Why can only the electric field (E) be measured?

[aphelix]

I would like to consult with you about something. Mobile phone towers transmit at relatively high frequencies. From a health perspective, on the internet :) all measurement values are interpreted in microteslas. However, with the Wavecontrol probe (WPF8) I have, only the electric field (E) can be measured. In fact, all of the company's high-frequency measurement probes can only measure the electric field. Is this a limitation? Is it pointless to measure the H value? Or can we calculate the H value through a general calculation like (H = E * 377)?

 

[BvU]

!​

There's a thread on this:

Hi. A electromagnetic wave consists of an electric and a magnetic component. I believe that the electric field strength is measured in volts per meter. The magnetic field I think is measured in Tesla. Let's imagine that I measure the electic field strength of two different radio stations and the electric field strength measured is 100 microvolts per meter. However, let's say one station is employing a dipole and the other a magnetic loop antenna. My question is, would the strength of the magnetic component of the wave be the same for each station? Thanks.

$\$

 

[Baluncore]

Antennas are modelled based on the currents that flow in the conductors. Those currents generate magnetic fields that are summed in different directions to establish the far-field pattern of the antenna. That is why the magnetic field is specified for a transmit antenna.

I suspect the probes used with the Wavecontrol instruments are interfaced to the meter through a known impedance, probably 50 ohm. It is the voltage appearing in that impedance, that is reported. Wavecontrol also make a couple of H-probes. Most probes are quoted as having a range of; V/m, and an equivalent; A/m.
https://www.wavecontrol.com/rfsafety/images/data-sheets/en/SMP2_Datasheet_EN.pdf

 

[sophiecentaur]

In fact, all of the company's high-frequency measurement probes can only measure the electric field.

At a distance of just a few wavelengths from an antenna, the wave propagates as if it's a plane wave in free space. The fields closer in are only of interest for health and safety.
The H field and the E field are at right angles to one another and the impedance of EM waves in free space is

which is about 377Ω. You can measure either but measurements of E are usually preferred at frequencies of more than a few tens of MHz because it's very convenient to use and calibrate a short dipole probe. For longer wavelengths , the local impedance and fields are often very 'site specific' and subject to local bits of metal. The impedance can be very variable. Magnetic probes are often. used here because measurements often tend to be more reliable (not affected by the user, holding the meter etc.)

mV/m tends to 'mean something' to Engineers, more than H fields. I'm sure there are exceptions to that statement; EE is a wide and varied topic!

 

[vanhees71]

It's $\vec{E}$ and $\vec{B}$ that belong together (or $\vec{D}$ and $\vec{H}$)!

 

[sophiecentaur]

It's $\vec{E}$ and $\vec{B}$ that belong together (or $\vec{D}$ and $\vec{H}$)!

My experience in RF has been that E and H fields are the pair used universally. I looked 'all over' to find E and B being used in antenna work. I couldn't find anything. Googling "E and H" fields provoked the expected shower of hits. It could be because H refers to free space / air but B (the magnetic induction) is relevant when dealing with transformers and motors.

Horses for courses and chacun à son goût. I think

 

[vanhees71]

In some branches of the electromagnetic community obviously the relativistic formulation hasn't arrived yet. This is a historical confusion, which explains why $\epsilon$ and $\mu$ are not in the same logic, when it comes to in-medium electrodynamics ;-)).

 

[sophiecentaur]

In some branches of the electromagnetic community obviously the relativistic formulation hasn't arrived yet. This is a historical confusion, which explains why $\epsilon$ and $\mu$ are not in the same logic, when it comes to in-medium electrodynamics ;-)).

That makes me feel like a plumber in a room full of coronary surgeons.

 

[WernerQH]

In some branches of the electromagnetic community obviously the relativistic formulation hasn't arrived yet. This is a historical confusion, which explains why $\epsilon$ and $\mu$ are not in the same logic, when it comes to in-medium electrodynamics ;-)).

I think the confusion is on your part. Have a look at section 3 of the Optics volume in Sommerfeld's Lectures on Theoretical Physics.

