[Antennas] Back to Basics

Fri, 4 Jul 2003 17:21:45 -0500

Right on, Harvey - I posted at length on this topic back in May with the
following excerpt:
"Fundamentally Zo is a complex quantity for the general transmission line.
It is entirely frequency
dependent both by definition and from the math used to arrive at an equation
for it
based upon first principles.
"Fortunately, at HF the reactive component of the Zo of most practical lines
is very small to the point of being negligible in most cases. However, as a
general rule, this is not the case. At audio frequencies for example, a line
may have a large reactive component to its characteristic impedance. The
transmission line equations take all this into account.
"Again, at HF, Zo is essentially resistive and independent of frequency for
all practical matters. My comments were addressed only to the transmission
line and made no reference to terminating impedances or the like, since Zo
does not depend upon such issues.
"It is solely a function of the distributed resistance, capacitance and
inductance of the line itself. It happens that when certain relationships
among these are satisfied then frequency ceases to have more than a very
low-order effect on line Zo."
As an example, RG-213 has a Zo which ranges from 50-j50.28 ohms at 500 Hz to
50-j0.219 ohms at 30 MHz.
Good to hear from you again!
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: "Harvey&Bessie" <[email protected]>
To: "Adam Farson" <[email protected]>
Cc: "Antennas Reflector" <[email protected]>
Sent: Friday, July 04, 2003 4:51 PM
Subject: Re: [Antennas] Back to Basics
> The statement that "Characteristic impedance is frequency-independent"
should be taken
> with a little grain of salt, so to speak. The reason being that the
dielectric constant of
> some materials does vary somewhat with frequency, thereby changing the
Z-sub-o.
> Harvey/W4TG
> - - -
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