EMF meters? Something seems wrong... - Page 1
Forum section: electronics / testgear

author: Jolle
author_rank: Contributor
timestamp: November 03, 2016, 09:53:02 pm
content: I am working with EMC standards (on a literature basis, not a lot of hands on) and thought it would be good to get a feeling of the amplitude and frequency allocation of the EM waves we are surrounded by. Optimally I would get my hands on a spectrum analyser+antenna so I would be able to see which frequencies the EM waves are positioned at, but that is unfortunately not within my price range. But then I found EMF meters which are more price friendly and should be able to measure the strength of the fields present in the real world.But searching for EMF meter reviews reveal a myriad of paranormal activity homepages and people being afraid of microwave ovens, and not a lot of engineers using them for anything?! Even a search on eevblog gives very few result!And it made me wonder, are they good for anything serious?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 04, 2016, 03:53:25 pm
content: A properly calibrated device can measure something meaningful, but it's still not all that useful as the bandwidth is unspecified. If you're measuring everything, then you don't know what frequency it was from... it could be anything... noise, a signal, a nearby radio station, who knows. That can be tricky, for RF purposes.The simplest meters are also peak reading, which means one signal dominates, and if it's rather peaky, it comes through much more strongly than the rest. Whichmight be what you want-- say, to simulate the response of a radio receiver with AGC (which takes some time to respond to peak levels, but follows them nicely).Offhand, most airborne signals are in the mV/m range or less, which means, for a roughly resonant length antenna, you get about as many mV at the terminals. Distant signals go into the uV, or under the noise floor. High gain (very directional) antennas will get more, but not hugely more (The biggest, Arecibo, is 73.5dB, which is only ~three orders of magnitude -- okay, to be fair, it is pretty huge gain... but it's a pretty huge antenna, too!).You can observe signals of this level with merely a scope (most can resolve a few mV), but you might not be able to get a good trigger on it (peaky signals dominate the trigger, and stable displays only come from periodic waveforms -- that is, anything with a strong spectral peak).EMC measurements are rather more specific; they're done with a specific detector, bandwidth and sweep rate. The fixed bandwidth means you'll get a very different number from broad signals, and converting between the two measurements isn't simple (you have to RMS over the entire spectrum to find what a wideband voltmeter would measure).Now, in combination with a tunable filter, you basically have a spectrum analyzer -- so, while the voltmeter may be kind of dubious by itself, it's the heart of quite a lot of powerful devices! (And how would you make a tunable filter? You use one very good filter, and hetrodyne the input signal around it, of course!)Tim

author: billfernandez
author_rank: Regular Contributor
timestamp: November 05, 2016, 01:06:12 am
content: Amazon.com has a wide range of EMF meters, some of which are cheesy "paranormal" or "natural health" detectors, others are true measurement instruments: but those start at $200 - $300 each. In all cases EMF meters are generally broadband so you never know which specific frequencies you're detecting. And depending upon what you're looking for you need the right frequency range (e.g. 60 Hz housecurrent fields vs 2.4GHz microwave oven and Bluetooth fields, vs 2.4 to 6GHZ WiFi fields). For example Extech has several different ones. It also depends a lot on whether you just want to poke around and see what's in the air, or if you want to make some sort of meaningful measurements.The German company Aaronia got their start making small, handheld measurement instruments for various frequency ranges, field strengths and field types (E vs H fields), and I used to have fun poking around with them -- sometimes being amazed at what I found; so it can be fun. But now they just make spectrum analyzers with non-trivial prices.But if you really want to do real EMC measurements you need calibrated equipment, the correct antennas, and to be able to isolate frequencies.

author: Someone
author_rank: Super Contributor
timestamp: November 05, 2016, 01:37:10 am
content: The biggest problem with the simple EMI measurement tool is that its sensitivity will vary with frequency, this can be calibrated if you know the aggressor frequency but if you are trying to measure sources of unknown or multiple frequencies then you need a tool which has a known flat frequency response.

author: HKJ
author_rank: Super Contributor
timestamp: November 05, 2016, 04:50:39 pm
content: Instead of spectrum analyzer, look for SDR (Software defined radio). They can receive a very wide range of radiation, making graphic displays of it and demodulate it (I.e. receive radio).

author: MrWolf
author_rank: Regular Contributor
timestamp: November 05, 2016, 06:26:29 pm
content: Gigahertz Solutions makes top quality meters:https://www.gigahertz-solutions.de/en/cat/index/sCategory/137I use this one for checking new hardware and for general electrical troubleshooting:https://www.gigahertz-solutions.de/en/emf-meters/low-frequency/me-series/397/me3951a?c=159Excellent all-around device. For example I pinpointed groundloop problem in the apartment below mine thru floor. Went to check it out on site and bingo - there was small groundloop thru water heater.These meter-type devices are good for very fast problem source identification.As for SDR - look for one with big spectrum window,for example AirSpy R2 has 24 – 1800 MHz range and 10 MHz spectrum bw.http://airspy.com/airspy-r2/http://www.rtl-sdr.com/using-the-airspy-as-a-low-cost-spectrum-analyzer-with-spectrum-spy/https://greatscottgadgets.com/hackrf/HackRF 1 MHz to 6 GHz range, 20 MHz bwRegular cheap "dongles" are crap compared to these.