Re: NEC-LIST: Linear Phased Antenna Array

Hideho YAMAMURA wrote:
>
> Dear All,
>
> Could anybody tell me why the characteristic impedance of 50 ohms in
> coax. cables became a standard value ?
>
> I have believed that this is the minimum loss condition, but I found
> in an old Mushiake's book that it is around 77 ohms for air dielectric
> solid metal coax, and even higher when dielectric material filled.
>
> I highly appreciate any information.
>
> I apologize if this is not an suitable subject for the list, but I
> believe someone else may want to know too.
>
> ==================================================
> Hideho YAMAMURA,
> Senior-Engineer,
> Hardware Technology Engineering Department,
> Advanced Product and Technology Development Operation,
> Enterprise Server Division,
> Hitachi, Ltd.
> ==================================================

Hi,

Actually, the characteristic impedance of most coaxial cables is
between 50 and 75 ohms.

The differences of the various impedance values are related to the
importance of which are the most important characteristics of the coax
cable :

               Attenuation
               voltage strength (of the insulation)
         and maximum transmitted power

for a coax cable with air insulation (epsilon = 1), the optimum
impedances are :

      approx. 75 ohms for minimum attenuation
              60 ohms for maximum voltage strength
              30 ohms for maximum transmit power

the 50 ohm coax cable characteristics are a compromise between max.
voltage strength and max. transmitted power.

Of course, the size of the cable (inner to outer conductor) depend
very much on the type of the dielectric (insulation) being used.

Historically, coax cables with full insulation had an impedance of 70
ohms, whereas an air-styroflex cable has approx. 50 ohm impedance.

hope this will answer your question.

Max Schmitt
Cons. Eng.
Received on Tue Mar 06 2001 - 23:59:40 EST