# Help with radio waves and electromagnetic fields!

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1. Aug 28, 2015

### Moe123

I need help explaining that this assumption is not correct or correct:
A product uses Radio waves to turn on a LED. Radio waves can be a form of electromagnetic fields therefore, a Hall Effect sensor could be used in this device to turn on that LED.

2. Aug 28, 2015

### nsaspook

Very unlikely a normal Hall sensor could detect (unless very close) or even have the frequency range to detect most RF fields from consumer devices.

3. Aug 28, 2015

### Moe123

Are Radio Waves a form of electromagnetic field?

4. Aug 28, 2015

### nsaspook

Yes, a radio-wave is an electromagnetic field.

5. Aug 28, 2015

### Moe123

I am confused with the definitions of electromagnetic fields and electromagnetic radiation
Are these 2 forms the same thing or different? Is radio waves an electromagnetic field or is it electromagnetic radiation?
I read that electromagnetic radiation consists of electric and magnetic fields, yet it goes on to state that not all configurations of electric and magnetic fields are describes as radiation...like the earth's magnetic field.
Therein lies the question of what physicists define a 'field' to be?

6. Aug 28, 2015

### nsaspook

It's not completely black and white. With a component like a tuned coil in a radio circuit to amplify RF signals it's designed to operate in the near field region zone of space where the EM fields around it store energy but are mainly reactive, out of phase and don't dissipate energy into space. The antenna on the radio is designed to interface from the near field reactive region to the far field resistive/dispersive in phase region where we consider those EM fields as radiation.

A simple example of EM field zones. Where one ends and the other begins is far more complicated.

Last edited by a moderator: May 7, 2017
7. Aug 29, 2015

### davenn

Hall sensors respond to reasonably close and strong magnetic fields, not EM fields

Dave

8. Aug 29, 2015

### Moe123

Thanks for the input but I am still not clear on the difference between electromagnetic radiation and electromagnetic field

It seems from text books definition that radio waves are a type of electromagnetic radiation. From your definition, my inference is that a EMF is a separate component:” RF signal operates in the near field region where EMF around it “

Would not the antenna on the radio respond not to the position but rather to the wavelengths’ frequency?

I also read that electromagnetic waves are formed when an electric field couples with a magnetic field. Magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave. Therefore, lies my confusions since is this statement is correct, how can your statement that radio waves are an electromagnetic field be correct? Then, my man Maxwell makes this statement: “This velocity is so nearly that of light, that it seems we have strong reasons to conclude that light itself (including radiant heat, and other radiations if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws”

I need help understanding why a radio wave is considered an electromagnetic field!

9. Aug 29, 2015

### nsaspook

Electromagnetic radiation are electromagnetic fields (the electric and magnetic field are a part of a single more complete object, an electromagnetic field) that are considered detached from the source. If you have an antenna the near EM field energy will stop when the power source is off but the EM field energy we consider radiation (far field) will continue to propagate into space. Near the antenna there is a complex interaction of fields coupling back to the antenna that can greatly modify the pattern of radiation into space and It's tricky to express exactly mathematically but for a simple dipole antenna it's fairly simple.

Last edited: Aug 29, 2015
10. Aug 29, 2015

### Moe123

Wow, thank you for this info!!
But, you Nsaspook, said that a hall effect sensor,(only if it is very close), be used in this gadget that uses RF signals
The definition of the hall effect sensor that I have is the following: "A Hall effect sensor is a transducer that varies its output voltage in response to a magnetic field. Hall effect sensors are used for proximity switching, positioning, speed detection, and current sensing applications. In its simplest form, the sensor operates as an analog transducer, directly returning a voltage.:"
Could this sensor be used then per your definition of electromagnetic field?
Thank you!!

11. Aug 29, 2015

### ZapperZ

Staff Emeritus
This thread is a jumble of confusion. Is it on EM wave/fields/radiation, or is it on Hall effect/probe?

Have you looked up the concept of a Hall probe? The standard Hall probe, as davenn has stated, measures the magnetic field strength. In fact, it is effective for a STATIC magnetic field strength! And EM wave, by definition, will have an oscillating electric and magnetic field. Unless you have a very "low frequency" EM wave, or a very fast-response probe, I don't think using a standard Hall probe will measure anything in this case. So using the Hall effect is ineffective.

Zz.

12. Aug 29, 2015

### nsaspook

You forgot this part "or even have the frequency range to detect most RF fields from consumer devices"
Hall sensors are used to measure changing magnetic fields. The cheap ones I use have a bandwidth of DC to 250kHz with a di/dt of up to 70A/usec.
https://s3.amazonaws.com/amploc/PDFs/HANDBOOK+web.pdf

250kHz is likely not in the RF range you need.

Last edited: Aug 29, 2015
13. Aug 30, 2015

Thank you!