US Ground Conductivity Map

Have been doing a lot of work this winter on the various mediumwave FCC databases. I've also torn apart the published Canadian database offered by Industry Canada, and added all licensed entries to the combined database which I keep. Primarily the work this winter has been constructing groundwave and skywave pattern plots for all stations.

In the process of doing this, I also tidied up the FCC's M3 sequence file, which defines the distinct areas of ground conductivity across mainland U.S., Hawaii, Canada, and Mexico. Missing Great Lakes summer conductivities were also ferreted out from other sources. With that in hand, I created a nice Google-based HTML map of U.S. ground conductivities and thought I would share it with you. It's fully zoomable like all Google maps.

It can be downloaded from the link at the upper right of this blog. It's self-contained like my pattern maps. Unzip the file and click on the HTML file to view.

Hope you enjoy it.

Static rendering of the FCC's M3 ground conductivity map.

The dBµ vs. dBu Mystery: Signal Strength vs. Field Strength?

We've just talked about Tecsun's use of the term dBµ (Greek letter µ 'mu') in a previous article. They use it as a measure of received signal strength on their DSP radios. But there is another dBu (lowercase 'u' this time), also a measurement of strength, and more commonly used. What's the difference?

We first need to identify dBu's cousin, millivolts per meter.

You may have seen the term mV/m, or millivolts per meter, used as a measurement of field strength. The common unit used in measuring E-field "strength" is volts per meter, or V/m. Volts per meter is a lot when we are dealing with small received signal levels, so millivolts per meter 'mV/m' (one-thousandth of a volt per meter) is usually used. We have all seen mV/m used in a receiver's sensitivity specs, or to represent a station's received field strength at a given distance. Defined, an electric field of 1 mV/m is an electrical potential difference of 1 millivolt existing between two points that are 1 meter apart, perhaps along a one meter length of wire or between two parallel planes placed in the path of a signal. Technically, a millivolt per meter (mV/m) is achieved if a voltage of 1 millivolt is applied between two infinite parallel planes spaced 1 meter apart.

dBu (yes, lowercase 'u'), in reality is another improper contraction - a shortened version of dBµV/m (there's that Greek letter µ 'mu' again). dBµV/m is commonly and usually written nowadays as dBu, using the lowercase letter 'u'. It is the term used worldwide by engineers and the FCC for measuring electric field strength of AM, FM, and TV broadcast stations at prescribed distances. dBu is directly related to mV/m (mV/m = 1000 times µV/m), and is the logarithmic representation of mV/m.

Graph depicting measured dBu levels for KOA-830 (1934)

Have a look at the 1934 graph above depicting various dBu levels for station KOA-830, Denver. Confusing things even more, in the old days dBµ was indeed used as the shortened version of dBµV/m, and not dBu. (note: KOA currently is allocated to 850 KHz).

It is interesting to see what the different dBu values represent in terms of field strength. Several levels are represented. I have converted the dBu values to millivolts per meter:

88 dBu: 25 mV/m (urban)
74 dBu: 5 mV/m (residential)
54 dBu: 0.5 mV/m (rural)
36 dBu: 0.06 mV/m (atmospheric noise level)

For propagation aficionados, some other interesting things are of note here, in terms of propagation of this 50 KW signal over the excellent ground conductivity of the mid-west:

1. The ionospheric signal (the skywave) is strongest in the 300-500 kilometer range.
2. At about 200 km distant, the skywave strength essentially matches the groundwave strength, at about 63 dBu (1.4 mV/m).
3. The skywave signal level rises above the atmospheric noise level at just 30 km distant.
4. The groundwave signal level doesn't drop below the atmospheric noise level (36 dBu) until about 550 km distant.

radio-locator.com, a site most are familiar with, uses the following mV/m values to represent different reception zones:

2.5 mV/m (68 dBu, local, red line)
0.5 mV/m (54 dBu, distant, purple line)
0.15 mV/m (43.5 dBu, fringe, blue line)

They are essentially in agreement.

radio-locator.com

Confusion continues to exist between Tecsun's dBµ (their version of dBµV), and dBu. They are constantly confused as the same thing, though they are very different. Note, however, that dBµV is indeed indirectly related to mV/m, and dBu.

