The length of a wave....

[Pro-95]

What is the number for the measurement of a wave as it pertains to frequency?

Are there differences depending on what range/style of collection?

In pertaining to antennas I found this on a 2m site:

Wavelength in inches = 984 * 12 / frequency in Mhz for full wave.

So 1/2 wave = 492 * 12 / frequency in Mhz.
and 1/4 wave = 246 * 12 / frequency in Mhz.

Others indicates that 234 = 1/4 wave.

Where is this difference coming from? I remember reading about 5% somewhere and 5% ~= the difference between 246 and 234 (233.7). If this is true, what is the 5% factor?

 

[N_Jay]

What is the number for the measurement of a wave as it pertains to frequency?

Are there differences depending on what range/style of collection?

In pertaining to antennas I found this on a 2m site:

Wavelength in inches = 984 * 12 / frequency in Mhz for full wave.

So 1/2 wave = 492 * 12 / frequency in Mhz.
and 1/4 wave = 246 * 12 / frequency in Mhz.

Others indicates that 234 = 1/4 wave.

Where is this difference coming from? I remember reading about 5% somewhere and 5% ~= the difference between 246 and 234 (233.7). If this is true, what is the 5% factor?

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

As an aproximation, Coax cable is 5% less than air, which is aproximatly the same as "Free Space".

Again these are rule of thumb mumbers.

 

[SCPD]

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.

 

[loumaag]

Sorry to requote, but needed to address line by line.

Wavelength in inches = 984 * 12 / frequency in Mhz for full wave.
So 1/2 wave = 492 * 12 / frequency in Mhz.
and 1/4 wave = 246 * 12 / frequency in Mhz.

This is true for theoritical antennas in free space. Not actual antennas.

Others indicates that 234 = 1/4 wave.

Where is this difference coming from? I remember reading about 5% somewhere and 5% ~= the difference between 246 and 234 (233.7). If this is true, what is the 5% factor?

The 234/F in MHz = 1/4 Wave Length in Ft is actually one leg of the 1/2 wave antenna and takes into account the ~5% factor you mention. It applies to wire antennas up to about 30Mhz and is a function of a two factors. One is the diameter of the wire and the second is something called end effect which is additional loading added on wire antennas by the insulators and loops used to attach the wire to the insulators at the ends of the antenna. These add a slight capacitance to the system resulting in a shortening of the antenna length from the theoretical "1/2 wave in space." This effect is extant in all 1/2 wave wire antennas without regard to feed system.

In addtion to length, you must consider diameter of the radiator in regard to the frequency. Typical tubing used to construct VHF/UHF base antennas are large in respect to wire at HF frequencies and hence the antennas become smaller then the formula would indicate. There are scaling algorithms; however, it is way beyond discussion in this format. I would suggest that you either get an anntenna refernce guide such as the ARRL Antenna Book or you stick with published guides for construction projects you can find on the Internet. 8)

 

[N_Jay]

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.

Hmm, have to do some thinking on that.

I think that any conducting material in the same dielectric medium (air) will have the same surface propagation velocity.

Someone correct me (with a source) if I am mistaken.

 

[loumaag]

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.

Hmm, have to do some thinking on that.
I think that any conducting material in the same dielectric medium (air) will have the same surface propagation velocity.
Someone correct me (with a source) if I am mistaken.

Well, other than the differnece in actual resistance between steel and copper of a given diameter (which is negligible in short (less than several 1000 feet) runs) I think you are right. I don't have the steel numbers, but stranded 14 Ga copper wire has a 2.575 ohm loss over a span of 1000 feet. I can't imagine steel would be that much different. Perhaps frootydawg was thinking of something else.

 

[N_Jay]

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.

Hmm, have to do some thinking on that.
I think that any conducting material in the same dielectric medium (air) will have the same surface propagation velocity.
Someone correct me (with a source) if I am mistaken.

Well, other than the differnece in actual resistance between steel and copper of a given diameter (which is negligible in short (less than several 1000 feet) runs) I think you are right. I don't have the steel numbers, but stranded 14 Ga copper wire has a 2.575 ohm loss over a span of 1000 feet. I can't imagine steel would be that much different. Perhaps frootydawg was thinking of something else.

Resistance has NOTHING to do with propagation velocity.

 

[n8gni]

Wavelength = The speed of light in meters divided by the frequency.
300,000,000 divided by the frequency
300 divided by the frequency in MHZ
Wavelength = 300/146mhz = 2 meters
Wavelength = 300/27mhz = 11 meters
Frequency is the inverse of time so as the frequency increases the time and wavelength will decrease.
½ wave dipole antennas in inches are 468/frequency in Mhz
This would be the total lengh, so you would divide by 2 and this would be the length in inches for each side of the dipole
There, now even I'm confused!

 

[loumaag]

Resistance has NOTHING to do with propagation velocity.

Actually it has everything to do with propagation velocity. I could go in a long explanation of why; however, a simple example will suffice to demonstrate.

