Thanks to everybody who responded so helpfully and so quickly!
A number of people have asked for the same information, so here it is.
First from Tom, N1MM...
>From the RF-1 manual:
Determining R + jX
As discussed above, R can be determined by canceling X with a
series capacitor or inductor. But, R & X can also be calculated
directly using SWR & Z measured by the RF1! The formula is:
(16) R = ((2500 + Z*Z) * SWR ) / (50 * (SWR*SWR + 1))
where SWR is relative to 50 ohms, as in the RF1 and R is in ohms.
(15) X = SQRT(Z*Z - R*R)
Then X is determined by equation (15) above. The sign of X is
easily determined by increasing the frequency slightly and
watching Z. If Z decreases when the frequency increases, then
X is negative (capacitive). If Z increases, X is positive (inductive).
Use a small frequency change so that Z does not go through a
maximum or minimum. The sign of X changes at a max or min Z.
(Remember that a feedline can also change the sign of X.)
We believe this simple method of determining the sign of X will
work in all cases, but there could be a rare exception due to a
rapid change in R with frequency... perhaps when R is very large.
As a double check, you could connect a small (5 pf?) capacitor in
parallel with the load. If Z increases, then X is positive. If it
decreases, X is negative. Use the smallest C that shows a
measurable Z change.
Equation 16 is error-free, and your RF1 is the most accurate SWR
and Z measuring device available unless you have a laboratory
impedance bridge. However a small error in SWR or Z can cause
a large error in R. You can only depend on equation 16 when:
a. SWR is greater than 1.2 but less than 6:1, preferable less
b. The ratio R/X is not too large or too small. This ratio
should be between 0.2 and 5, or the impedance will be
dominated by either R or X, and the other will be
However, even outside these limits, equation 16 can give
sufficiently accurate estimates for many purposes. Just use
caution. (This note was first included in RF1 instructions about
Sept. 1994. If you know of owners with older instructions, please
inform them of this new use for the versatile RF1 that they're
probably not aware of.)
I hope this helps.
Tom - N1MM
You will also have seen the BASIC program from Brian, K3KO.
Once again, thanks everybody!
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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