So let's define them again, concisely:

dBu (letter 'u') from (dBµV/m): the decibel (logarithmic) representation of electric field voltage above or below one microvolt per meter.

dBµ (mu 'µ') from (dBµV): the decibel (logarithmic) representation of voltage above or below one microvolt across a load.

All you have to do is remember two things:

1. dBu (letter 'u') is actually another name for dBµV/m, related to mV/m. It came into common use many years ago.

2. dBµ (mu 'µ') is somebody's shortening of dBµV. Tecsun re-coined this one.

Important! You cannot convert dBµV as shown on the DSP radios to mV/m or dBu! The values are not interchangeable. The difference is found in what is called Antenna Factor, or the ability (actually efficiency) of the antenna to convert the passing field to an electrical voltage which can then be received by the detection process. As each antenna is different, each will transfer a different signal voltage to a radio's input. Each antenna (a ferrite loopstick is an antenna) will have a different Antenna Factor.

It matters little whether (at reception time) the received signal is ultimately impressed on a ferrite bar or rod, or a long wire, or a bed spring, in that the receiver will take whatever tiny voltage induced and convert it into intelligible audio if it is strong enough. Remember, as stated before, the iron core ferrite rod is basically a signal concentrator. The longer the rod and thus the more iron ferrite, the more the concentration, and the greater signal voltage, at least to a point.

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

If you'd like to figure it yourself, you can 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

(Log is the common logarithm, or base 10).

How, then, do we go about measuring millivolts per meter, mV/m?

Millivolts per meter (mV/m) is a way of defining a station's expected (or measured) field strength at a receiving location. Field strength can be measured by a device specifically designed to measure the strength of the passing wave. Potomac Industries makes the model 4100, a device which measures field strength. It was the subject of a previous blog post. Formulas to calculate approximate field strength can also be used.

Potomac Industries 4100 measurement.

Please note that a mediumwave station's expected field strength at a receiving location depends on many factors. One is transmitter power. Two, the distance from the transmitter. Three, the ground conductivity variations along the path between the transmitter and receiver. Four, the frequency of the wave. There are other factors too.

I did some articles on signal measurements and ferrite antennas on my blog a couple of years ago. Maybe they will help with introducing some of this field strength material.

Field Strength Calculations (3 parts)

An Unassuming Antenna - The Ferrite Loopstick

Field Strength Calculations: A History

I hope this helps in identifyng the difference between dBµ and dBu.

The Ultralight dBµ Mystery, S-Meters, And Field Strength

On our little ultralight DSP receivers like the Tecsun PL-380 and PL-310, etc. you may have noticed the dial face has some numbers which vary with the received signal strength. Next to them are marked the letters "dBµ". It seems like a field strength meter of some kind. Just what is that? Well, it is indeed an indicator of signal strength. Let's dig into what it means and see how it is directly related to the S-meter of old, the field strength meter.

National Radio Company

Tecsun's use of dBµ (a funny-looking, backwards 'u', which is the Greek letter µ 'mu', meaning micro, or one-millionth) is really an improperly-used, shortened version of the term dBµV. Warning! You may have also seen the term "dBu" (lowercase "u") written in various publications, associated with field strength also. It refers to something different (actually the E-field of the passing wave). We can handle that one in a separate article, so be sure not to confuse Tecsun's dBµ with dBu!

Back to Tecsun's dBµ. Let's break it down:

dB = decibels, or simply a way of expressing magnitudes of a value, like voltage, logarithmically
µV = microvolts, or millionths of a volt

Consequently, dBµV is a voltage expressed in dB above (or below) one microvolt. This is measured across a specific load impedance, commonly 50 ohms. Important! Here we have a real received voltage measured across a specific load impedance like a tuned circuit!

The 'dB' or decibel measurement is a logarithmic ratio as you may know. In terms of voltage, or more correctly dB as a field amplitude ratio 20 * Log (V2 / V1), an increase of 6 dB is a doubling of voltage. The power increase is 4-fold. So, if our little Tecsun receives a signal at 28 dBµ and it increases to 34 dBµ, the received voltage has doubled. Coincidentally, this is also an increase of one S-unit! Now we are getting somewhere.

