Field Strength Calculator One

Field Strength Calculator One is a program which will calculate expected received ground wave signal strength at longwave and mediumwave frequencies.

Click image to enlarge.

DOWNLOAD

To download, see the link at the top of the right sidebar under LATEST PROGRAMS. The sidebar at the top right will have the most current link in case the program is updated. The link will change in the case of an update, so I would avoid copying and pasting it into a forum or other web page. Come to the main page of this blog instead.

DISCUSSION

Being a mediumwave DXer and particularly a daytime mediumwave DXer, I wanted a way to determine a "ballpark" signal strength for various stations not only in my immediate area, but out to 100, 200, or even more miles distant. I was unhappy with virtually all the web-based signal strength calculators found on the internet, as they use the vanilla Inverse-Square-Law to calculate signal attenuation. Fine, if you are in an outer space vacuum or on a perfecting conducting surface, but not even close in accuracy for normal people here on Mother Earth.

The few stand-alone programs out there were either wildly expensive, too complicated to use, inaccessible, or plainly won't work on the Windows platform. I set out to accumulate information, formulas, and data to start writing the field strength calculator program. Investigating the history and ferreting out the pertinent information to arrive at a simplified formula that was reasonably accurate took some time.

The result was and is Field Strength Calculator One. It is based on the work of numerous engineers and mathematicians, who, starting about 1909, spent some 50 years developing the extremely complicated formulas to predict accurate signal strength at mediumwave frequencies. The basic, simplified formula has been known since the 1930s, being slightly modified by various people and agencies since then. It is accurate to within a couple of percent of the big programs that calculate field strength - those using additional input like the transmit and receive array heights above average ground, and the earth's topographic elevation changes along the signal path.

Simplified ground wave electrical field intensity calculations can be made by the introduction of a shadow or diffraction factor in the Sommerfeld-Norton planar earth expression. A mouthful! This simply means that a factor is computed and introduced to account for the additional attenuation caused by wave diffraction out beyond the radio horizon. It permits one to calculate the ground wave E (electrical) field well beyond the geometric and radio horizon, where E field values are close to the atmospheric noise level.

Be sure to read about the history of how this fascinating formula came about in the recent article on RADIO-TIMETRAVELLER: Field Strength Calculations: A History. Many of the terms used in the previous and next paragraphs are explained.

The simplified formula used by Field Strength Calculator One takes into account Sommerfeld's original plane earth theory, modified by diffraction factoring. It uses an exponential function which takes into account the spherical earth diffraction effects, and acts on the planar earth equation even before the radio horizon is reached, so the resultant E field values, as a function of distance produce a continuous curve, thus rounding-in difficult intermediate distances.

The long-accepted concept of "numerical distance" (p0) and "phase angle" (b) are used in all calculations, two variables determined by frequency, distance, and dielectric constants of the ground as a radio conductor. Numerical distance depends not only on frequency and ground constants, but also on the actual distance to the transmitter. Phase angle is the measure of the power factor angle of the earth.

Field Strength Calculator One returns expected received field strength in millivolts per meter and dBu (also known as dBµV/m), based on ground conductivity, earth dielectric and several other input constants. It also displays the distance to the radio horizon and the signal path loss in dB, along with several more technical parameters. The resulting output of Field Strength Calculator One should be accurate in most cases to a couple of percent in the longwave and mediumwave bands. It compares favorably to ITU program GRWAVE and currently available FCC Ground Wave Conductivity graphs.

Field strength calculations by Field Strength Calculator One are based on the works of A.Sommerfeld (1909), B.Rolf (1930), K.A.Norton (1936), H.Bremmer (1949, 1958), NTIA Report 86-203 (1986), ITU-R P.368-7 (1992), and NTIA Report 99-368 (1999).

For further information on how field strength is calculated see the Field Strength Calculations Series previously published on RADIO-TIMETRAVELLER.

INSTALL

Install is simple. Download the .zip file and unzip. Click on the FieldStrengthCalculatorOne.exe file to run. This program makes no registry changes and saves no data to your hard drive. It has been developed and tested in Windows 7. It should work fine in Windows Vista and XP environments, and Windows 8. It is written in the old standby Visual Basic 6.

Included in the .zip is a readme.txt file. Be sure to have a look.

I hope you enjoy this program and find it useful.

