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smith.doc
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1982-10-14
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DOCUMENTATION FOR THE DIGITAL SMITH CHART PROGRAM `SMITH' BY AA4L
Smith calculates resistance and reactance at any point along a
transmission line. It also calculates reflection coefficient `rho'
and vswr.
Resistance and reactance as measured at some point on the transmission
line are entered in response to prompts. The program then calculates
R & X for each n degree point for the next half wave length TOWARD THE
GENERATOR. `n' is user selectable. Values for points toward the load
may be derived indirectly as follows: Assume that a transmission
line is 230 deg long, and that R & X have been measured at the trans-
mitter end. It is desired to find the R & X at the antenna. Since all
values are the same for each half wave length of a (lossless) line,this
line is equivalent to a line which is 230 -180 = 50 deg long. Hence
the R & X at the antenna is equal to the R & X at a point 180 - 50 or
130 deg toward the hypothetical generator from the point at which the
measurements were taken. Think about it!
The equivalent of a Smith chart gamma swr circle may be created by
asking for a listing for a line which has a known point of R=swr*Z0
or R=swr/Z0 and j0.
Conductance `G' and susceptance `B' are also calculated for each n
degree point. This data is very useful for the calculation of
parallel matching stubs.
To avoid divide by zero errors, enter R=0.000001 or such for a shorted
line and R=999999 or such for an open line. Values near 90 deg are
somewhat invalid under some conditions. (The tangent of 90 deg is
infinite.)
The following formula is used by SMITH to calculate physical line
lengths: theta = length in ft * 0.367 * freq in mhz / velocity factor.
73 es gl de AA4L Bob Johnson 11305 Rums Hill Raleigh NC 27614
919 847 5606 10/10/82