Talk:dBm

Opening sentence

The very first sentence in the article is misleading by use of the word 'ratio' and I'll change it and see the reaction. Regards, -- Steve -- (talk) 15:56, 13 November 2020 (UTC)

Untitled

The dBm is still a ratio between two quantities, as are all decibel measurements, this on relative to 1 mW, so shouldn't it also have no units, simular to Sound intensity level? --LeakeyJee 14:57, 31 May 2006 (UTC)

what do you mean by "it also have no units"? dBm is a shorthand for the power in Decibel relative to 1mW. it *does* have units in the name. that's what the "m" is for in dBm.

I think what he means is that this unit should be dBmW, as it appears like someone just forgot to add the final W. The "dBm" is just a multiplier (an exponent and 1/1000), without actual unit. Technically "dBm" is dimensionless. The missing W is the unit. It appears as if dBm is more like some shorthand for dBmW that have been adopted by the industry and wide public. (Just because EVERYONE is repeating a mistake does not make it right. It's still a mistake.) — Preceding unsigned comment added by 98.128.172.177 (talk) 09:30, 29 October 2020 (UTC)

I realize it would be nice to have a very obvious and unambiguous two letter suffix, but that is not how things are always done in practice. In the two way and cellular and other RF industries I have interfaced with, the dBm is commonly used for a power level in dB relative to a milliwatt. It has been so for at least the 50 years I've been involved. The dBk, without a "W", is used in broadcast TV and radio relative to a kW and still has no "W". The dBmv (ref milivolt) in the cable TV industry is an outlier using two letters for the reference power and most likely was done to distinguish it from the already widely used dBm; IN ADDITION it is assumed to be in the cable TV impedance of 75 ohms. The others have used the single letter all along. -- Steve -- (talk) 22:19, 22 November 2020 (UTC)

Impedance dependent?

The first paragraph states that dBV is impedance dependent:

dBm (or dBmW) and dBW are independent of impedance (as opposed to dBV which is dependent, for example).

I disagree. As it says on the Decibel page:

dBV - voltage relative to 1 volt, regardless of impedance.

Surely the impedance is only important when converting between dBm and dBu/dBV? Pediacycle 04:48, 14 August 2007 (UTC)

Because the reference is a specific voltage, the actual *power* is most definitely dependent on the impedance. P=E^2 / R

That section is referring to measurements of voltage, NOT power, using the log representation and calling it dB (reference to a volt). This is still a power measurement in a varying signal or a system where impedance is constant for all measurements.

In other areas this is actually an incorrect measurement because the dB is only defined for power. HOWEVER, that said, it is very common to use the log scale for voltage-only measurements around a system and call them dB and those in the engineering world understand what it actually means. An OP-AMP with a very high input impedance and very low output impedance will typically be measured iwith a voltage measuring device on a log scale using 20Log() and it is understood that this in no way relates to power gain because the impedances are different. However, measuring the frequency response with a voltmeter, those numbers and a graph will show the variation in power over frequency. Those in the industry know what is being measured, or at least should. -- Steve -- (talk) 22:19, 22 November 2020 (UTC)

A dBm is referenced to 600 ohms and is a power measurement. 0 dBm = 1mW across 600 ohms. (as defined by the I.E.E.E.) Combining Kirchoff's voltage law (V=IR)and the power formula (P=IV) you get: V = sqrt(PR); for a 600 ohm circuit, V = sqrt (0.001 * 600) = 0.7746 volts For a 50 ohm circuit, V = sqrt (0.001 * 50) = 0.2236 volts A dBu is not reference to impedance and is a voltage measurement. 0 dBu = 0.7746 volts. dBu's came later in electronics so that is why you see the same voltage as 0 dBm, but it's only the same at 600 ohms!

70.70.3.5 03:09, 24 August 2007 (UTC) John Fulton - sorry don't have any web links the two formulas are laws and the rest is math.

The 600Ω sounds like it is in the context of telephone lines. However dBm is used for RF, inlcuding power levels on transmission lines and also in free space. Here the dBm is a power referenced to milliwatt and not to any particular impedance. When it comes to a radio wave the concept fo a volt is not relevant, you would have to measure volts per meter or watts per square meter, and even this is a different concept. Graeme Bartlett 04:20, 24 August 2007 (UTC)

Every place I've ever worked (radio xmitter repair/installation) always referenced 0dbm to a 600 ohm load. I was taught 0dbm has no context without a fixed, standard impedance. My experience was in the military but I've noticed commercial industry also follows this practice.

