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., 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.