Propagaion velocity is dependent on the dielectric constant of the medium through which the RF passes. For this explanation, we will assume air is at unity. The dielectric properties of wire are such that resistance is a component of that along with any insulation or even oxidation. These are the factors which must be considered when designing an antenna for a given frequency in addtion to length and diameter of material (we are assuming a uniform diameter, not a tapered situation.) Since wire presents a resistance to the flow (more than air) this is why a wire antenna is shorter (even ignoring end effect) then a free space antenna. The actual physical length of a wire antenna is:

Length in Feet = (491.8 * K ) / frequency in MHz 
Where K = Ratio of free space wavelength to conductor diameter

Energy applied to an antenna is dissipated in two ways, the desireable effect of radiated power and heat caused by resistance in the wire and/or dielectrics. Both of these effects are due to resistance. Whether it be radiation resistance (impedance) or actual resistance (wire losses) it is all a resistive load to the source. With no load there is no flow. Hence, resistance has EVERYTHING to do with propagation velocity. :wink:

 

[Voyager]

½ wave dipole antennas in inches are 468/frequency in Mhz
This would be the total lengh, so you would divide by 2 and this would be the length in inches for each side of the dipole

NO, that gives you the length in FEET. Multiply x 12 for inches.

Dividing by two, a quarter wave would be 234/frequency=Length in feet


Joe M.

 

[N_Jay]

Resistance has NOTHING to do with propagation velocity.

Actually it has everything to do with propagation velocity. I could go in a long explanation of why; however, a simple example will suffice to demonstrate.

Propagaion velocity is dependent on the dielectric constant of the medium through which the RF passes. For this explanation, we will assume air is at unity. The dielectric properties of wire are such that resistance is a component of that along with any insulation or even oxidation. These are the factors which must be considered when designing an antenna for a given frequency in addtion to length and diameter of material (we are assuming a uniform diameter, not a tapered situation.) Since wire presents a resistance to the flow (more than air) this is why a wire antenna is shorter (even ignoring end effect) then a free space antenna. The actual physical length of a wire antenna is:

Length in Feet = (491.8 * K ) / frequency in MHz 
Where K = Ratio of free space wavelength to conductor diameter

Energy applied to an antenna is dissipated in two ways, the desireable effect of radiated power and heat caused by resistance in the wire and/or dielectrics. Both of these effects are due to resistance. Whether it be radiation resistance (impedance) or actual resistance (wire losses) it is all a resistive load to the source. With no load there is no flow. Hence, resistance has EVERYTHING to do with propagation velocity. :wink:

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. :twisted: :wink:

 

[loumaag]

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. :twisted: :wink:

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. :twisted:

 

[scanfan03]

I have a question

What is the difference between quarter wave and half wave?

I am not new to scanning, just new to antennas. (I've always used Radio Shack's 800 MHz antenna). And I know that usually the higher the frequency the shorter the antenna.

 

[Pro-95]

What is the difference between quarter wave and half wave?

1/2, sorry coudln't resist. quarter wave is 1/4 of the frequency wavelength in distance, 1/2 is 1/2 the frequency wavelength in distance. Now someone can tell how the full signal is completed.

I am not new to scanning, just new to antennas. (I've always used Radio Shack's 800 MHz antenna). And I know that usually the higher the frequency the shorter the antenna.

The higher the frequency the more "waves" through a given segment of time. The cycle of a wavelength is then much shorter the higher in frequency you go, there fore to receive the entire signal the antenna can be shorter as the entire wave duration is also shorter.

Now let's see how good my explanation is. :lol:

 

[scanfan03]

The higher the frequency the more "waves" through a given segment of time. The cycle of a wavelength is then much shorter the higher in frequency you go, there fore to receive the entire signal the antenna can be shorter as the entire wave duration is also shorter.

Now let's see how good my explanation is. :lol:

Yeah, I knew all of that, i was just wondering, if i wanted to make my own antenna for 866 MHz, would i use 1/4 or 1/2 wave? Which one has more sensitivity?

 

[Pro-95]

Which one has more sensitivity?

Sorry man I'm just now learing the whole elctromagnetics of antennas so I'm not the one to answer that.

 

[Pro-95]

Here's a page full of links pertaining to antennas.
>>>>> CLICK HERE <<<<<

 

[scanfan03]

Wow thanks for the link, looks to be a lot of info. I will be going through it tomorrow.

 

[crayon]

It would seem as though this thread is taking on the commentary hues of information that was copied-n-pasted from a google search.

Building a simple antenna to receive is the easiest, and coolest thing anyone can do that is into scanning!! So it is good to talk about this stuff.

What is the number for the measurement of a wave as it pertains to frequency?

As has already been mentioned, the velocity of a radio wave in space is considered to be 300,000,000 meter per second. Actually, its 299,792,462 m/s. Stated another way for the metrically imparied, 186,282 miles per second or 983,569,000 feet per second.

I saw a sign that looked like a highway speed limit sign and it said: 186,000 MPS ... it not just the limit, its the law! Should have bought it, but I didnt. :?

So anyway, the main point that I wanted to make is, when solving for wavelength, the formula varies depending on what frequency is used in the equation. n8gni somewhat alluded to this in his post.

To summerize:

wavelength = 300,000,000/frequency
wavelength = 300,000/frequency in kHz
wavelength = 300/frequency in MHz

As you might guess, the terms in the equation can be rearragined. Google if your lost.



The following is really more suited to a discussion of radiating antenna's.

As an aproximation, Coax cable is 5% less than air, which is aproximatly the same as "Free Space".

I have no ideal where this could have come from, and I'm sure that this applies somewhere, but in the context of this discussion is does not.

loumaag's post about this is somewhat correct ...

End Effect as it relates to the '5% rule' of half-wave antenna's is generally thought of as the dielectric effect of the air at the end of the antenna. This effectively lengthens the antenna longer than it actually is by about 5%. To overcome end effect the antenna must be cut to a physical length about 95% of the computed half wave length.

If the antenna wire is close the ground, or buildings, trees, etc., the end-to-end capacitance increases and end effect increases, requiring still shorter lengths.

 

[crayon]

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 ..