Potomac FIM-71 Field Strength Meter

Field Strength Calculations: A History

A previous three-part series on RADIO-TIMETRAVELLER delved into Field Strength Calculations. It covered ground conductivity's effects on signal strength, measurements quantifying signal intensity, and how to use the FCC Groundwave Conductivity Graphs to calculate expected received signal strength. Mathematical formulas, somewhere, produced those graphs. What is their history? Might we use a simplified formula to calculate expected received signal strength for our DX purposes?

Let's continue with the story behind Field Strength Calculations and explore the 50 year quest for accuracy in calculating signal strength by mathematical formula. It is an interesting tale. We will finish with a handy field strength calculator program I wrote using a simplified formula.

When we talk about field strength, we are really talking about radio propagation - the behavior of radio waves when they are transmitted or propagated from one point on the earth to another, or into various parts of the atmosphere. In our formula quest, we will mostly be concerned with those signals that hug the ground, or "ground wave". It may surprise many who are new to the hobby of mediumwave DXing that daytime ground wave range for a mediumwave signal might extend out to as much as several hundred, and in extreme cases, nearly 1000 miles!

Accurate formulas for calculating expected signal strength at mediumwave and longwave frequencies took many years to develop. Radio originally inhabited the longwaves in its infancy. Many of Marconi's early broadcasts, including his 1905-1906 transatlantic tests, were sub-100 KHz. The trend would be decidedly upward in frequency and downward in wavelength.

At the end of World War I, a fierce battle ensued between the US government and the Department of the Navy over control of the airwaves. The Department of Commerce eventually won and became master of the air and the regulatory agency for commercial radio. They started by establishing two broadcast frequencies: 833 KHz (360 meters) and 619 KHz (485 meters). The Federal Radio Commission took charge in 1926, lasting until 1934 when the current Federal Communications Commission was formed.

Throughout the early years of radio, interest mounted to quantitatively determine the service area of broadcast stations. Engineers redoubled their efforts to derive an accurate attenuation formula. The radio world was focused on accuracy of measurements at broadcast frequencies.

"Accuracy" is the key word here. The Inverse-Square Law, as applied to physics, had been commonly known since Isaac Newton's day in the 1600s. Applied to radio, it stated that the power density of the wave is proportional to the inverse of the square of the distance from a point source. In other words, doubling the distance from a transmitter means that the power density of the radiated wave at that new location is reduced to one-quarter of its previous value. But did it apply?

"Free-space" formulas calculating signal loss in the vacuum of space or "perfectly conducting earth" using the so-called inverse-square law are indeed accurate for those environments. But the Earth is not a perfect conductor, nor does it represent perfect-world conditions. Free-space formulas alone are not usable for our purposes. You will find many of them on the web, even calculators, purporting to deliver a signal strength solution for a given transmitter-to-receiver distance. They can be ignored as inaccurate. In fact, they are not even close.

Arnold Sommerfeld, 1868-1951

Mathematicians started with a "plane earth" (flat earth) theory when they first envisioned a signal attenuation formula. Brilliant, German-born genius Arnold Sommerfeld, nominated a record 81 times for the Nobel Prize during his lifetime, solved the plane earth general problem by 1909, publishing signal attenuation graphs in 1911. Bruno Rolf, basing his work on Sommerfeld's findings, published more attenuation graphs in 1930, some 21 years later. From this information, the Federal Radio Commission compiled formulas and curves, published in 1931. They were used in hearings and allocation matters at least until 1933. It was just the beginning.

In the intervening years from 1909 to 1930, four more scientists obtained independent solutions of the Sommerfeld problem which agreed with the 1909 solution. That is, except for one difference - an inverted mathematical sign. Apparently none of these authors noticed this discrepancy until the FCC's K.A. Norton, in a letter to the editor of "Nature" in 1935, pointed it out and showed that it was responsible for the anomalies in propagation predicted by the Sommerfeld-Rolf graphs. Norton in 1936 was able to construct a universal curve for prediction of field strength at relatively short distances.

Focusing on the plane earth theory, Sommerfeld expected that the surface or ground wave would be only slightly affected by the curvature of the earth since it is guided around the earth's curve in much the same manner as an electric field can follow around the bend in a wire with a comparatively small loss of energy. This explains the general success of the Sommerfeld plane earth formula at distances far beyond the line of sight. However, two major roadblocks to accuracy still existed.

The first, and most important, was "diffraction". The other, "intermediate distance".

