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[TowerTalk] Autek RF-1

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Subject: [TowerTalk] Autek RF-1
From: (Paul Ferguson)
Date: Fri, 16 May 1997 12:51:29 +0000
> According to QST for September 1996, the recent version of the Autek
> instruction manual gives a formula on Page 10 for determining R +/-
> jX, which isn't in my older manual.
> Please could somebody let me know the details?
> 73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
>From my RF-1 manual:

(Note, in equations below -- Z2 = Z squared, SWR2 = SWR squared, R2 
= R squared.)


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 and Z measured by the RF1! The formula is:

(equation 16)       R = ( (2500+Z2) (SWR)) / (50 (SWR2 + 1 ))

where SWR is relative to 50 ohms, as in the RF1, and R is in ohms.

Then, X is determined by X = Square root of (Z2 - R2). 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, preferably less than


b. The ratio of 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 inaccurate.

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 9/1994. 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.)



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