The length of a wave....

[DonS]

The wavelength is dependant on the medium which it propagates through.

While this is a correct statement, to a point, I disagree because of its overly broad implications.

As radio wave's propagate through the air and into my office or my house, the wavelength, or frequency, is not affected.

As an electromagnetic wave travels through different media (air, water, glass, perfect vacuum, copper, etc.), its speed changes. Its frequency remains constant. Since speed, wavelength, and frequency are related by speed= wavelength x frequency, if speed changes and frequency remains constant, then wavelength must change.

That's why, when calculating the length of a half-wave antenna for a given frequency, we cannot use the speed of light in a vacuum - we must use the (slower) speed through the antenna medium. If we don't, we'll calculate an incorrect wavelength.

 

[N_Jay]

The wavelength is dependant on the medium which it propagates through.

While this is a correct statement, to a point, I disagree because of its overly broad implications.

As radio wave's propagate through the air and into my office or my house, the wavelength, or frequency, is not affected. Instead what one finds is that the radio wave signal strength may deteriorate significantly.

However, it does not shift in frequency so that it ends up being a completely different frequency.

Sorry Charlie .. seems like its pick on N_Jay day.

Hey .. that rhymes ..

Sorry, I said Wavelength, meaning the phyisical length of the wave, not the commonly used (but literally inccorrect) meaning "frequency".

Geeze. :wink:

 

[N_Jay]

By the way, did some checking.

Velocity of Propagation is determined by the Dielectic Constant, not the resistance nor frequency. (Don't have a link to quote right now)


The 5% number I gave was wrong fofr cable.
5% is a rule of thumb to take into effect "end Effects, and other physical anomolies in antennas. (as others have said)

 

[crayon]

As an electromagnetic wave travels through different media (air, water, glass, perfect vacuum, copper, etc.), its speed changes. Its frequency remains constant. Since speed, wavelength, and frequency are related by speed= wavelength x frequency, if speed changes and frequency remains constant, then wavelength must change.

That's why, when calculating the length of a half-wave antenna for a given frequency, we cannot use the speed of light in a vacuum - we must use the (slower) speed through the antenna medium. If we don't, we'll calculate an incorrect wavelength.

DonS,

Thanks for posting! It is really hard to talk in generalities about RF when it is so dynamic in nature. I should have explained myself more, but at 3 am, it sure made sense me.

I am in total agreement with what you said. I was trying to turn the converstation away from radiating antenna theory to something that is more of use as it applies to antennas, coax and a scanner.

What I wanted to point out is that since a scanner's antenna is surrounded by air, it's okay to use c to represent speed. Generally, most people are unconcerned with velocity factors when building an antenna to only receive radio signals.

:wink:

 

[N_Jay]

As an electromagnetic wave travels through different media (air, water, glass, perfect vacuum, copper, etc.), its speed changes. Its frequency remains constant. Since speed, wavelength, and frequency are related by speed= wavelength x frequency, if speed changes and frequency remains constant, then wavelength must change.

That's why, when calculating the length of a half-wave antenna for a given frequency, we cannot use the speed of light in a vacuum - we must use the (slower) speed through the antenna medium. If we don't, we'll calculate an incorrect wavelength.

DonS,

Thanks for posting! It is really hard to talk in generalities about RF when it is so dynamic in nature. I should have explained myself more, but at 3 am, it sure made sense me.

I am in total agreement with what you said. I was trying to turn the converstation away from radiating antenna theory to something that is more of use as it applies to antennas, coax and a scanner.

What I wanted to point out is that since a scanner's antenna is surrounded by air, it's okay to use c to represent speed. Generally, most people are unconcerned with velocity factors when building an antenna to only receive radio signals.

:wink:

Many antenna designs use cable to creat matching transformers between the natural impeadence of teh antenna design and the desired impeadance for terminating the line.

In this case it is important that those playing with building their own antennas should at least understand that this effect exists.
(and has nothing to do with the resistance of the conducting material. (sorry, just had to toss that in))

 

[Pro-95]

Many antenna designs use cable to creat matching transformers between the natural impeadence of teh antenna design and the desired impeadance for terminating the line.

In this case it is important that those playing with building their own antennas should at least understand that this effect exists.
(and has nothing to do with the resistance of the conducting material. (sorry, just had to toss that in))

Speaking of which perhaps you can explain 5 loops, 5" diameter for decoupling. ???As an inductive coil?????

 

[N_Jay]

Many antenna designs use cable to creat matching transformers between the natural impeadence of teh antenna design and the desired impeadance for terminating the line.

