How to measure dBm ?
Category: Science > Instruments and Methods
Asked by: tulsaoc3guy-ga
List Price: $42.00
13 Nov 2005 14:14 PST
Expires: 13 Dec 2005 14:14 PST
Question ID: 592556
How can I measure the power in dBm emitted from my IEEE 802.15.4 device at a certain distance from the transmitter? 1) I do not have an RF test facility or lab. Does it make sense for end users to try to measure received dBm with an RF power meter? ($10 for answer.) 2) If so, what kind of RF power meter could be used? A Rhode and Schwartz meter? ($7 for answer.) 3) My device emits only IEEE 802.15.4 packets. Can you still get a received dBm measurement by somehow measuring the EM field strength of the packets? In other words, would I need to modify my IEEE 802.15.4 device so it emmitted some sort of continuous 2.4 GHz wave, instead of packets, to get an accurate dBm measurement? ($25 for this answer.) Note: The detailed sensitivity requirements as defined in the 802.15.4 standard for sensitivity testing are in IEEE reference P802.15.4/D18-6.1.6, 184.108.40.206. This is not the answer I am looking for. I am wondering how IEEE 802.15.4 device manufacturers determing dBm received at locations around their device.
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Re: How to measure dBm ?
From: formica34-ga on 14 Nov 2005 08:29 PST
3) Generally you wnat to emit a continuous 2.4GHz tone (CW, or Continuous Wave) transmission. You'd probably want to do it at the band edges, and in mid-band. It would be almost impossible to get an accurate reading otherwise, unless you had some extremely expensive 802.15.4 protocol analyzer/spectrum analyzer combo equipment. There is usually some way to get the device to do this, to make FCC testing easier. You can make these measurements with home-made equipment (many amateur radio operations would have some of the pieces, try www.arrl.org for an intro to that). However, if you want accurate measurements this is usually done with a calibrated antenna, connectors, coax cable, etc. That would be difficult to insure without some expensive equipment to boostrap yourself. A local university might be willing to help you calibrate your own antenna, coax, RF power meter, etc. Once that was done you'd have a chance at making real measurements, although they probably still wouldn't satisfy the FCC.
Re: How to measure dBm ?
From: fubini-ga on 12 Dec 2005 20:45 PST
I'm not really familliar with dBm, in fact, I don't know what that stands for, but I know how decibels (dB) are calculated, so I think this might be applicable. Bels are a measure of intensity of a sound wave, not of the volume. This is a critical distinction. The way bels are caclulated are Log(I/Io) Where I is the measured intensity and Io is the reference intensity which is commonly accepted to be 10^-12 W/(m^2)(Which I think is such because it's the lower threshold of human hearing). A decibel is (as you might have guessed) is almost the same as bels, but put your measured intensity as I^10. Thus, Db=10*Log(I/Io). Intensity is measured as Power (in Watts, in the case of sound) per unit area. Power is defined to be the energy transferred per unit time. So really the units on Db are Joules/((seconds)(meters squared)). I don't know how to apply these concepts to electromagnetism, but I hope it helped.
Re: How to measure dBm ?
From: microacer-ga on 14 Dec 2005 13:46 PST
For field strenght measurements, the general concern is how much power is being received from a certain distance. Normally, the units that you would use for this is in V/m (volts per meter - electric field) or the actual received power in Watts. However, in practical applications, this actual received power is normally less than 1 Watt. That's why the dBm unit is used to translate the really low power readings into a unit that can be grasped easily. 1). It would make sense for the end users to measure the received power to determine the signal characteristic. For instance, if your receiver can process a certain minimum power level, then you can use the RF power meter to determine how far you can go away from the transmitter in order for your receiver to work correctly. Think in terms of a radio station. The farther you move away from the station the more your signal quality deteriorates. In terms of a transmitter (emitter), you want it to emit a signal efficiently so that it can still fulfill the required power level at certain distances. This emitted signal depends on the device design (power output, antenna characteristics, etc.). 2). For portable applications, I suggest this company http://www.digifield.com/. However, if you are willing to shell out big bucks, you can use try http://www.ztechnology.com. 3). It doesn't matter if you are sending packets, audio, or what kind of data. The signal being emitted can still be characterized in terms of signal strength by the RF power meter. In terms of measuring received power, you need to have a control environment such as an anechoic chamber. The reason why you don't want to do it outside is because there are different factors that affect the signal quality such as weather conditions, obstacles, time of day, etc.
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