[AMPS] Re: Two Questions.

[km1h @ juno.com]

On Tue, 9 Sep 1997 09:32:08 +0100 Peter Chadwick
<Peter.Chadwick@gpsemi.com> writes:



>What value of shunt resistance is used in the W8JI type suppressor? 

It should vary depending on the tube used in a conventional single
resistor suppressor with n turns of copper wire. 

>Why
>can't you get the same lowering of impedance by dropping the value of
>the shunt resistor? My (possibly faulty) understanding of the way the
>suppressor works is that the inductance is added to increase the 
>amount
>of inductance in the circuit so that the shunt resistor is then 
>damping
>a higher proportion of the parasitic circuit, thus reducing its gain.
>The higher the stray inductance in the parasitic circuit ( bad layout
>etc), the bigger the suppressor choke has to be. The bigger the
>suppressor choke, the more volts of fundamental appear across it, and
>the more fundamental power gets dissipated in the resistors. If the
>value of the resistors is  too small compared to the reactance of the
>choke at the parasitic frequency, then the effective impedance that 
>the
>suppressor offers drops too far, and the assembly looks like a lossy 
>bit
>of wire. So there's an optimum value for the impedance of the assembly
>at the parasitic frequency, and too low is just as bad as too high.

Absolutely....in a conventional suppressor Peter. In Rich's and my
approach the resistor value appears to be less critical with the L
material playing a bigger role.
My approach is somewhere between Rich and Tom and appears to be very
tolerant.

>After doing a few sums, I'm not convinced that the nichrome approach
>doesn't have advantages in some circumstances - at least you aren't
>searching for high power (5-10watt) low capacitance, low inductance
>resistors.

Maybe not available in the UK Peter.  A good 5W is $.50 in single qty in
the Colonies.

73.....Carl   KM1H

>73
>
>
>Peter G3RZP
>
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