EEVblog #1273 - EMC Near Field vs Far Field Explained

An explanation of near-field vs far-field in relation to EMC compliance testing.
H-field magnetic probes vs e-field electric probes.
Explanation extraction from video #1176

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[Richardincancale]

10:00 onwards - Very helpful explanation of the electrical versus magnetic fields, and why and when the far field starts - thanks!

[Ziferten]

Build your own antenna! There's a ton of ham radio literature about log periodic antennas. No doubt it'd be a challenge, but I bet you could get it done under 100 USD. I specifically recommend the ARRL Antenna Handbook.

[eugenepohjola]

Howdy. Nice presentation. Some comments.
Near field. The E and H components are 90 degrees out of phase. Almost all field energy collapses back into the antenna periodically.
Far field. The E and H components are in phase. The energy has broken free and travels forward.
The intermediate field or the Fresnel field. The phase shift is between 90 degrees and 0 degrees. Some of the energy collepses and the rest propagates.
What is the mechanism that splits the components into collapsing parts and propagating parts ?
Collapsing takes time. Field lines closer to the antenna have time to collapse. Field lines farther away do not have time to collapse completely they will encounter the build up of the next half wave field at the antenna. The ends of the field lines disconnect from the antenna and interconnect to form loops that travel forwads.
Regards.

[pegr]

This was a very good video about EM radiaton. Thanks Dave!

[johnwest]

Dave, I think you 'described' it without 'explaining' it. I always figured that a layman's explanation for the difference between near-field and far-field signals was that near-fields are the portion of the field that is close enough (at the speed of light, ) to collapse back onto its source as the signal's source polarity reverses, while far-fields are the portion of the field that is not close enough to get back before the reversal, and thus is 'pushed away', and must continue on as free-space radiation, an effect that essentially turns an antenna into an electromagnetic-field 'pump'. This explains why longwave near-fields can extend a mile from an antenna, while microwave near-fields are centimeters or less.

[zetacrucis]

Good one Dave. Being a physicist, I was gonna call you out on the H-field vs B-field thing but you included it in passing (which is plenty good enough!) 10:06 - nicely done!

[gincoba]

Hi Dave! Go for Schwarzbech, good antennas, and fair prices! But it depends if you want an antenna just to cover 30 to 250 or 300 MHz. In this case go for a biconical antenna. If you want to cover the band from 30 MHz to 1GHz with a single antenna, go for a so called hybrid antenna, the union of a biconical and log antenna, otherwise called bi-log. Happy EMCing!

[valpine]

HAM here. Look up Mag loop receiving antennas. Highly directional. 50khz to 50mhz with a 3ft loop.

[cheyenneriver]

Excellent! Dave = walking electronics encyclopedia!

[byronwatkins]

This "E field" probe is actually an electric potential (voltage) probe. Electric potential is a scalar and has no direction. In isotropic materials B and H are always proportional to each other. In fact it is quite difficult to find or to engineer materials having tensor magnetic susceptibility; materials having tensor electric susceptibility are numerous. In free space E=cB and B=mu H, but in materials mu is larger than 4pi X 10^-7. So whether we call the magnetic component of EM waves B or H is purely theoretical other than the proportionality constant with E.

[MrTomasMaliauka]

It would be iteresting to understand the difference between near field (NFC) and far field devices. As you said in one of your videos, near field devices work only in close proximity and in magnetic fields more like transformator with air core. I like your tutorial type videos.

[mohamedlanjri]

If you look at the datasheets of those expensive antennas you'll apreciate how much variation is on parameters like the antenna factor, antenna gain, directivity and so on. The reason for those prices, apart from high quality materials that imporve the performance, are the calibration data that you get with them in order to be able to calculate with accuracy radiated and received power (Basically same concept as you explained in your video about why Fluke meteres are so expensive). So basically you're buying parameters to fill in your Friis ecuation, and certifications needed if you want to be a EMC certification lab. I think you can build your own antenna but you'll end up with the same problem of having several cheap antennas that cover all the frequencies range, the only advantage I see is that you won't need to switch from one antenna to other while sweeping the entire band.

[DrMarkVaughan]

You were asking about lower freq antennas. Mil461 describes designs for biconical antennas that cover the lower end of the radiated field. Not overly difficult to make. I don’t believe they detail it also requires a 4:1 balun.
The bigger wide range hyperlogs are practically a log periodic crossed with a biconical of course.

[johnclawed]

Excellent. Thank you. Now, a little more on the difference between H and B would be nice.

[Unsensitive]

I had to learn the basics about Nearfield effects of RF transmitters for MRI Safety.

It's absurd that a Magnetic Resonance Imaging scanner, designed for imaging humans, can cause RF burns.

If a patient has any part of their body within 5mm of the inside bore of the MRI scanner, nearfield effects are possible.

This would be like building a car with holes on the floor, then blaming the driver then they lose a foot by putting it through the hole.

Hoping to understand better from this video :)

[Mystickneon]

Dave, you have said in the past you're not a radio guy, but you should look into an antenna called the Quad Loop. Basically, a simple square(or any shape, really) antenna with a variable capacitor inserted at the feedpoint. I have built several of these(including the caps), and while the dimensions and cap values are important, they are not critical to function, as it's a tuneable antenna. Bonus: it has a fairly narrow(bilateral - dependent upon shape) gain pattern, so isolation is easier. One I manufactured for 20 meters(14MHz) measures only about 2/3 of a meter in its largest dimension. Getting down to 30MHz or low-VHF should not be a problem at all, and it would be even smaller. I used a simple SDR as a frequency domain analyzer to verify my tuning range, but I'm pretty sure you have one of those laying around the shop. As you tune the cap, you'll see the spike in the signals move around the frequency domain.

Barring that, a coil of wire of the correct length or one wound around the correct ferrite rod will get you reception in high HF/low VHF. Or even lower - this is what most AM receivers use and I have seen those measuring only inches long; albeit with sometimes strange gain patterns, but it you put on in a Faraday Bucket(a cage with an open side), you may get good results.

PS, I'm trying to get down to the site for the Solar Road demo in Baltimore and get some pics for you. I try to avoid that place, as it's all a "bad neighborhood"...

[DiyMech]

Welcome to EMC hell !!, wait till you package it in your designed box, send it to the very expensive test house and it leaks like a siv, both radiated and conducted. keep up the excellent vids

[trainfan]

EEV Community....thank you for your Arrogance!! I asked why the testing is done but I received not one response. Thank you!!

[MrTomasMaliauka]

I am totally not a EM radiation hobbyist ... I have never heard abaut near field zone, where separate electric and magnetic fields are converging in to a point where they become an electromagnetic field/ radio wave. Very interesting info. Could you make more video tutorials about EM fields and radio wave basics? Maybe viewers could build up some kind of cool and usefull radio devices after these tutorials...

[friedmule]

15:01 Last time I saw a smile like that on anyone, was when at boy was told he could eat as much candy he would! :-)