[Antennas] Back to Basics

Tue, 8 Jul 2003 01:11:23 -0500

Well, let's really get back to basics in a few words.
First, the Zo of a transmission line is inherently a complex quantity
Ro+jXo. Under certain special conditions we can safely neglect those aspects
of the line which make it have a complex Zo and treat it in a more simple
fashion. For example, we can calculate the Zo solely from the geometry of
the wires, etc.
More exactly, if the conductance of the dielectric in the environment of the
conductors - the plastic in coax, for example - is negligible and if the
per-unit resistance of the conductors is negligible, then the Zo can be
considered as real and no longer a function of frequency.
For most HF purposes, common transmission lines are close enough to having a
purely resistive (real) Zo that we can ignore the tiny reactive component.
For many other applications, though, it has to be considered.
Audio transmission lines, for example, can have Zo values where the reactive
component is equal to the real component. For example, RG-213 at 51 Hz has a
Zo of 50-j50 ohms. Even at 14 MHz, it is 50-j0.315 ohms.
For 99% of typical amateur applications, though, we can ignore the small
reactive component and proceed as if the Zo were truly only resistive.
Saves a lot of additional calculations!
73/72, George
Amateur Radio W5YR - the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:[email protected]>
----- Original Message -----
From: "Ronald KA4INM Youvan" <[email protected]>
To: "a" <[email protected]>
Sent: Monday, July 07, 2003 9:16 PM
Subject: Re: [Antennas] Back to Basics
> > Since impedance is dependent on the frequency involved, what is the
> > basis for 52 ohm coax, and how does it differ from 75 ohm Coax?
>> Well this is true for reactive items, such as antennas, but not for
> transmission lines, for coax it is the result of the ratio of the
> outer diameter of the inner vrs the inner diameter of the outer
> conductors modified by the the dielectric surrounding the conductors.
> For twin lead it the ratio of the conductor's diameter to the
> distance between the conductors modified by the dielectric
> surrounding the conductors. Or some such.
> --
> 73 (= Best Regards) de: Ron [email protected]
> 100% Slack. since July, 1997 (still free!) SENT D&T are UTC
> Visit my HAM Web SITE at: http://www.qsl.net/ka4inm
>> - - -
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