"Typical power"

The typical power of an FM broadcast transmitter is given in the article as 100kW. While this is true for a transmitter with a desired range of 30-40 miles, many local radio stations have an Effective radiated power (ERP) of 1kW or less, since this is enough to cover an area a few miles across. Also, FM transmitters with mixed polarization may have an ERP higher than this. The wording has been altered to reflect this. See also [1]. --♦IanMacM♦ ^{(talk to me)} 09:30, 11 November 2007 (UTC)

I find the reference to Typical maximum output RF power from a ham radio HF transceiver too vague. As a Swedish ham I am allowed to use anything from 1 W (ERP or e.i.r.p depending on band) up to 1 kW (usually in terms of p.e.p. fed to the antenna system) on HF bands. This is a range from 30 to 60 dBm. Johan Adler (talk) 11:10, 20 February 2015 (UTC)

References

Signal strength in dBm?

I understand the concept of transmitted power, and I can understand power at the receiver end too, once it has been converted to electricity. But what is the "power" of a radio signal? Take for example the description (from the table of typical values) "Typical maximum signal strength (−10 to −30 dBm) of wireless network". I am similarly puzzled by "Typical RF power inside a microwave oven". Can someone explain this to me? Thunderbird2 12:25, 11 November 2007 (UTC)

Any statement involving the use of the word "typical" is open to debate. Radio transmitters, microwave ovens etc have variable power and they are subject to the inverse square law governing the strength of electromagnetic radiation at a distance. Broadcast radio transmitters have a wide range of power outputs, and microwave ovens can range from 500 to 2000 Watts. The figure that I gave for the typical maximum received strength of a wireless network is based partly on research from PassMark Wireless Monitor [2] and also from my own tests of a wireless router with the receiver next to the transmitter (maximum received signal around -25dBm for a 100mW transmission.) PassMark gives the figure of maximum strength for a wireless network transmission as ranging from -10dBm to -45dBm, and this may vary as the maximum strength for a wireless network is dependent on a range of factors. Any further comments here would be welcome. --♦IanMacM♦ ^{(talk to me)} 18:38, 11 November 2007 (UTC)

I wasn't questioning the values. Rather, my question was a conceptual one. I will try to phrase it differently. Imagine a radio transmitter of power W in free space. At a distance R, the intensity of an expanding spherical wave, neglecting absorption, would be W/(4 pi R^2). It then seems natural (to me) to measure signal strength at that position in watts per square metre rather than watts. Or for the microwave oven, I would think more in terms of energy density (joules per cubic metre). So, another way of asking the same question is: How do I convert the intensity for the radio transmitter (or energy density for the microwave) into power? Thunderbird2 19:02, 11 November 2007 (UTC)

I don't think it's an indication of the field strength at any given location inside the oven. It's simply the total output power of the microwave elements. Oli Filth^(talk) 19:08, 11 November 2007 (UTC)

I see. That clears up the question about the oven. For the wireless network is it also the transmitted power? Thunderbird2 19:15, 11 November 2007 (UTC)

The transmission power of a wireless router for home and small office use is usually around 100mW (20dBm). There are several ways of measuring the received signal strength of a wireless network, and there is a good explanation at [3] (PDF format). --♦IanMacM♦ ^{(talk to me)} 09:01, 12 November 2007 (UTC)

I still don't understand. The pdf tells me how to convert power (in watts) to power level (in dBm). But how do I get to power (from intensity) in the first place? Thunderbird2 09:31, 12 November 2007 (UTC)

In general, it's not a simple relationship. It's a function of power density (power per square metre), antenna aperture, effective receiver resistance, etc. Luckily, this article doesn't mention "intensity". Oli Filth^(talk) 11:01, 12 November 2007 (UTC)

OK, but we're getting closer because power per square metre is the same thing as intensity. You say the signal strength (in dBm) is a function of receiver resistance. Does that mean it's the power in the electrical circuit after reception? That would make more sense to me. Thunderbird2 11:32, 12 November 2007 (UTC)

With certain assumptions, the received power (i.e. the signal strength in the receiver) is given by the free-space path loss equation. Obviously, this won't be the same as the signal strength at the transmitter, which is what the numbers in this article are geared towards. Oli Filth^(talk) 11:38, 12 November 2007 (UTC)

I think there tends to be some confusion because a lot of field strength detectors report dBm, but then you need to know what antenna you used to get field strength. In that case, dBm is the power in the detector. I don't see why the table lists "typical maximum received signal power (−10 to −30 dBm) of wireless network". It seems what you would care about is the maximum possible signal power (which should be on par with the transmission power) and the smallest detectable power for a typical receiving device. You may also be interested to know that for an ideal resonant circuit, a small antenna will still have an effective aperture of lambda squared (divided by 4pi if the following link got it right), and antenna gain is compared to this ideal. So for small antennas, power in the device can be related to field strength in a way that doesn't depend on specific antenna size (as long as it's small and the device is ideal). See e.g. pg. 63 of http://www.kaltmancreationsllc.com/SPECTRAN-HF2_V1.1_EN.pdf Physicsjock (talk) 20:26, 3 April 2010 (UTC)

