| Author |
Message |
    
Brain_damage
Member
Username: Brain_damage
Post Number: 81
Registered: 4-2005
| | Posted on Wednesday, February 06, 2008 - 3:03 am: | 
|
what is velocity factor and is it base on 36 ft which is a full wave.some coax states 78% velocity factor.so my question is,with shorter runs of coax does the velocity factor increase as the lengths of coax get shorter 18ft- 1/2 wave and 9ft-1/4 wave |
Romstar
Intermediate Member
Username: Romstar
Post Number: 118
Registered: 3-2007
| | Posted on Wednesday, February 06, 2008 - 6:26 am: |
|
Well, isn't that a great question to ask. Because this always gives some people brain damage.
Generally speaking, "velocity factor" is a measurement of the speed of a signal through a conductor as compared to transmission through free space. Your 36 foot number is directly related to a 27 MHz, or 11 Meter signal in free space. You can determine the distance it takes any signal to complete a full wavelength in open space with the following formula:
One wavelength in open air = 984 / Frequency in MHz.
CB channel 20 = 27.205 MHz, so if you divide 984 by 27.205 you get a result of 36.16982.
Radio signals travel through different materials at different speeds. All conductive materials, especially coax cable introduce resistive and capacitive loads to the signal as it passes through the conductor. The effects of these loads, measured against the the movement of the same signal in open air gives us our velocity factor.
To answer your question in a nutshell, no. The velocity factor is constant through the entire length of the conductor.
What is being measured is a TIME duration. If we take a piece of RG-8 with a velocity factor of .66 we can determine the length of cable required for one full wave length with the following:
[984 x .66] ( the velocity factor) / 27.205 (frequency in MHz) so multiply 984 by the velocity factor, then divide that result by the frequency.
(984*.66) / 27.205 = 23.87208
As you can see, it requires a shorter length of cable to complete a full wave length.
So, the lower the number for velocity factor, the shorter the cable will need to be to give you a full wavelength of propagation.
The real thing to remember here is that it really doesn't matter unless you are creating phased arrays where you need to have very specific lengths of line to achieve the proper phase between the antenna elements.
There are a number of excellent web sites where you can learn about antenna and coax as well as the ARRL handbook and other publications.
Good luck,
Frank |
Tech237
Moderator
Username: Tech237
Post Number: 957
Registered: 4-2004
| | Posted on Wednesday, February 06, 2008 - 6:14 pm: |
|
Velocity factor (VF) is a caused by the construction of coax cable ( and in fact all cabling)and doesnt change no matter what the length of cable is BUT is does change the electrical length of the cable.
This means that not matter if the RC-8 is a full wave, a halfwave or a some unknown number of wavelengths the velocity factor will always remain as 0.66 or 66%. Other impedences and type of coax will have a different VF, which will remain constant for that cable.
Now Romstar is not completely correct instating it does not matter unles creating phasing harnesses. Using an incorrect length of cable can throw the apparent swr off on even a simple antenna (see the article Coax Cables – Facts and Fantasy in the Copper Talk » Subscriber (Preview) section) for a better explanation.
Simon
Tech237
N7AUS
|
Romstar
Intermediate Member
Username: Romstar
Post Number: 119
Registered: 3-2007
| | Posted on Thursday, February 07, 2008 - 7:21 pm: |
|
Simon,
I guess I should have stated that a little better.
What I mean by saying that it does not matter is that it generally won't affect the performance of the radio or antenna by using an arbitrary length of coax.
As you rightly stated though, what it can sometimes affect is the apparent SWR. It is this very reaction that caused the rise of the special length coax myth in CB radio.
Frank |
Captian_radio
Intermediate Member
Username: Captian_radio
Post Number: 371
Registered: 9-2004
| | Posted on Tuesday, March 11, 2008 - 7:25 am: |
|
If when changing the length of coax changes the SWR it mostly means that there is something wrong with the antenna system.I once was using a radio antenna etc at a friends place and was not able to hear squat on 10 meters and SWR was flat but everything was fine on another antenna,now the antenna was fine so we changed the coax and wham the signals were fine ,turned out that the coax was very lossy and was acting as a dummy load,that coax wasn't long for the trash bin.
Bob CEF451/VE1CZ
Robert L. Spicer The days of radio are just beginning!
|
|