Out beyond what is called the "radio horizon", radio signals undergo atmospheric and ionospheric diffraction, that is, refraction and scattering caused by atmospheric irregularities. This enables AM radio signals in low-noise environments to be received well after the transmitting antenna has dropped below the horizon. It has been shown theoretically that the ground wave attenuation factor at mediumwave frequencies is very little affected by diffraction at distances less than about 55 miles, the approximate "radio horizon" for mediumwave.

Norton, also in 1936, provided curves for greater distances in the diffraction region. These curves, however, were based on an incompletely developed theory. Mathematical solutions were being developed in Europe, and were two years away from completion. Europeans van der Pol and Bremmer published their paper in 1938, offering a more complete solution of the radio diffraction problem for propagation. Never-the-less, the calculation of field strength beyond the radio horizon still proved troublesome, though Norton's remarkable work clarified Sommerfeld's ground wave propagation theory.

The radio horizon at the longer wavelengths, including mediumwave, can be calculated quite simply.

For example, the radio horizon for a station transmitting on 600 KHz is about 59 miles.

By 1940 the FCC, through the work of K. A. Norton, had developed a practical method for constructing curves approximately representing the theoretical predictions. The method used the flat earth theory of Sommerfeld out to a distance of about 80 kilometers, and the diffraction theory of van der Pol and Bremmer at relatively great distances, those in excess of 200-300 kilometers depending on frequency and ground constants.

The gap in the curve was still intermediate distances. The Watson transformation, a theory originally described in 1918 by English mathematician G.N. Watson, was an attempt to connect the two. How to incorporate it into the general theory, to calculate the intermediate distances, was still the final problem. In the curves published in 1940, the gap was simply sketched in by a draftsman.

In 1952, George A. Hufford of the National Telecommunications and Information Administration provided a basis for unifying the ground wave prediction methods of Sommerfeld with Watson's diffraction transformation. It had been 43 years since Sommerfeld's 1909 thesis. There was finally light at the end of the tunnel. New curves were added in 1954 for very low conductivity. These were quite accurate, although freehand drawing was still necessary to join the Sommerfeld curve segment to the curve segment calculated for the diffraction field at relatively great distances.

Then in 1958, Hendricus Bremmer, the same Bremmer who in 1938 brought the general solution to the diffraction problem, provided correction terms which completed the search for the practical formula. Engineers could finally calculate ground wave field strength with accuracy. It had been 50 years in the making. The formula was born.

The FCC curves were considered satisfactory for regulatory purposes until it became necessary to convert to metric units toward the end of the 1970s. In a 1979 FCC report, it was recommended that a computer program be written for recalculating the curves using the methods in Bremmer's 1949 book. The program was subsequently used to produce new FCC curves in 1985 which agree within 1 to 2 decibels with the previous curves. However, the 1979 computer program was mathematically deficient in its ability to cover all the range of intermediate distances, and the great distance values it computed were shifted upward to force a match in the middle. FCC curves drawn for the X-band, 1605-1705 KHz, are the most recent. They are the result of precise calculations of field strength over the full range of distances of interest, including the previously troublesome intermediate distances.

And thus we have the short version of the history to achieve accuracy in field strength formulas. Stay tuned for the next installment, a program to calculate field strength, based on a simplified formula.

Next up: Field Strength Calculator One

Original measured vs. calculated f/s
values for KOA, Denver, 1934

Cross Country DX, Fall 2012

Greetings from the great southwest!

It's been awhile. Had a good trip across country during the last three weeks of October. Been busy getting life back in order the last few weeks, so apologies for no posting. Had some good DXing moments while crossing the mid-section of the country which I'd like to relate. Long distance daytime reception along the I-70 route from Columbus, OH to Denver, CO was good.

All reception on the road using 2006 Ford Ranger truck radio with 24 inch extension to whip antenna.

October 10.

WSM-650, Nashville, TN (50KW) hung in there from Ohio clear into central Missouri at mile marker 154, near Kingdom City, a distance of 346 miles. Final reception was at 13:30L. Very weak at the end, with long fades.

The next day, traveling through eastern Kansas on I-70, the following were heard with positive ID. All daylight hours, mid-morning, 9:00-10:00L. Sunrise was 06:35L.

October 11.

KOA-850, Denver, CO (50KW) at mile marker 343 (476 miles) Weak.
KKOB-770, Albuquerque, NM (50KW) at mile marker 336 (643 miles). Weak but steady.
KGAB-650, Cheyenne, WY (8.5KW) at mile marker 286 (444 miles). Very weak.
KHOW-630, Denver, CO (5KW) at mile marker 272 (416 miles). Weak.