I also have the impresssion that dBm is not a well defined unit for signal strength in the article. As I physicist, I can only embrace W for source strength and W/m^2 for signal strength (i.e. intensity). To use the same unit dBm for both entities is, at least, problematic. In fact, in the article dBm is only well defined for source strength and not signal strength.
Still, wikipedia is a encyclopedia and not a Standards Bureau, it has to cope with such problematic, widespread units and cannot just define better ones. If the definition of dBm for signal strength is really the maximum power a small antenna can receive ideally (as Physicsjock says, which however has to be verified) this should be quoted in the article, since this is a complete definition. Peter.steier (talk) 14:44, 13 February 2011 (UTC)

See my remarks above in the first section "Rename this article dBmW". A dBm, for example, a well defined term, can be used for any power level desired. It can be the power out of a stage in a complex circuit. It can be a transmitter's full output power. It can be slightly less than that power delivered (due to some loss) via a coax transmission line to a physical antenna. It can be the tiny bit of power captured by a quarter wave ground plane antenna at some distance from that transmitter. It can also be the greater amount of power captured by a Yagi-Uda antenna at that very same distance from that same transmitter antenna.

The very first sentence in the article is misleading by use of the word 'ratio' and I'll change it and see the reaction. Regards, -- Steve -- (talk) 15:54, 13 November 2020 (UTC)

Unit conversions (table)

Many of the examples in this table are not sufficiently specific. For instance, the allowable power "from a ham radio transceiver (rig)" varies depending upon the frequency band, operating mode, and operator class.

http://www.arrl.org/files/file/Hambands_color.pdf —Preceding unsigned comment added by Rwestafer (talk • contribs) 11:39, 12 October 2010 (UTC)

There is at least one severe error

 | −174 dBm || 0.004 aW = 4 zW|| Thermal noise floor for 1 Hz bandwidth at room temperature (20 °C)

in Johnson–Nyquist_noise the same value is used at a different temperature and is also wrong

possibly the whole table may have severe errors.

--AK45500 (talk) 21:49, 8 December 2020 (UTC)

let's continue the discussion at Johnson–Nyquist_noise. Constant314 (talk) 23:16, 8 December 2020 (UTC)

Standards

The 802.11a power levels (at least) are wrong. According to the standard at http://standards.ieee.org/getieee802/download/802.11a-1999.pdf it is (Maximum output power with up to 6 dBi antenna gain(mW))

5.15–5.25 GHz: 40 (2.5 mW/MHz)

5.25–5.35 GHz: 200 (12.5 mW/MHz)

5.725–5.825 GHz: 800 (50 mW/MHz)

ntg (talk) 09:41, 21 June 2012 (UTC)

Equation units

The "Unit conversions" section of this article presents four equations for expressing the "power P in watts as x in dBm." I am fairly certain that in the first and third equations the power P should be in milliwatts, not watts. If I am correct, the first and third equations should be introduced by saying they express the "power P in milliwatts as x in dBm." The second and fourth equations should remain with the existing introduction, where P is indeed in watts.

Here are the first and third equations from the article, where P should be entered as milliwatts, not watts:

${\begin{aligned}x&=10\log _{10}{\frac {P}{1\mathrm {mW} }}\\\end{aligned}}$

and

${\begin{aligned}P&=1{\text{mW}}\cdot 10^{\frac {x}{10}}\\\end{aligned}}$

Using P = 1W as an example, the first equation gives a value for x of 0 dBm. This is clearly not correct; if P is 1W, x should equal 30 dBm. Only a value for P of 1000 mW in the first equation yields the correct value for x, 30 dBm.

Similarly, in the second equation above, a value of 30 dBm for x yields the correct value for P, 1000 mW, not 1000 W.

Gary63 (talk) 04:35, 25 April 2014 (UTC)

Explanation is sought

All of this sounds very nice for those who are in the know. But for me, as one of the ignorant masses, it doesn't explain how -50dbm relates to the strength of my Wi-Fi connection. My gratitude goes to anyone who cares to elaborate on a Grade 1-2 level. — Ineuw talk 18:00, 21 May 2017 (UTC)

Tsar Bomba comparison

Article states: "203 dBm | 2.1×10^17 W | Upper estimate for total power output of the Tsar Bomba"

I am removing this row from the table because it makes no physical sense. The number 2.1×10^17 W is unsourced, and it does not appear in the text of the wiki article about Tsar Bomba. A number of 209-243 Petajoules is mentioned, but that's energy, not power -- 2.1x10^17 joules fits very nicely into the lower end of that estimate range, but Joules and Watts are not interchangeable. The total energy output of a bomb is measured in Joules. "total power output" implies a sustained rate of power transfer which is not consistent with an energy impulse like a detonation. Power (Watts) is energy over a period of time. What time period is to be assumed? One could say that "2.1×10^17 W is the average power output of the bomb if all its energy is assumed to be delivered uniformly over a period of one second after detonation" but that really becomes arbitrary.

Happy to have a discussion and put a revised version back in, but as written I think it misunderstands the relationship of energy and power. Twredfish (talk) 03:36, 14 July 2021 (UTC)