In this case it is important that those playing with building their own antennas should at least understand that this effect exists.
(and has nothing to do with the resistance of the conducting material. (sorry, just had to toss that in))

Speaking of which perhaps you can explain 5 loops, 5" diameter for decoupling. ???As an inductive coil?????

If it is truly for decoupling, then you are building a high impeadance path to prevent RF from traveling down the sheild of the cable.

 

[gr8amp]

N_Jay wrote:
It has much to do with the dieletric constant of the medium which it is propagation through.
This is still not "RESISTANCE" as it refers to a conductor.

Find me a link, and one (or both) of us may learn something.
Okay.

Although this link is specific to cable and and finding cable faults, it does define the relationship of propagation velocity and resistance. Look toward the bottom of the first page of the pdf document and read from there.
http://www.radiodetection.ca/docs/part3.pdf
Now, if you find fault with this, you find a link where it clearly states that resistance has NO effect on propagtion velocity.

I find fault with this. First, providing a link to a users guide for TDR's does little to prove your point. The document merely makes a statement without proof, explanation, or sources. It was written for users of their product, not as the final word on electromagnetics. I'm not saying that everything written in the document is false, but it doesn't prove anything more than the previous posts. Simply saying "find me an article that says otherwise" is a childish argument at best, and only hurts your argument.

Furthermore, the article is discussing wave propagation through a guided medium (coax line), which is ONLY comparable to unbounded propagation for lossless cases. This post begins by discussing antenna dimensions, not transmission line theory. I'm not sure how we arrived at this point where the two became the same, but I believe it began with this:

And don't forget the type of material the antenna is made of. A copper antenna, for a specified frequency, would be of a different length than for a stainless steel antenna for the same frequency.

I'll take a guess on the basis of this statement, as it may hold some truth. If you consider the antennas impedance at the feedpoint, it consists of real and imaginary components. The imaginary component is mainly a function of the antennas length relative to the propagating wavelength. However the real part of of this impedance consists of radiation and loss resistances, and these are functions of the conductivity, length and magnetic permeability of the elements used. Therefore, theoretically, since the conductivity and permeability are unique to a specific material, proper matching can be achieved for various materials through adjusting the length. However, the permeability and conductivity differences between copper and steel are relatively low, making the difference in resistance insignificant.

Keeping the above statements in mind, the reasoning behind the 5% difference initially discussed in the first post is likely due to impedance matching of the half wave antenna. The imaginary component of the impedance is significantly reduced by shortening the antenna approximately 5%, making the impedance purely real. This makes matching significantly easier.

What type of antenna are you trying to make?

-Jim

 

[N_Jay]

N_Jay wrote:
It has much to do with the dieletric constant of the medium which it is propagation through.
This is still not "RESISTANCE" as it refers to a conductor.

Find me a link, and one (or both) of us may learn something.
Okay.

Although this link is specific to cable and and finding cable faults, it does define the relationship of propagation velocity and resistance. Look toward the bottom of the first page of the pdf document and read from there.
http://www.radiodetection.ca/docs/part3.pdf
Now, if you find fault with this, you find a link where it clearly states that resistance has NO effect on propagtion velocity.

I find fault with this. First, providing a link to a users guide for TDR's does little to prove your point. The document merely makes a statement without proof, explanation, or sources. It was written for users of their product, not as the final word on electromagnetics. I'm not saying that everything written in the document is false, but it doesn't prove anything more than the previous posts. Simply saying "find me an article that says otherwise" is a childish argument at best, and only hurts your argument.

Furthermore, the article is discussing wave propagation through a guided medium (coax line), which is ONLY comparable to unbounded propagation for lossless cases. This post begins by discussing antenna dimensions, not transmission line theory. I'm not sure how we arrived at this point where the two became the same, but I believe it began with this:

And don't forget the type of material the antenna is made of. A copper antenna, for a specified frequency, would be of a different length than for a stainless steel antenna for the same frequency.

I'll take a guess on the basis of this statement, as it may hold some truth. If you consider the antennas impedance at the feedpoint, it consists of real and imaginary components. The imaginary component is mainly a function of the antennas length relative to the propagating wavelength. However the real part of of this impedance consists of radiation and loss resistances, and these are functions of the conductivity, length and magnetic permeability of the elements used. Therefore, theoretically, since the conductivity and permeability are unique to a specific material, proper matching can be achieved for various materials through adjusting the length. However, the permeability and conductivity differences between copper and steel are relatively low, making the difference in resistance insignificant.