Let's translate our received dBµV into actual received voltage:

dBµV   µV(millionths of a volt)
-------------------------------
 94    50000.0
 84    15810.0
 74     5000.0
 64     1581.0
 54      500.0
 44      158.1
 34       50.0 (the S-9 of old!)
 28       25.0
 22       12.5
 16        6.3
 10        3.2
  4        1.6
 -2        0.8 (less than 1 µV sends the dB ratio to a negative value!)
 -8        0.4
-14        0.2

The following formula is used to convert dBµV to millionths of a volt:

µV = (10 ^ (dBµV/ 20))

To convert millionths of a volt back to its decibel representation:

dBµV = 20 * Log(µV)

(Log is the common logarithm, or base 10).

The modern DSP receivers like the Tecsun PL-380, 310, etc. which employ the Silicon Labs chips, measure and display dBµV as received at the tuned front end across a load. They call it the RSSI indicator. Our radio's antenna, the iron core ferrite rod, is basically a signal concentrator. The longer the rod and thus the more iron ferrite, the more the concentration, and the greater the signal voltage transferred to the radio's tuned input.

So what exactly is this so-called dBµ indicator on our DSP radios telling us?

Some time ago, more than a year ago, I posed this question to Scott Willingham, who was on the design team for the SiLabs DSP receiver chips used in these radios.

He stated:

"The RSSI (dBµ) readings are referred to the pins of the chip, which are the inputs to the LNA. In the Tecsun radios operating in the MW band, this is also the voltage across the loopstick. In SW bands, the Tecsun ULRs use a preamp/LNA on the circuit board between the whip antenna and the Si4734. In that case, the RSSI readings reflect the signal at the output of Tecsun's external LNA."

Essentially for mediumwave, the received signal is measured in microvolts right off the loopstick and then converted to dBµ, which is decibels above a base of one microvolt. Remember again, dB is just a logarithmic ratio. Of course a PL-380 is not going to read the same dBµ as a PL-310 or a PL-398, etc., because the antenna setups (loopstick lengths, whip extension, tuned circuit efficiency) are different and each will induce different received voltage levels to the radio.

A curious measurement, yes, but there is also some meaningful information here in comparing signal strengths within the same radio just like an S-meter did, and in fact there is a direct correlation to the S-meter.

The analog S-meter us old guys remember in now ancient receivers was based on S-9 indicating a 50 µV (microvolt) input signal to the antenna circuitry, at a load impedance of 50 ohms. That is, the S-meter read S-9 if the receiver S-meter was calibrated right, as the meter was further down the IF chain, and usually responded to the AGC (automatic gain control) level. Each S-unit is 6 dB apart, meaning a signal reading S-9 is 6 dB stronger than a signal reading S-8. S-9 +10dB is 10 dB greater than S-9, or one S-unit plus 4 more dB.

What does this mean? An S-9 signal is twice as strong as an S-8 signal. The received voltage is double. An S-9 signal is four times as strong as an S-7 signal. The received voltage is doubled twice.

Some direct correlation can be attempted with the SiLabs DSP chip dBµ readings used in the Tecsun radios.

S-unit        µV  dBµV  dBm
---------------------------
S9+60dB  50000.0   94   -13
S9+50dB  15810.0   84   -23
S9+40dB   5000.0   74   -33
S9+30dB   1581.0   64   -43
S9+20dB    500.0   54   -53
S9+10dB    158.1   44   -63
S9          50.0   34   -73
S8          25.0   28   -79
S7          12.5   22   -85
S6           6.3   16   -91
S5+4.9dB     5.6   15   -92
S5           3.2   10   -97
S4           1.6    4  -103
S3+1.9dB     1.0    0  -107
S3           0.8   -2  -109
S2           0.4   -8  -115
S1           0.2  -14  -121

Look at the S-unit and dBµV columns. As can be seen, a 34 dBµV signal (again, the Tecsun DSP radios label it dBµ) is essentially equivalent to an S-9 signal on the old S-meter setup. The 25 dBµ signal shown in the picture below represents a signal halfway between S-7 and S-8.

