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R: Velocity Factor



Hi Howard,

Please read my comments between your original lines

73 de i8CVS Domenico

----- Original Message -----
From: Howard Long <howard@howardlong.com>
To: Howard Long <howard@howardlong.com>; el bachir zantou
<el_bachir_z@hotmail.com>; <amsat-bb@AMSAT.Org>
Sent: Thursday, February 07, 2002 8:07 PM
Subject: RE: [amsat-bb] Velocity Factor


> Folks - I may have misunderstood what el bashir was saying but here's what
I
> think is being asked...

I believe that you understood  the right question.

> Take a piece of coax. Separate the braid from the inner for 1/4
wavelength.
> Stick the inner 1/4 wl vertically and use the separted braid as part of a
> ground plane (or alternatively a counterpoise).
>

I believe that you are correct  and infact as a proof  i have in my hands
a 1/4 wavelenght antenna as per your above description.

This 1/4 wavelenght antenna was originally used for the Navy Navigation
 Satellite System at 400 MHz  i.e. at a  wavelenght of  0,75 meters

The manufacturer separated the braid of a semirigid coax cable for about 1/4
wavelenght or 17,5 cm in this case sticking the 17,5 cm inner conductor
vertically living over it the insulating  dielectric polyethilene foam
only for salt protection on the sheep.

A separated pipe of the same lenght was used as a counterpoise to form
a vertical dipole suitable to be supplied by an unbalanced trasmission line

The inner conductor 1/4 wavelenght long extracted from this coax cable is
about 4 mm diameter solid copper wire and the outside diameter of the foam
is about 18  mm and this represent the radiating element of the antenna.

At 400 MHz a free space 1/4 wavelenght is 0,75/4= 0,1875 meters or 18,75 cm
so that,owing for the 4 mm wire diameter the radiating 1/4 wavelenght
long element to resonate at 400 MHz has been shortened by a factor of
17,5/18,75=0,93 according to the ARRL Antenna Book

This piece of  coax cable 17,5 cm long in this case is used as a  radiating
element of the antenna and NOT as a transmission line to feed the antenna.

The velocity factor of  any coax cable i.e. for example 0,66 for the well
know RG213 must be taken in to account  ONLY when the coax cable is used
as a transmission line because internally of  it the speed  of the RF
current is 0,66 time lower than the speed of the corresponding
electromagnetic wave running in to free space.

As soon we remove the outer conductor from a coax cable,as  they did
for the above 1/4 wavelenght antenna,the inner conductor of this coax is any
more part of a coax cable and since the inner conductor  radiates
electromagnetic waves in to the free space,the velocity of propagation of RF
current over its surface approach  that of the light been limited only and
by a very small extent by the diameter of the wire and not by the velocity
factor of the original coax cable from wich the radiating element or inner
conductor has been extracted.

> That's why I say the dielectric in the exposed radiating vertical 1/4 wave
> has a VF of 100%, as the electric field is mostly going through air and
not
> the dielectric sheath.
>
>             |
>             |<-inner in dielectric sheath
>             |
>             |
> coax        |
> oooooooooooo+------------- Braid
>
> So I say the length of the inner uses 100% VF but with the usual
shortening
> factor making it 95% of a 1/4 wl in free space.
>
I aegree with you and The NAVY NAVIGATION SATELLITE  SYSTEM
antenna size aegree also with  your drawing.

> Phew, this is hard work, and I'm not sure I've understood the question
yet!!

I believe that you understood very well the question and i feel that
your explanation is correct.


 73" de i8CVS Domenico

> 73 Howard G6LVB ;-)
>
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