• I
garryA
So, when a radiowave strike a surface, whether the wave get reflected or not depend a lot on the characteristic impendence of it. If the radiowave travel from free space to a material. Any mismatch from the approx. 377 ohms impedance of free space (Z0) will result in a reflected wave; the greater the mismatch, the greater the amount reflected. Since conductor such as metal has very low impedance therefore good at reflecting radiowave. But how about insulator?. Could an insulator have the impendence high enough (much higher than free space value) that make it also reflect radiowave?

Mentor
Could an insulator have the impendence high enough (much higher than free space value) that make it also reflect radiowave?
AFAIK, the ##Z_0## of free space is as high as you can get. You can get lower values of ##Z_0## with different dielectrics that you propagate through.

The value to look at is the refractive index ##n## of the dielectric material, compared to whatever is surrounding it. For a vacuum, ##n=1##, and for water and other dielectrics, ##n>1##. See this Wikipedia page for more information on how to calculate the amount of EM reflected by the interface:

https://en.wikipedia.org/wiki/Fresnel_equations

Klystron, Delta2, DaveE and 1 other person
Gold Member
AFAIK, the ##Z_0## of free space is as high as you can get. You can get lower values of ##Z_0## with different dielectrics that you propagate through.

The value to look at is the refractive index ##n## of the dielectric material, compared to whatever is surrounding it. For a vacuum, ##n=1##, and for water and other dielectrics, ##n>1##. See this Wikipedia page for more information on how to calculate the amount of EM reflected by the interface:

View attachment 302829
https://en.wikipedia.org/wiki/Fresnel_equations
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/refr.html

Delta2
Homework Helper
You should be careful with the index of refraction. It is not a constant valid for the whole Em spectrum. The index for visible light does not have to be similar to that for low frequency radiowaves. It makesmore sense to look to the reflectivity in the specififc frequncy domain. Or look for the dielectric constant of the dielectric in that frequency domain.

Klystron, dlgoff, vanhees71 and 1 other person
garryA
AFAIK, the ##Z_0## of free space is as high as you can get. You can get lower values of ##Z_0## with different dielectrics that you propagate through.
So in short insulator can still reflect radio wave, just not as good as metal?

Delta2
Homework Helper
Gold Member
So in short insulator can still reflect radio wave, just not as good as metal?
Yes insulators are poor reflectors, I think up to 90% of the energy of the incident wave passes through.

garryA
Yes insulators are poor reflectors, I think up to 90% of the energy of the incident wave passes through.
Someone told me that insulator can have very high surface impedance much higher than the 377 ohms value, therefore they can be good reflector. What do you guys think?

Homework Helper
Gold Member
Someone told me that insulator can have very high surface impedance much higher than the 377 ohms value, therefore they can be good reflector. What do you guys think?
This is not correct according to mainstream physics. Maybe some unknown experimental material can have i don't know.

garryA and dlgoff
Mentor
Someone told me
That is not a valid reference for discussion at PF.

And as you can see from the EM reflection equation that I posted from Wikipedia, the DC resistance of a material does not enter into that calculation. Reflection from conductors is governed by other equations, but you were asking about insulators/dielectrics, so that's the question that I addressed.

dlgoff
Homework Helper
Someone told me that insulator can have very high surface impedance much higher than the 377 ohms value, therefore they can be good reflector. What do you guys think?
How do you define the surface impedance for insulators? The formula for metals contains the conductivity and skin depth. But anyway, the reflection coefficient for insulators depends on what is called the wave impedance or just the impedance of the medium.

garryA
garryA
That is not a valid reference for discussion at PF.
Iam not trying to use that as a reference, I simply trying to understand the reflection behavior of material.
Since my knowledge in this field is a bit limited

Homework Helper
Maybe it is good idea to start by looking at each case (ideal insulator and ideal conductor) separately.

Gold Member
If we make a prism out of paraffin wax, we can demonstrate total internal reflection at microwave frequencies by using an insulator.

vanhees71, garryA and nasu
garryA
How do you define the surface impedance for insulators? The formula for metals contains the conductivity and skin depth. But anyway, the reflection coefficient for insulators depends on what is called the wave impedance or just the impedance of the medium.
I found here that surfaces impendance affect reflection
https://hal.archives-ouvertes.fr/ha...h-Impedance_Surface_Design_Considerations.pdf
"High-Impedance Surfaces (HIS) have been extensively investigated in the field of antennas. Such metasurfaces exhibit an in-phase reflection of incident waves, that makes them to behave like an Artificial Magnetic Conductor (AMC) within a limited frequency range. Consequently, HIS can be used as efficient reflectors"

vanhees71 and Delta2
garryA
If we make a prism out of paraffin wax, we can demonstrate total internal reflection at microwave frequencies by using an insulator.
I didn't know paraffin wax can reflect radiowave

Homework Helper
Gold Member
I found here that surfaces impendance affect reflection
https://hal.archives-ouvertes.fr/ha...h-Impedance_Surface_Design_Considerations.pdf
"High-Impedance Surfaces (HIS) have been extensively investigated in the field of antennas. Such metasurfaces exhibit an in-phase reflection of incident waves, that makes them to behave like an Artificial Magnetic Conductor (AMC) within a limited frequency range. Consequently, HIS can be used as efficient reflectors"
Though that paper doesn't explicitly state from what material HIS are constructed from, I think is is left implied that they are made by good conductors and not insulators.

garryA
Though that paper doesn't explicitly state from what material HIS are constructed from, I think is is left implied that they are made by good conductors and not insulators.
Isn't conductor supposed to have low surfaces impendance?

Homework Helper
Gold Member
Isn't conductor supposed to have low surfaces impendance?
I think you are confusing the electrical impedance or ohmic resistance with the characteristic impedance of a material.

davenn and dlgoff