KKOB-770, Albuquerque was the real surprise, with exceptional distance for the time of day.

More coming soon! An article on field strength calculations is in the works, plus a field strength calculator program for mediumwave. Stay tuned.

Tower Talk

WSM-650 tower, Nashville, TN, 1943

Let's take another look at towers in the mediumwave band across the U.S. I did a previous series on RADIO-TIMETRAVELLER entitled Mediumwave Oddities in which one installment covered tower oddities. Oddities turned out to be fairly popular judging from the number of hits.

I've always been interested in towers, and specifically mediumwave towers. Recently I've been in touch with Dan Goldfarb, owner of the Yahoo group MW Masts, and we have been exchanging information on mediumwave towers in the U.S. Dan is attempting to document every mediumwave tower in the world, and has made great progress to that end. Be sure to check out the MW Masts group. You will find interesting discussion and informative files available for download.

As I drive around the country, it occurs to me that there is a lot of mediumwave metal up in the air. I wondered how much? A few code tweaks to my Radio Data MW program gave me the answer, supplying a comprehensive and interesting fact list of tower info. We'll get into the statistics toward the end of the article.

But first let's talk about the different types of mediumwave masts in the U.S. A tower is a tower, right? Not necessarily so. There are four different types, and actually five counting the odd Franklin antenna which I'll describe shortly. Officially, the FCC defines tower types by numbers - 0, 1, 2-9 and A-Z. Sounds like a lot of varieties, but it's really not. In practice Types 0, 1, and 2 designators are used, in which Type 2 can be one or several kinds of sectionalized antenna. So there are basically three tower varieties plus the Franklin, which the FCC lumps into the Type 2 category.

By far the most common tower out there is the FCC Type 0 (some 93%), a normal mast without any frills. It is simply a standard tower almost always insulated at its base. Stations strive to erect a tower at least one-quarter wavelength tall for the frequency at which they broadcast. At the low end of the mediumwave band this can be a tall tower - a quarter wave at 540 KHz is some 455 feet tall. At the high end of the band, in the western hemisphere at least, 1700 KHz, the quarter wave mast height would be only 144 feet tall.

The aim in all cases is to push as much RF energy across the ground as possible in order to get the best signal out to the target audience. Gone are the days when stations prided themselves in a nearly cross-country audience (during darkness hours of course). It's all about local and regional target audience now, as that's where the ratings, and thus the money, comes from.

So the object is to transmit as much signal near 0 degrees elevation (the ground) as possible, known as the "elevation angle" of the signal. The quarter wave vertical mast does a pretty good job, however by extending its length a little it gets even better. Excepting the Franklin type, a mast 5/8 wavelength tall does the best job, producing the strongest signal at the lowest angle. Only some 90 towers out of more than 7000 have heights in the range of 5/8 wavelength or taller. Few stations can afford this height, or may be prohibited from erecting such a tall tower due to various restrictions. Stations strive for at least a quarter wave for efficiency's sake.

To the rescue came an old electrical innovation called top-loading, discovered by Thomas Edison and put into first practical use in 1905 and 1906 by Fessenden and Marconi in their transatlantic tests on 82 KHz. A simple flat-top method of top loading creates the necessary capacitive effect to electrically extend the length of the mast. The resultant antenna is sometimes known as an umbrella antenna, top loaded with a "capacitance hat" looking like an umbrella. Some 550 towers (a mere 7%) use the top-loaded scheme. The top-loaded tower is the FCC's Type 1 tower.

A fine example of a mediumwave top-load "hat" is that of Australia's 3WV, 594 KHz, in Western Victoria, pictured just below.

So what is a Type 2 tower, you ask? The answer is a sectionalized tower, which is a tower having insulated sections stacked atop one another.

A scant 10 towers across the U.S. are sectionalized, an idea adapted for radio during World War I by Arthur O. Austin who already held the patent on sectionalized power transmission towers. The war ended, and Austin's sectionalized tower scheme was temporarily forgotten. Then station WHK, Cleveland, Ohio's pioneer broadcaster, erected a sectionalized tower. Though hampered by a power restriction of 1000 watts and a shorter wavelength, the station was suddenly being heard in faraway Honolulu, New Zealand, and all over the United States, achieving distances comparable to its super-power brothers.

