumwave station. radio-locator divides the signal coverage area into three distinct ranges.

1. Local (red line), the area in which the field strength is 2.5 mV/m or greater, where " should be able to receive the radio station on almost any radio with moderately good to very good reception".

2. Distant (purple line), the area between 0.5 and 2.5 mV/m, where "....the signal of the radio station may be weak unless you have a good car radio or a good stereo with a good antenna. You may not be able to receive the station at all on Walkmans or other portable radios".

3. Fringe (blue line), the area between 0.15 and 0.5 mV/m, where "....the station's signal will be very weak. You may be able to receive this station if you have a very good radio with a good antenna, but it's possible that interference from other stations may prevent you from picking up these stations at all." This seems to agree with Hatfield and Dawson's report.

Another important factor in our quest to predict received signal strength is transmitter antenna efficiency. Size does matter. The better the antenna, the better the station gets out. Returning again to Hatfield and Dawson, now discussing mediumwave antennas:

"MW antenna heights are referenced to a wavelength (this only includes the radiating portion of the tower). In MW broadcasting, 5/8 wavelength (or 225°) antennas are more efficient than ¼ wave antennas. A ¼ wavelength (or 90°) antenna is near the lower end of acceptable antenna heights. Antenna heights much below ¼ wavelength are undesirable, as efficiencies decrease dramatically below this height. A 225° antenna provides the maximum coverage and is the theoretical maximum. The efficiency decreases for antennas taller than 225°, which results in reduced coverage. In summary, taller towers are more efficient and shorter towers decrease coverage and are more difficult to work with."

So back to our quest of received signal strength prediction. Few documents are available which demonstrate how to predict "real-world" surface propagation distances at mediumwave frequencies. Many analysts use vanilla formulas for calculating free space path loss using the inverse square law, computing the path loss (in dB) from transmitter to receiver. The path loss, in turn, is used to determine the received field strength value.

Unfortunately, free space path loss assumes a perfect environment, like you would find out in space or over a perfectly conducting earth. Be wary of cute little field strength calculators on the web, their resultant field strength figures are almost an order of magnitude higher than what is seen in real life. Formulas exist which will predict field strength at mediumwave frequencies, but the math is well beyond the average person's ability.

A simpler method exists to ballpark the figure we want. We need but one more piece of information to complete the puzzle. That is the millivolt per meter level at one kilometer from the station transmitter, emanating in your direction. Using that with the FCC's Ground Wave Field Strength Versus Distance graphs and the M3 conductivity map we can estimate the received signal level in mV/m to fairly good accuracy at our location for any station.

The next part of this article will show you how to do that. But let me leave you with this before we go. What information can we derive from the FCC graphs and other station data to maximize our daytime DX distance?

1. The lower frequencies propagate better per unit of distance. Stations near the low end of the broadcast band propagate nearly three times farther than ones at the highest end, for the same power output and same ground conductivity. The lesson: listen to the lower end of the MW band if you want extreme distance in reception.

2. Look for over-water paths. A local example: Ground conductivity around here in the Rochester, NY area is about 8 or a little less, tending towards the 6 mS/m figure judging from reception experience. Without passive loop assistance, normal daytime reception limits seem to max out at about 150 miles. That is, except for two stations: WJR-760, Detroit, MI and CKLW-800, Windsor, Ontario, Canada, near Detroit. These two are out at about 250 miles or about 400 kilometers. Why are they receivable? Most of their signal path is across water over the east-west length of Lake Erie into Rochester. Reception is routine on most simple radios with medium or better sensitivity.

3. Study station antenna patterns. The FCC web site is great for this. Virtually all stations with multiple towers have directional patterns and the FCC makes the pattern plot available in .PDF form. Study the plot and pay particular attention to your azimuth away from the station. Pick stations that push a lot of signal in your direction. They are the best prospects. Example: WXXI-1370 pattern

4. The radio-locator "fringe" 0.15 mV/m boundary seen on their plots is just that - fringe. Many stations are copyable at this boundary and beyond on a sensitive radio. Don't hesitate to try for them. Example: Denver stations KHOW-630 (only 5KW), KKZN-760 (50KW), and KOA-850 (50KW) all have excellent strength east to well past Hays, Kansas (300+ miles, or 483+ km) despite the fringe zone ending miles back towards the Kansas/Colorado state line.

The jury seems to be out on how far ground conductivity plays a role in daytime propagation. Some documents refer to the 55 kilometer figure, others to 150 kilometers. My bet is it is well beyond the 150 km figure as I have routinely heard stations at the 800 kilometer range during high daylight hours at all times of the year.

Next up: Calculating

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