 

[vanhees71]

I'd rather refer to paragraph 2 of vol. 3, where Sommerfeld very clearly states that E and B on the one hand and D and H on the other belong together. In fact in the relativistic formulation, either of these are the components of antisymmetric four-tensor fields. Nowadays there's no doubt about this anymore.

 

[WernerQH]

Nowadays there's no doubt about this anymore

Of course not. You've simply missed the point of what the previous posts were about.

 

[WernerQH]

It's $\vec{E}$ and $\vec{B}$ that belong together (or $\vec{D}$ and $\vec{H}$)!

Incidentally, the Poynting vector is a mixture, $\mathbf{E} × \mathbf{H}$, and not $\mathbf{E} × \mathbf{B}$.

 

[vanhees71]

Still, $(\vec{E},\vec{B})$ are the components of one four-tensor and $(\vec{D},\vec{H})$ that of another. So these "pairs" of components belong together. That you can build other quantities, which "mix them", is of course not excluded. It's clear that the energy-momentum tensor of the em. field in matter is by far not as simple as this really simple issue ;-).

 

[WernerQH]

Why don't you actually read what I suggested? You are just adding to the noise by reiterating things that everybody knows, and that are not relevant in the context of this thread.

 

[vanhees71]

I have no clue, what you mean. Just leave it at that. It's simply wrong that you claim that Sommerfeld would say anything different than I did.

 

[WernerQH]

It's simply wrong that you claim that Sommerfeld would say anything different than I did.

In your post #5 you seemed to disagree with @sophiecentaur's post #4.
His definition of impedance in terms of E and H agrees with what you would find in Sommerfeld's derivation of the Fresnel formulas. (If you would care to look.)

 

[vanhees71]

I don't understand, what this has to do with the simple mathematical fact that E and B as well as D and H belong together. It's stressed by Sommerfeld in Paragraph 2 of his electrodynamics book. H doesn't claim anything else in his optics book too. His derivation of the Fresnel formulae is the standard one.

 

[sophiecentaur]

the simple mathematical fact that E and B as well as D and H belong together.

I think it depends on what you mean by "belong"
We all reach for Feynman when the gloves are off and he writes, in this lecture:
$S=ϵ_0c^2E×B$,

In my respected text book by Panofsky and Philips, Classical Electricity and Magnetism (from my ancient Uni course), they say
$N = E×H$
Clearly you can choose the pair to work with.

In the context of practical EM Engineering, it saves a lot of time to avoid writing that constant - same as avoiding 2π by using angular frequency ω. Why get cross about it?

 

[vanhees71]

You still seem not understand my argument. It's a fact that $F_{\mu \nu}$ is a four-tensor with components consisting of the components $\vec{E}$ and $\vec{B}$ (in the used reference frame). The same holds for $H_{\mu \nu}$ with $(\vec{D},\vec{H})$.

 

[WernerQH]

si tacuisses ...

 

[vanhees71]

I don't understand, why you deny this simple fact. Obviously you are not even convinced by Sommerfeld's statements, you've quoted yourself.

 

[WernerQH]

I don't understand, why you deny this simple fact. Obviously you are not even convinced by Sommerfeld's statements, you've quoted yourself.

I'm not denying this simple fact. I also agree with Sommerfeld.
You simply don't understand that you are talking past everybody else on this thread.

 

[vanhees71]

I give up.

 

[sophiecentaur]

That is why the magnetic field is specified for a transmit antenna.

To help with context, that statement may be true for performance acceptance levels of physically large antenna (MF or HF, perhaps). That's because, as already stated, the local impedance variations tend to give less variation in measured H. But, Broadcasters and comms Engineers pretty much universally use the E field , measured in the service area of the transmitter. V/m is what's used to describe received signal strength and 'exposure'. It's all about repeatability, when you get down to it.

In my limited experience of field strengths in the near vicinity of transmitting arrays (RF Hazard protection), it's V/m that is specified. A typical low cost meter.

But, for higher frequencies, the transmitted Power in a given direction is often specified.