The cable type is a menu of common cable types that can be used.
In addition two user defined entries allow you to enter custom
cable types as described below.
You need to enter the frequency in the first text field. The label
"Cable Length in Feet" can be changed to "Cable Length in Meters"
which will change all the length units from feet to meters. You
then enter the length in the cable length field. Similarly, the
"Load Resistance" label can be changed to "Input Resistance" so
that measured impedances at the input end of a cable can be converted
to antenna impedances. Enter the resistance and reactance (positive
for inductive, negative for capacitive) in those fields.
When your input is set, press calculate. If there is an input error
a message will be displayed in the field to the left of the stop
button. The output side only changes when you press calculate,
and will display the characteristic impedance Z0 of the line,
the impedance at the other end of the line, the velocity factor,
the SWR at each end of the line, the matched loss of the line,
and the total loss with your load impedance.
The bottom part of the applet is a plot. The x axis is the position
along the line; x=0 is the input. The default shows the power,
current and voltage on the line all on one plot. To make them
all appear on a single plot, I take the current to have magnitude one
at the load, I divide the voltage by the characteristic resistance
of the line, and I divide the power by its value at the load. At the
top of the plot is a choice menu that allows you to also select
the power in watts, voltage magnitude in volts,
or current magnitude in amps with 1500 watts input to the line. You
can also plot the impedance in ohms seen along the line. The red curve is
the resistance and the blue one is the reactance. The plotting
is implemented using a stripped down version of
ptplot by Edward A. Lee and Christopher Hylands,
copyright University of California. One of the nice features of this
package is that you can zoom in on a portion of the plot by holding down the
left mouse button on that portion and move down and to the right to highlight
the region with a little square. You can do this multiple times too.
If you hold down the left mouse button and move up and to the left, you
can unzoom. The easiest way to get back to the original is to click
calculate again.
Two user defined cable types are provided.
For user 1, you enter the resistive part of the
characteristic impedance, the velocity factor and the
attenuation in dB/100ft at some frequency. If you want to
extrapolate to other frequencies, the exponent would be
exactly 0.5 if the loss mechanism were purely conductor losses, and
that should be good enough for a first guess. Otherwise, the loss
at a new frequency will be the ratio of the new frequency to this
frequency raised to the exponent power times this attenuation.
The reactive part of the characteristic impedance is calculated
assuming only conductor losses.
User 2 does not scale the results with frequency. Instead you input
the characteristic impedance both resistive and reactive parts,
the attenuation and the velocity factor. These characteristics are
assumed to not change with frequency.
© July 1, 2000
Michael A. Lee and Kevin E. Schmidt
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