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Author Topic: Google Goo 2  (Read 259 times)
K5PEW
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Posts: 223




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« on: March 06, 2008, 12:54:47 PM »

Dear Elmer,

Again, I'm combating the the Google grunge, the quagmire of mis, dis, and irrelevant information.

My Google search for information regarding the Velocity Factor of aluminum tubing brings mostly information about coax. There doesn't seem to be wording that I can conjure up that brings responses that are relevant to my interest.

Do you happen to know where on the internet I can find a table, calculator or rule of thumb regarding the velocity factor of various tubing diameter, and in particular how much shorter various tubing dimeters need to be cut by comparison to a wire.

Thanks for your time.

Graham Welch - K5PEW (In memory of Paisley Elizabeth Welch)
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AC0JX
Member

Posts: 13




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« Reply #1 on: March 06, 2008, 02:06:23 PM »

I don't have a specific reference, but most of what I have read regarding velocity factors of tubing uses a velocity factor of 0.95.  My guess is that is a good starting point from a design standpoint, and that actual element lengths are then determined through experimentation and tuning.  Hope this is helpful.
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K1BXI
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Posts: 812




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« Reply #2 on: March 06, 2008, 02:20:07 PM »

The shortening effect of tubing on a dipole has to do with the length/diameter ratio rather than VF, assuming they are both surrounded by air.  There are tables in most antenna handbooks to figure this K factor. (492*K / f(MHz). On HF it varies from around .94 to .98   So you might try to Google something to include "length/diameter ratio" in the search.

John.....K1BXI

 
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K5LXP
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Posts: 4448


WWW

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« Reply #3 on: March 06, 2008, 02:25:36 PM »

EZNEC will take conductor size into account, if all you're trying to do is calculate element lengths.

Mark K5LXP
Albuquerque, NM

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KF6ZLB
Member

Posts: 101




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« Reply #4 on: March 06, 2008, 03:11:18 PM »

On his website, Mr. Cebik (W4RNL) discusses almost everything you would ever want to know about antennas.  He is a big proponent of using antenna simulation software (for example, EZNEC).  A couple of his articles show the change in length of elements when changing element diameter.  The two examples are for the 20 meter band.  Rather than a "rule of thumb," however, he strongly suggests re-simulating.  If you try to extract a "rule of thumb" from his results, remember that ALL of the geometry needs to be scaled if changing to another frequency.  In other words, a 1" diameter tube used in a design for the 20 meter band corresponds to using a 0.1" diameter conductor in the 2 meter band.

http://www.cebik.com/wire/wirel.html

http://www.cebik.com/yagi/taper.html

Somewhere, he also investigates round tubing "equivalents" of other shapes (for example, aluminum angle, or square tubing).  I have not found that information on his website, but I am certain it is there.

Far from being theoretical in nature, the investigations are very empirical.  Still, there is a lot of detail to absorb.

To use Google to search this specific website, include the following text on the search line, after your keywords:

site:http://www.cebik.com/

Good luck,
Doug

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VK1OD
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Posts: 1697




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« Reply #5 on: March 06, 2008, 06:27:20 PM »


"There doesn't seem to be wording that I can conjure up that brings responses that are relevant to my interest."

That might be because your concept is not widely shared.

Velocity factor is the ratio of phase velocity to c0. I don't this that is the explanation of shortening of the resonant length of a half wave dipole in the case of an aluminium tube in air.

As others have advised, NEC based models will give you a good, and probably the best, prediction of the effect of conductor diameter on length and feed point impedance.

In a practical situation, there will be other influences as well, so a calculated value is only a starting point.

Owen
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W5FYI
Member

Posts: 1044




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« Reply #6 on: March 06, 2008, 07:20:01 PM »

The VF of aluminum is fairly close to 1. The wavelength-to-diameter tables in antenna books apply the so-called "end effect" to conductors to make them resonant. If that is what you are looking for, search for "dipole K factors."

If you are looking for the velocity factor of aluminum tubing used as a transmission line, it is quite simple to determine its physical length by measuring its actual length, calculating a physical length resonant frequency, and dividing that frequency into the actual resonant frequency.

For example, a 15' length of transmission line is shorted at one end, and its resonant frequency is found to be 13.28 MHz by use of a dip meter, noise bridge or antenna analyzer. The physical length frequency should be 983.6/(4x15')=16.39 MHz, where 983.6 is the velocity of light in free space in feet per second. The velocity factor is then determined by dividing the actual resonant frequency by the line's physical frequency, that is 13.28/16.39, giving a velocity factor of 0.81. (983.6 is the speed of light, in free space, in feet per second).

FYI, Stew
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WA3SKN
Member

Posts: 5440




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« Reply #7 on: March 07, 2008, 05:06:12 AM »

The reason you are having trouble is that the type of material used (aluminum tubing) has nothing to do with the Velocty Factor!  The type dielectric does.
The diameter of the tubing will affect it.  The spacing between the tubing elements will affect it.
So, you are chasing the wrong variables, so to speak!
73s.

-Mike.
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K9YLI
Member

Posts: 850




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« Reply #8 on: March 07, 2008, 06:55:14 AM »

 your google search  and all other search engines that I have tried.

 The logic of a search argument is a logical ' OR'..

 thus it looks ate every word you type and includes it in the list.

 a proper search engine,  (IBM retain360)  used a logical "and"  for its default.
   Thus the result had to contain ALL the argument words.
 If I remember right putting a comma between words made it and "and" function. (Ive been retired 14 years)

 but you can reduce your hits by  typing the word
   and   between each argument word

as in " velocity and factor and aluminum and rf"

some wher I pulled a whole list of the  logic  functions you can use in your argument and how to invoke them..
 commas  quotes  brackets  etc.

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