[TowerTalk] Autek RF-1

[Tom Wagner]

>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
    than 4:1.
  *and*
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 
    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
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


>At 08:18 AM 5/16/97 +0100, you wrote:
>>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'
>>                          'In Practice' columnist for RadCom (RSGB)
>>
>>--
>>FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
>>Submissions:              towertalk@contesting.com
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>>Problems:                 owner-towertalk@contesting.com
>>
>


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