Hi I Recently need to define some of my own content actually, which is not quite unusual I needed to link to it and it turns out it was I Wanted my explanation of near field versus far field EMC or electromagnetic conformity in regards to EMC testing and what is the difference between near-field and far-field Let's try and explain it. and as it turns out, I've done like four maybe five videos on various aspects of EMC testing, emissions and probing, near field probing and stuff like that and it was actually buried away I had to do some finding it was buried away inside one of my videos. so I thought I'd just split that out and do a separate video here. It's mostly recycled content, but it'll be better for people trying to find information for near-field What's the difference between near-field and far-field testing? And as it turns out you, you've no doubt seen my previous videos.

if you haven't I'll link them in the hall down below. These aren't near field probes. these are H field or magnetic field probes of various different sizes. And I've done a video on how you can manufacture your own for like 10 bucks or less.

although these are like sets of these are pretty cheap these days. I might link some sets down below on Aliexpress you can pick up a whole set so you know you can argue it's probably not worth making your own. If you already got the rigid coax, you can do that and you can make one for like 10 bucks. Really cheap and you get a little preamplifier.

you can get those really cheap as well. But anyway, these are near-field probes designed to probe directly onto your PCB and then you've got your electric field or a field probe like this. And there is a huge difference between electric field and magnetic field probes. But when it comes to measuring the far field ie.

what you get if you send your product away to an EMC test house to be compliance tested versus a very the various Sis per standards or whatever standard you're actually testing against for electromagnetic conformity, then they're going to test it in the far field. They don't use near field probes like these. These are only for your own. like a debugging and pre-compliance troubleshooting and stuff like that.

Extreme handy And you should have them and you should know how to use them. But far-field testing is significantly different. and I've now got a far field probe. Yes, it's a probe.

a far field antenna because when your electric fields and magnetic fields combine at a certain distance, information come in shortly. You need a regular antenna like this. In fact, this isn't a regular antenna. this is a log periodic design.

This one's designed for about just under 300 megahertz up to one gig. And unfortunately, the SIS per standards that you might generally test against and for compliance for product compliance generally might be say thirty megahertz up to a one gig for example. So you need a much bigger antenna for this. So anyway, I Do plan on using this incent for doing some far field measurements in a future video, so watch out for that eventually.

So the whole idea is that you take your product and you take it to your Emc test house or an open area test site and oats and you whack this on your table. You power it up and you put your far-field antenna a certain distance away from it at a certain orientation and you can rotate your product as well around like this to get the different axes and stuff like that. But then you can measure the electromagnetic far field emissions of your product. So anyway, there is a good quite a substantial difference between a near-field and far-field and it's very important to understand.

So if you are looking at like getting a like a real antenna for example, am I for far field testing like they're quite expenses, this is like a five thousand dollar one. but this will go down to 20 megahertz for example so it'll cover that entire compliance range pretty much. but different products have different compliance standards and different frequency ranges you need to test over and things like that. It's actually quite complex.

So yeah, if you want to do it properly, of course you've got to go to an EMC test house. They will tell you exactly what standards that your particular product is applicable for and they'll test against those particular standards. But yeah, if anyone knows where you by a a far-field EMC test antenna that goes down to say 30 megahertz, please let me know because I have been trying to find a cheap one and I cannot do it. This one is only 50 bucks and I'll link it on Aliexpress down below.

And as I said, it's an order of magnitude higher than what I want? It's about 300 megahertz enough I Want to go down to 30 megahertz for your various basic product standards? but of course, then lower the frequency that goes the physically bigger antenna you've got. So this is a log periodic designed. for those who don't know, There's a little SMC connector up here and the center pin of that it goes to one side of this. even though the silkscreen shows both, there's actually only one PCB trace there and then it'll alternate the next PCB trace and then this side and then that side and the other side contain is hooked up to the ground of the antenna.

so it'll be the opposite one. So the center conductor will be this one here and then the ground will be this one down here like this and it goes down for the different wavelengths like this. So once you get down to 30 megahertz has to be, you know you have to start doing you know, more convoluted designs something like this. to get those low frequencies.

You can see that. Yeah, this one's got a combination of the log periodic and the bowtie approach antenna down there. So yeah, if anyone knows we can get one of these cheap, please let me know. I Haven't been able to find the infos but I looped in this one.

It's like 50 Yankee bucks or something on Aliexpress So I haven't tried it out yet, but I plan on giving this a bowl to do some far-field testing on my new 4 layer inner conductor outer grounded board, so that should be interesting, but that's a future video to come within the next month or two. I Guess it might be immediate, but I'll eventually get around to doing that. So here with is the explanation: near field versus far field. Thanks.

Catch you next time. And the thing with these H field magnetic probes and it's not like an issue with them. In fact, it's a feature is that they are dependent upon the Orion tation. They work in the plane.

so if you've got your coil like this, it's picking up magnetic fields that are in that flat plane there. So you'll notice that if we take this, there's a spectrum and that's over 250 megahertz. And if we simply rotate that like that, it picks up different components. Look at that so you can actually use that as a feature.

Using a smaller diameter wind, you can get down there and you can trace down are your offending components and traces better and things like that. So I probably have to do a whole separate video on this. But yeah, it does make a difference. The orientation We've seen quite a significant difference here between the four layer and the two layer board makes a heck a difference.

like typically like broadband noise. In this particular case about you know, 15 DB or so and that's a lot. But does that translate If you measure, say a 15 DB difference here? Does it actually with your near-field probes? Does that actually translate to a 15 DB difference on your EMC testing when you put it through the test house and you test it against the compliance standard? Well, the answer is unfortunately not these near field probes. both the H field magnetic field and the electric field probes.

