[AMPS] Another arc question

Ian White, G3SEK G3SEK@ifwtech.demon.co.uk
Sun, 10 May 1998 19:42:16 +0100

Rich Measures wrote:
>>Rich Measures wrote:
>>>How could Wes' have possibly measured 166 ohms of Rp for a suppressor 
>>>that uses a 109 ohm resistor?  
>>Easily - the effect is due to a small series lead inductance external to
>>the parallel R-L network.
>It is easily seen that Wes also made separate measurements for the 
>various suppressor inductors alone, without a shunt R, and that he 
>measured Rp at each of the various test frequencies.  If, as you claim, 
>Rp is the suppressor's shunt R, how was Wes able to measure Rp when no 
>shunt R was used?
Because the coil has finite Q. If you take any measured Lp value from
the table, convert to reactance at the measurement frequency, and
multiply by the measured Q, there's the Rp value. 

>>Let's assume that 109 ohms was an accurate value at all frequencies, and
>>the parallel inductance that Wes used was exactly 100nH (the target
>>value for all the samples measured). Let's also assume that the
>>inductance of straight #14 wire in free air is about 20nH per inch (from
>>standard formulae, and it's not very dependent on wire diameter).
>>It turns out that lead lengths of only 0.3in between the parallel R-L
>>network and the binding posts of the impedance analyser would be enough
>>to push the measured Rp value up to nearly 140 ohms at 10MHz - 
>40 ohms at 10MHz from maybe a dozen nH?   XL calculates at 0.75 ohms, and 
>I doubt that the resistive component could be more than a few milliohms.  
>.  Did the 40 ohms of explanatory R perhaps come from a hat?  .  . 
The additional Rp is generated from an impedance-transforming effect,
which exists whether you doubt it or not. Take a break from the keyboard
and do the calculation yourself. 

>>which is
>>almost exactly what Wes DID measure (140.85). 
>Yes, Wes measured an Rp of 140-ohms for the copper-wire suppressor at 
>10mHz.  However, the resistance-wire suppressor measured 51-ohms of Rp.  
>Did the similar lead-lengths add 57-ohms of resistance to the 109-ohm 
>suppressor R, and subtract 49-ohms of resistance from the 100-ohm 
>resistor in the resistance-wire suppressor?  
The nichrome suppressor behaves differently because there is very
significant distributed resistance throughout the inductor and its
leads. You do some calculations for a change, and tell us what you find.

>The answers do not appear to wash, Mr. White.   
>>Even a very small amount of stray series
>>inductance can push the measured Rp way above the physical value of the
>>resistor in the network.
>>I chose 10MHz because the components are more likely to behave in an
>>ideal way down there. At 100MHz this very simple model requires a little
>>more lead inductance to fit the data - but that's still only about
>>0.5-inch leads. A model with better representations of the values and
>>stray reactances in the main resistor and inductor would fit Wes's data
>>at all frequencies.
>>I said:
>>>>2. My terminology and analysis agree exactly with Wes's. 
>> - and they still do.
>When there was has No shunt R whatsoever on "Ls" (the suppressor 
>inductor), Wes measures Rp. .  .   According to your terminology and 
>analysis, Mr. White, this would be quite impossible.   
Impossible for the idealized components we were talking about then -
perfectly possible for the real-world components that were measured. 

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)

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