[TowerTalk] RG-149

[Peter FORBES]

Hi Mike,

It might be worth searching through the forum archives, as I am pretty sure
that this topic has come up before.  Mind you, it may have been on another
forum.

Once you know the diameter of the inner conductor bunch and the diameter of
the dielectric, it is an easy matter to determine what impedance it is,
using the following formula:

Since the dielectric constant (permittivity)  for PE is approximately 2.25,
and  square root of permittivity = 1.5

Z = 138/1.5  x   log (D/d)    where D = diameter of the dielectric (or the
inside diameter of the braid = same thing approx.)   and d = diameter of the
bundle of wires making up the inner conductor.


>From Allied wires data   D = 0.285 inches,  d = 0.0480 inches


Z = 138/1. X log(0.285/0.0480) =   92 x log(5.9375)   = 92 x 0.7736 = 71.2
ohms   = near enough, because there is a finite distance between the
dielectric and the braid that must be accounted for that will slightly raise
the impedance.


As you say,  if you are getting it for free, you are surely able to press it
into service for something around the shack!


Cheers and beers

Peter  VK3QI




 

-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Mike
VE3YF
Sent: Friday, 13 December 2013 8:16 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] RG-149

Tnx Guys:

I am going to have the current owner double and perhaps triple check with
the old fashioned method. The more I listen hear and the internet, it has to
be
75 ohm cable and not 52 ohm. The couple sources I found are either a
misprint or like someone said 52 Ohm hasn't been made in years. Still for
almost free I can surely find a good use for it. Mni Tnx Guys especially to
Peter.

Tnx


73 De Mike
VE3YF

http://www.ve3yf.com
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