On the Tecsun PL-380 (at least the version I own, which registers from 15 dBµ - 63 dBµ), somewhere around 15 dBµ seems to be the signal detection threshold which translates to just below the old S-6, at 6.3 microvolts of signal. As noted elsewhere, these modern drug store consumer radios are not as sensitive as the old communications receivers we remember. S-6 on an old vacuum tube receiver was virtually "arm chair" copy. This is where an FSL or passive loop brings up the weak received signal to similar levels in the DSP radios.

Hamwaves has an excellent page on how decibels are used in power and voltage measurements, and how they are related to the S-Meter.

So there you have it. Keep this chart handy and you can convert between Tecsun's dBµ and S-units.

Stay tuned for the second article in this series: The dBµ Versus dBu Mystery: Signal Strength vs. Field Strength?

25 dBµ, or between S-7 and S-8

2015 US And Canadian Pattern Reference

Editor's note: The newest version of the pattern set (2024) is available. See the link at the upper right of this page.

A new US Mediumwave Pattern Reference, produced by Radio Data MW, has been uploaded. You will find it at the top of the right sidebar. Radio Data MW, a program I have been working on for the last few years, accomplishes this mapping process.

Included is a complete set of GoogleMap-based, HTML-driven maps which show the most current pattern plots of all licensed US mediumwave broadcast stations from 540 - 1700 KHz. The set includes all frequencies for the indicated services: Unlimited, Daytime, Nighttime, and Critical Hours. Individual maps are grouped by channel frequency: 540, 550, 560 KHz, etc.

I will attempt to make this a regular feature on RADIO-TIMETRAVELLER, with regular yearly updates. The sidebar at the top right will have the most current links. The link will change for each new posting, so I would avoid copying and pasting it into a forum or other web page. Come to the main page of this blog instead.

INSTALLING

The maps are HTML-based, so no regular install is necessary. Simply unzip the downloaded file and click on the individual map file to run. The map will open up in your web browser. They are self-contained, with image icons embedded right into the code. You must have an internet connection to view the maps.

HOW THEY ARE PRODUCED

Using the actual FCC database files Radio Data MW will auto-generate an interactive HTML pattern map, showing the pattern plots for all stations included at the discretion of the user. A complete set of mediumwave pattern maps can be generated in a matter of minutes. Radio Data MW generates a real pattern plot based on ground conductivity, ground dielectric constant, and can display actual (but approximate of course) signal level boundaries for Local, Distant, Fringe, Extreme mV/m levels, or any custom mV/m level chosen by the user.

The online Google Maps API is used to generate and plot each station on a map of the US. An accurate flag pin is placed at each transmitter location, and in satellite view may be zoomed in to see the actual transmitter site. Map flags are color-coded to indicate Unlimited, Daytime, Nighttime, and Critical Hours services. Each flag has a tooltip-type note, and when hovered over with the mouse will display a note on the station.

A pattern plot for each station is generated and displayed. Each pattern is calculated using standard formulas used by the FCC to compute the base values at one kilometer, and field strength formulas at distance based on the works of many people over the years. See Field Strength Calculations: A History and Field Strength Calculator One, previously posted on RADIO-TIMETRAVELLER.

Finally, an accurate ray path can be drawn from all transmitters to a user-specified receiving location by inputting latitude-longitude coordinates. Super-imposed on the pattern plots, the ray paths show the listener where he or she falls on each station's pattern, a handy guide to knowing where you stand.

Note that these maps are web-based. As stated, they use Google Maps and thus require access to Google. In order to view them you need a connection to the internet. In desktop or laptop use, they have been tested in the Internet Explorer, Firefox, Chrome, Opera, and Safari browsers. If using Internet Explorer, best results are had with the latest version. Chrome works best.

These maps will work on some tablet or phone browsers. I have tested them on an Android device and it's handy to be able to display them while DXing outside or on the road. Some browsers will not allow pinch-to-zoom, where others will. Some browsers don't render the map controls correctly. Response is fair to poor on the tablet or phone due to the sheer number of HTML lines and processing required to render the maps. Such is the current state of tablet and phone browser rendering.

Hope you enjoy these pattern maps and find them useful.