With a sectionalized tower, high angle radiation can be minimized to give better ground wave performance than is possible with a simple vertical radiator. Calculations for a 120 degree (one third wavelength) sectionalized tower versus a standard 120 degree tower show the unwanted high-angle radiation at 60 degrees above the horizon to be nearly 2.5 times more for the standard, non-sectionalized tower. So why aren't sectionalized towers more commonly used today? Probably cost, and the fact that super long distance reception is irrelevant in today's markets. It's an idea relegated to the past as far as mediumwave broadcasting goes.

There are several variations of sectionalized towers within this type. The two main varieties are non top-loaded and top-loaded, both falling into the Type 2 designation. They may be fed at the bottom or the center of the tower depending on whether the tower is grounded at the base or not. Top-loading on a sectionalized tower has the same effect as the standard tower - increasing the effective electrical height without building a taller tower.

KNBC top-loaded Franklin tower, 1949

Now we have come to the Franklin antenna, actually a variation of the sectionalized tower and another Type 2 variety. One of the drawbacks to the efficient 5/8 wavelength tower described above is an unfortunate lobe of power pushed skyward at an approximate angle of 60 degrees. During daylight hours this hardly matters as it is absorbed in the atmosphere. At nighttime this skywave reflection can cause the station to interfere with itself out at certain critical distances within the broadcast target area.

The true Franklin antenna is composed of two half wave radiators mounted one above the other and insulated from each other. The tower base is insulated in the usual way. The difficulty is that the feed is in the middle of the tower instead of at the bottom. Feed line is run up inside the lower section, and a phasing network is used to feed both sections so that the currents in each are in phase, and the efficiency of two half-wave radiators is realized in terms of increased ground wave and decreased high angle signals. Thus, we have the Franklin antenna, a skyscraping one wavelength tall.

Several stations claim to be using Franklin antennas. There is one true Franklin antenna array in the U.S., that of California's KFBK-1530 in Sacramento. It consists of two towers, each a full wavelength in height, each fed in the middle. The claim is that 50KW daytime KFBK-1530 has the highest published millivolt per meter level of any station in the FCC AM system - an average of 3545.89 mV/m at 1 kilometer distance. It is the highest figure found in the FCC's database, at least.

Little known is that one other station, KSTP-1500, St. Paul, Minnesota, misses the perfect Franklin definition only by a hair, being just shy of a perfect full wavelength. Its single tower is comprised of two stacked 0.498 wavelength sections, making 0.996 wavelength height in all. It's computed (not FCC figure) millivolt per meter level is actually higher than KFBK's - 3618.76 mV/m. The difference is that the FCC publishes 1 kilometer field strength values for single towers based on a standard 1KW output, only 511.77 for KSTP. In actuality, 50KW KSTP-1500 is the champion.

The WHO-1040 tower (50 KW) out of Des Moines, Iowa,, a Type 2 center fed shortened "Franklin" is grounded at the base and fed at the center with only the top section excited. It is also top-loaded. It's curious omni-directional pattern pushes a mammoth 15.8 dB gain lobe out at 41 degrees elevation to the horizon, probably the highest gain lobe of any station in the U.S. The effective radiated power (ERP) in that lobe is some 2.1 million watts! There we have the high angle lobe described in paragraphs above. The single hop bounce off the ionosphere at this angle illuminates mid-America farm country with two million watts in a perfect ring around Des Moines ranging from about 150-350 miles out. Surprisingly, distant DX from WHO is not as good as it might seem, and in fact worse than the usual 50 KW monopole.

So, what does analyzing the FCC's tower data tell us? Here are the results.

Data has been compiled from the 09-26-2012 FCC database. 4744 station facilities are represented. Since the ASRN (Antenna Structure Registration Record) is not recorded in many cases, tower heights in this list are necessarily based on the FCC's figure of radiating height of the tower, which is always indicated. In almost all cases it comes very close to actual tower height, the difference being the base insulator and top lighting.

Daytime

4740 stations are on the air during daylight hours (99 listed as silent). 7176 towers are transmitting some 27,782,249 watts. How about that for an electric bill?

6666 are normal masted towers, that is, without top loading. 500 are top loaded. 10 are sectionalized.

The tallest daytime tower belongs to WKY-930, Oklahoma City, OK at 951 feet tall (290 meters). Interestingly, the older WKY tower collapsed in a freak tornado back in June of 1998, and there is video of its collapse taken from an adjacent tower cam during the storm.

