Tuesday, February 24, 2015

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.


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.

Saturday, January 31, 2015

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, an increase of 6 dB is a doubling of voltage. 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
S9+60  50000.0   94
S9+50  15810.0   84
S9+40   5000.0   74
S9+30   1581.0   64
S9+20    500.0   54
S9+10    158.1   44
S9        50.0   34
S8        25.0   28
S7        12.5   22
S6         6.3   16
S5         3.2   10
S4         1.6    4
S3         0.8   -2
S2         0.4   -8
S1         0.2  -14

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.

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

Saturday, January 24, 2015

2015 US And Canadian Pattern Reference

Greetings, and Happy 2015!

The 2015 US and Canadian mediumwave broadcast pattern reference is now available for download. You'll find the links at the upper right of this page.

I've spent much of the last year working on mediumwave pattern mapping, when time permitted.

Included is a complete set (daytime and nighttime) of GoogleMap-based, HTML-driven maps which show the most current pattern plots of all licensed US and Canadian mediumwave broadcast stations from 530 - 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. Data for the plots in this offering is based on the current FCC and Industry Canada databases available at the time of its creation (January 22, 2015).


1. Groundwave pattern maps have been streamlined and made more accurate.

2. This current set of maps uses an enhanced version of the FCC's M3 Ground Conductivity data base. Many errors in the original database were found, things like segments not joining properly, missing data, odd values for the Canadian land mass, problems with Alaska and Hawaii. I have corrected all these, plus added the conductivity data for the five Great Lakes bodies of water which were missing from the original database. Even little Lake St. Clair near Detroit has been incorporated.

3. I have written from scratch nighttime skywave mapping code using the standard FCC formulas, and now nighttime skywave pattern maps are newly available in this download. Nighttime signal patterns represent the standard SS+6 (sunset plus 6 hours, or approximately midnight), 50% signal probability at 0.25 millivolts per meter (48 dBµV/m, a.k.a. dBu). Note that nighttime reception of signals out beyond the depicted pattern is very possible, and in fact likely for the DXer. The maps represent a signal strength between distant and fringe, a level generally easily received at night on most portable radios. I have chosen this signal level to give a good representation of what should be fairly easily received by most DXers on an average evening. The nighttime signal probability of 50% means that the signal will be received at this level approximately 50% of the time at that location for the sunset+6 hour time. Also included in the nighttime map series is a web-based HTML table listing all nighttime stations in the US and Canada. It has clickable links which will take you directly to the FCC pages for that station (US stations only).

4. The daytime map series shows expected groundwave coverage patterns for Unlimited, Daytime, and Critical Hours operations. Daytime signal patterns represent groundwave coverage out to the 0.15 millivolts per meter contour (43.5 dBµV/m, a.k.a. dBu). Note that daytime reception of signals out beyond the depicted pattern is very possible, and in fact likely for the DXer. The contour line represents a signal strength at the station's fringe distance, a level usually received on a sensitive portable radio with a low ambient local-noise level. I have chosen this signal level to give a good representation of what should be fairly easily received by most DXers during sunlight hours. Also included in the daytime map series is a web-based HTML table listing all daytime stations in the US and Canada. It has clickable links which will take you directly to the FCC pages for that station (US stations only).

5. The Industry Canada Canadian database has now also been included in the daytime and nighttime pattern maps, showing each Canadian mediumwave station and its broadcast pattern. A lot of time was also spent incorporating the Canadian engineering data and coding up software to process it. Unfortunately, no Mexican patterns are available. I have elected to exclude them from the maps as the Mexican government does not provide technical engineering data via the internet. FCC Mexican data is redundant and inaccurate at best, though I may at some point offer some pattern maps of what is available.


Downloads are at upper right as always. Remember the download link changes each time I publish these files, so always come back here to get the latest files. Files at upper right are current as of the January 22, 2015 FCC and Industry Canada databases.

Be sure to read the readme.txt file contained in the .zips for tips on how to use the maps.

Click the image for a full-sized map representation.

760 KHz at night. 0.25 mV/m, 50% reception level.

Saturday, September 13, 2014

Fall Season, 2014


I must apologize for no blog posts this last year. I have gotten away from the writing due to other outside activities beyond radio, but perhaps will do some this fall or winter.

Getting ready for another cross country trip again (east coast to west coast), and will be taking the radios with me, as always. Will try to make some notes on any interesting receptions.

Have bought a Radio Shack 20-629 synthesized world AM/FM/SW receiver (PL-505 clone) over the summer and have been enjoying it very much. It has characteristics much like the Tecsun PL-600. It is a fun radio to use.

In other news, I've been working on mapping nighttime signal patterns over the last year and other program coding for the FCC database. I also have the full Industry Canada AM database unpacked and coded, which allows complete daytime and nighttime pattern maps. I am getting close on the skywave coding, and will shoot for some downloadable files on this blog in the next few months, as well as a groundwave pattern update.

The latest run on the FCC database, dated today, 09-13-2104, shows:

US Mediumwave:
4710 facilities. (FCC indicates 4715, though 5 of these are officially deleted entries).
300 facilities registered to transmit IBOC digital.
94 facilities are silent.

The latest run on the Industry Canada database, dated today, 09-13-2104, shows:

Canada Mediumwave:
161 facilities. 14 more (making 175 total) are listed as provisional and may or may not be operational.

More to come.

Take care and good DX to all,


WWL-870 nighttime (50 KW) and its competition to fringe .15 mV/m 50% signal probability