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Velocity Factor Calculator




 
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Author Topic: Velocity Factor Calculator  (Read 3453 times)
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aa5wg
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« on: March 22, 2015, 05:50:50 PM »

Hi to all,

I am looking for a open wire feeder velocity factor calculator.

Thank you.

Chuck
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KA2DZT
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« Reply #1 on: March 22, 2015, 07:29:12 PM »

Open wire feeder probably I would use close to 100% for velocity of propagation.  On second thought,  use something a little more than the constant used for dipoles which is 468 (492 is 100% for 1/2 lambda).  If it's the crappy brown OWL probably something less than that.

A few factors affect the length of a dipole, the length to diameter ratio of the element and end affect.  Wire antennas of multiples of 1/2 wavelength still only have one end affect correction factor.  These corrections are where the constant of 468 is arrived at.

Fred
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Pete, WA2CWA
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« Reply #2 on: March 22, 2015, 08:48:33 PM »

The VP for most store bought open wire line is about 91%.

DIY:
http://en.wikipedia.org/wiki/Velocity_factor
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
aa5wg
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« Reply #3 on: March 22, 2015, 09:29:37 PM »

Gentleman,

Excellent information!  Thank you.
I still would like to find a calculator.

Chuck
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aa5wg
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« Reply #4 on: March 22, 2015, 09:32:54 PM »

How would you measure velocity factor of open wire feeders?
Chuck
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Pete, WA2CWA
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« Reply #5 on: March 22, 2015, 10:12:08 PM »

Velocity Factor:
VF = 1/c*square root of L*C

VF=1/c√LC
where L is the distributed inductance (in henries per unit length), C is the capacitance between the two conductors (in farads per unit length), and c is the speed of light in vacuum.
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
aa5wg
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« Reply #6 on: March 22, 2015, 11:28:17 PM »

Pete,

Thanks for the formula.  This will help.

Do you know how to measure VF?

Chuck
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KA2DZT
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« Reply #7 on: March 23, 2015, 12:12:20 AM »

How would you measure velocity factor of open wire feeders?
Chuck

Seems you would have to experiment to find the VoP of a given OWL.  Every half wave the same set of conditions occur on a transmission line.  Every 1/4 wave the opposite set of conditions occur.  If you have a length of OWL and feed a signal on one end and had a RF volt meter connected on the same end you would read some RF voltage.  Now, if you adjust the RF signal up and down the voltage will change. Your RF meter would have to be broad banded.  The other end of the OWL is left open. When the adjusted frequency reaches a 1/4 wavelength the voltage will dip to a minimum.  This occurs because a 1/4 wave down the line there is an open, so a short occurs at the voltmeter.  

Now you know the frequency at which the piece of OWL is a 1/4 wave length.  Now if you measure your length of OWL and compare that length with what a 1/4 wavelength (in space) is for the same frequency you will know the VoP for that OWL type.  Divide the length of the OWL by the free space length (for a 1/4 wave length).

This is one way to do it, there may be other ways.

Fred
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aa5wg
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« Reply #8 on: March 23, 2015, 09:04:46 AM »

Hi Fred,

Thank you.

Chuck
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