Keeping the above statements in mind, the reasoning behind the 5% difference initially discussed in the first post is likely due to impedance matching of the half wave antenna. The imaginary component of the impedance is significantly reduced by shortening the antenna approximately 5%, making the impedance purely real. This makes matching significantly easier.

What type of antenna are you trying to make?

-Jim

just for those new to the thread, Lou posted that link not me

 

[loumaag]

just for those new to the thread, Lou posted that link not me

Hmmm, actually the first part of the quote is yours. The answer was mine.
I find it interesting that you have no objection to Jim saying the same thing I did in other words. Is it just me you want to pick on?

...However the real part of of this impedance consists of radiation and loss resistances, and these are functions of the conductivity, length and magnetic permeability of the elements used. ...

Jim, this is basically the same explanation I gave earlier in this thread and when I commented that the difference in resistance between copper and steel would be negligible N_Jay resonded with:

Resistance has NOTHING to do with propagation velocity.

While I know that is a false statement, N_Jay wasn't about to take my word for it, (he seldom takes anyone's word for anything.) N_Jay's favorite ploy is to make a statement such as that and then challenge someone to find a link to disprove it. Since I was not going to type in the entire text of anntenna theory from a book, I looked for any link that would use the word RESISTANCE in regard to Propagtion Velocity. The challenge in reverse will prove to be somewhat harder to find, since resistance is a component of impedance which does effect propagatoin velocity. Was it childish? Maybe; but no more so than making the challenge to begin with.

As for this entire discussion, I leave it to everyone else to continue, as I have more important fish to fry than a subject which is only interesting to a few of us and something I explored over 30 years ago. :wink:

 

[N_Jay]

Lou, I added that comment ONLY because the way the post was formatted it looked like the statement around the link and the link was posted by me.

And are you having trouble where gr8amp says "I find fault with this. First, providing a link to a users guide for TDR's does little to prove your point."

I agree with him and disagree with you.

His comment about the impeadance of an antenna is correct, HOWEVER this was not what we were discussing, what we were discussing was PROPAGATION VELOCITY and the fact that WAVELENGHTH is different depending on the DIELECTRIC CONSTANT of the medium it is propagating through ONLY!

As for a link,

http://www.allaboutcircuits.com/vol_2/chpt_13/3.html

Velocity factor is purely a factor of the insulating material's relative permittivity (otherwise known as its dielectric constant), defined as the ratio of a material's electric field permittivity to that of a pure vacuum. The velocity factor of any cable type -- coaxial or otherwise -- may be calculated quite simply by the following formula:

 

[crayon]

I have more important fish to fry ..

mmmmm, fish. good.

 

[crayon]

In this case it is important that those playing with building their own antennas should at least understand that this effect exists.

agreed.

Anyone reading this thread should, by now, have about twenty-five different ways to describe SWR.

 

[gr8amp]

Sorry guys, I screwed up the quotes.

His comment about the impeadance of an antenna is correct, HOWEVER this was not what we were discussing, what we were discussing was PROPAGATION VELOCITY and the fact that WAVELENGHTH is different depending on the DIELECTRIC CONSTANT of the medium it is propagating through ONLY!

Yes, I was not talking about propagation velocity, I was trying to get back to the point and make an attempt at answering Pro-95's initial question.

Since I was not going to type in the entire text of anntenna theory from a book, I looked for any link that would use the word RESISTANCE in regard to Propagtion Velocity. The challenge in reverse will prove to be somewhat harder to find, since resistance is a component of impedance which does effect propagatoin velocity. Was it childish? Maybe; but no more so than making the challenge to begin with.

Please dont take my statement the wrong way. I understand what you were trying to do. However, finding a "somewhat" reliable source would have been a way to break free from and rise above the juvenile path that these posts have a tendency to follow. I wasn't trying to throw gasoline on the fire, just making a suggestion and moving back to the point. If I came across the wrong way I am truly sorry.

-Jim

 

[N_Jay]

Please dont take my statement the wrong way. I understand what you were trying to do. However, finding a "somewhat" reliable source would have been a way to break free from and rise above the juvenile path that these posts have a tendency to follow. I wasn't trying to throw gasoline on the fire, just making a suggestion and moving back to the point. If I came across the wrong way I am truly sorry.

-Jim

Sorry, I was under the impression that if you KNEW something, people might not argue just because you don't have a source handy.

It had been about 20 some years since fields and waves class, and the radio propagation lab, but most if the time since has been spent in radio.

I should have been clearer. I was presenting what I knew, not what I thought! :wink: