[MRCA] AS-2259 mast impedance

[ersmar at comcast.net]

Gents:

     With help from the dimensions from Breck's AS-2259 mast (thanks, 
Breck), I calculate (below) that the impedance of the mast is around 46 
Ohms, depending on what I assume for the dielectric constant of the 
dielectric material in the mast.  So 50 Ohms might not be a bad number for 
me (us?) to use in developing matching circuits for the base.

     Here's the equation I found in the ARRL Handbook and on the Web ( 
http://www.cableworld.com/ct2/archives/0999/ct0999f.htm )for the surge 
impedance, aka characteristic impedance, of coaxial transmission line:

Zo = (138/sqrt(er))log(D/d)
where:
Zo = characteristic/surge impedance
er = dielectric constant of the material between inner and outer conductors
D = innder diameter of outer conductor
d = outer diameter of inner conductor.

     Also, the above web site defined er as approx 1/velocity factor of the 
dielectric material.  I found in the HB that hardline of nearly any diameter 
has a VF of 0.81.  I then applied (assumed) this number was near the VF for 
the AS-2259's material.  If I'm wrong (likely), my final number could be 
less than or greater than 46 Ohms by a small amount.  So I'll use 50 Ohms 
and sleep well tomight.

     Back to the Smith Charts!

73 de
Gene Smar  AD3F