All this is as I said, the near field. and whereas all of the compliance testing is done in the far field and I'll explain that in a minute because I have a Dave card. So what's the point of using are these near field probes if they're not sort of like quantitatively equivalent to what they do in the test house? Well, the good thing about it is that at the design stage or maybe if you fail compliance or something or you need to or you're doing some pre compliance testing, you can go around your board and sniff all around your board. And with the H field and the evil probe to see if there's any issues, see if there you know anything's radiating wildly and stuff like that you can.

You might be able to see a big spike or something at one particular frequency. you might go or we need to knock that down. Even though you don't even know you, it might be compliant. At the design stage.

You might go. Well, you know. I'm not gonna take any chances and I'm going to knock that problem on the head now before I send it across to the test house. So we'll briefly talk about near-field and far-field here and how it relates to the electromagnetic radiation.

Now A you might have heard the term electromagnetic radiation. It's electro and magnetic, contains electric and magnetic components, and you can look at it. This is like the standard visual representation of it. The electrical field might like would go up in the z-axis like this, and the H field is 90 degrees from that.

So they actually propagate in different orientations. And of course this is the wavelength. And here's a cute little animation just to show you how that works as it propagates down. Now what we actually have to look at though is what's called the wave impedance and this is where the difference between near-field is Everything on this side and far-field is everything on this side.

Now the wave impedance in ohms like this in there. for this particular scale, please excuse the crew did didn't have time to build up the scale. a lot of pain it from 10 ohms to 10,000 here. So this is where you have to define far field and near field.

Well, the electric field and the magnetic or H field. There is a difference between H and B. By the way, be as flux density. You might sometimes be here at chord B but it's actually H magnetic field as opposed to induced magnetic field as opposed to maintain any flux density.

Anyway, won't going to the details. so the H or magnetic field actually has a very low impedance source in the near field whereas the electric or a field has a very high impedance. It'll clarify that in a minute. but basically it all comes down to the wavelength lambda here and this is normalized to 1 here.

and it's Lambda on 2 Pi which is basically we're going to normalize to that value. And of course let's take for example, 100 megahertz is a wavelength of 3 meters. so Pi onto that's about 1/2 meter. So when you get to 1/2 meter away from your product, this is where the electric fields and the magnetic field actually start to converge.

It's not really clean like this. There's a bit of you know overlap here and this is like the transition. There's going to be like a transition region in here where the two fields eventually combine and anything over roughly half a meter away at a hundred megahertz. The electric and magnetic fields combined to give you a singular impedance which actually happens to be 377 ohms in free air.

So anything over the wavelength on two pi is deemed to be the far field and anything closer physically closer than that like we just did with our probes. Here is the near field. Now this is why we have two different types of probes. One is the H field probe, the magnetic probe, the other is the E field or electric field probe, and the Magnetic or H field is going to be generated by higher currents ie.

sources that have a very a lower impedance. So for example, if you've got a lot of current flowing in a in a particular art race either due to an actual like heavy current switching or even very fast switching that's dumping a lot of energy into the bypass capacitors and the capacitance between the power planes and everything else then that's generating. Typically be generating a magnetic field due to the low impedance and the high current, but very high impedance. Things that don't generate lots of current.

Then they generate electric fields and hence the bigger source impedance. So you could generate electric fields from, say, just like a static power supply for example, your 5 volt power supply. Whereas all you're switching stuff will dominate down in the H field here because there's lots of current being dumped into the trace, our the load capacitance, or the particular load itself when you switch into things. So that's why you need to use these two different probes and the magnetic field probes.

They are sensitive to orientation like this and like that as well as I talked about on the plane. Whereas the electric field is not sensitive, you can just put that in any orientation and it's not going to make a difference. So if I use my ear field probe like this and let's say I probe this power trace over here like this can see, it's really not going to make any difference. the orientation that I put that in.

it's just completely insensitive to that because there's no magnetic field coupling. it's electric field coupling and it's just purely the distance. But if you take a magnetic loop probe like this and I just change the orientation like that, Wow that makes a big difference. It really brings out the peaks if I put it vertically like that.

if I put it horizontal, it gets more of the current flowing through the trace. And if we use our smallest H field probe, let's just have a look at let's say this like blank area over here. This is our four layer board like this. or maybe right over on the edge of the corner of the board.

right over here like this. and let's compare that with our two layer board here. Bingo! Look at that because we've actually got a power trace actually running right around this corner as well, which we actually physically remove. and you can actually see that the power trace actually running all the way around there like that.

So that's just going to radiate like buggery. But even if we go over just the ground plane there, you can see it's much much higher than we get with the four layer board. And this is why at the EMC test house though tests in the far field here because it binds the electric and magnetic fields together and basically the typical testing distances would be like one meter, three meters, five meters, 10 meters, for example, away. It depends on the type of product they testing into which are standard they're actually testing too, but say, if you put it 10 meters away, then you can have a larger rotating turntable so that your product rotates around like this on the turntable.

and they can measure all the axes like this when they, while they have, they're super expensive. You know, buy a conical, super calibrated measurement antenna ten meters away measuring over say 30 mega Hertz to 10 gigahertz far-field for example might be a typical measurement ray. and then there will be our standard like envelopes that you have to get under and also Peaks and things like that and it gets. You know the standard gets are quite complicated, but yeah, just the near-field testing that we do here doesn't really translate to the far field, but you can certainly I get an indication of whether or not you've got any nasties on your board.