There are 5 towers exceeding 750 ft. in height. They belong to WNAX-570, KMJ-580, WSM-650, WKY-930, WHO-1040.

The shortest daytime tower belongs to KJNT-1490, Jackson, WY (listed as silent at the moment), a mere 50 feet tall (15 meters). The next shortest belongs to KBZY-1490, Salem, OR, at 52.8 feet tall. It is a top-loaded affair, the top-loading upping its effective height to a perfect quarter wavelength!

In all, there are 12 towers shorter than 75 feet. They belong to KYPA-1230, KCFM-1250, WKCY-1300, KFJL-1400, WLUX-1450, WEEO-1480, WIRB-1490, KBZY-1490, KLZN-1490, KJNT-1490, KVOG-1530, WPDC-1600.

Out of 4740 daytime stations, 3554 have omni-directional patterns, or a single tower. 1214 have shaped patterns using multiple towers.

The total height of all towers broadcasting during daylight hours is 1,775,871 feet (541,286 meters), or 336 miles (541 kilometers) of metal antenna structure up in the air!

Again, the highest daytime published millivolt per meter level goes to KFBK-1530 of Sacramento, CA with 3545.89 mV/m. KSTP-1500's actual calculated level is higher at 3618.76 mV/m.

The lowest daytime published (and actual) millivolt per meter level goes to two-tower, 250 watt WCTA-810 of Alamo, TN with 140.82 mV/m.

Nighttime

4158 stations are on the air during nighttime hours (81 listed as silent). 7689 towers are transmitting some 10,956,214 watts. Quite a drop in power.

7,335 are normal masted towers, that is, without top loading. 525 are top loaded. 9 are sectionalized.

The tallest tower "in use" day or night goes to nightime WRDT-560, Monroe, MI at 992 feet (302 meters). An interesting situation, as daytime and nighttime services transmit from different locations. WRDT-560 daytime service of 500 watts transmits from Monroe, MI (near Detroit) on 4 "short" towers of the same height (each 410 ft.). Nighttime service (a puny 14 watts) transmits from the huge 992 ft. Detroit Metro Media Center tower in suburban Oak Park. FCC records show WRDT uses the entire length of the tower to radiate its 14 watts. Even at the operating frequency of 560 KHz, the 992 ft. tower is still just 0.565 wavelength. The media tower is obviously used for multiple purposes.

There are 6 towers exceeding 750 ft. in height. They belong to WRDT-560, WNAX-570, KMJ-580, WSM-650, WKY-930, WHO-1040.

The shortest tower belongs to the nighttime operation of WSIV-1540, E. Syracuse, NY at 31.9 feet (10 meters) which appears to be operating out of a house in a residential neighborhood. It is not top-loaded either, transmitting a signal of 57 watts into a 0.050 wavelength radiator. It appears that nighttime WSIV-1540 takes the prize for the shortest wavelength antenna too.

There are 13 towers shorter than 75 feet. They belong to KYPA-1230, KCFM-1250, WKCY-1300, KFJL-1400, WLUX-1450, WEEO-1480, WIRB-1490, KLZN-1490, KBZY-1490, KJNT-1490, KVOG-1530, WSIV-1540, WPDC-1600.

Out of 4158 nighttime stations, 2530 have omni-directional patterns, or a single tower. 1655 have shaped patterns using multiple towers.

The total height of all towers broadcasting during nighttime hours is 1,965,079 feet (598,956 meters), or 372 miles (599 kilometers) of metal antenna structure up in the air! Nighttime wins over daytime with an extra 36 miles of antenna tower.

Wrap Up

Several things are apparent from these statistics. Fewer stations (582 fewer) are on the air during nighttime hours, but they use 513 more towers than the daytime group. This is because of the need for signal directivity to avoid interference.

The total power output of the nighttime group is only 39% of the daytime group, again for the same reason.

Nighttime KFBK-1530 of Sacramento, CA this time has the absolute claim to the highest computed millivolt per meter level: 3126.79 mV/m. KSTP-1500 switches to a 3 tower array at night and loses its place.

The lowest nighttime published (and actual) millivolt per meter level goes to two-tower, 1 watt WZRK-1550 of Lake Geneva, WI with a micro field strength of only 9.11 mV/m. WZRK takes the overall prize in this category.

The next time you take a drive, look for mediumwave towers. They are an interesting subject! More from the west coast. I am headed across country soon.

A nice shot of the WSM-650 tower as it appears today.