[AMPS] impedance of nichrome lower

Wed, 10 Sep 1997 15:39:36 +0000

Sorry Carl,

I figure anyone dishing it out as much as you should also be pretty 
thick skinned on the receiving end. ;-) I'll try to be more civil to 
you, and I sincerely hope you reciprocate. In any event, I'm sorry. 
The is no reason for anyone to get personally insulting with someone 
else.

Some (or many) network analyzers measure impedance, although you are 
correct. N7WS used what is described as an impedance measuring 
device.

In any event the test instrument uses a low power source.

Wes' data for the inductor indicates the equivalent parallel 
resistance across the UNLOADED inductor. Since this is a single 
component, the Q is rather high so the parallel resistance is very 
high (the series resistance very low).

I think the problem is Rich has tried to focus on Rp, but Rp by itself 
means very little. It is the terminal impedance of the suppressor 
expressed in reactance and resistance, and the impedance of the rest 
of the system, that really dictates what the suppressor does to the 
Q of the overall system. 

My point, that I might have not made clearly enough, was it matters 
very little what the unloaded Rp or Q of the suppressor is at VHF. 
No matter what the suppressor inductor's Q might be, it is the 
external resistor and the inductance value that mainly controls 
VHF Q. 

If we were going to operate the suppressor without a parallel 
resistor, nichrome would be great (at least for  VHF loading). 
Once the resistor is there, the Q of the material in the inductor  
(or the inductors Rp) matters very little. A very slight adjustment 
in turns and / or  resistance would easily make the two 
methods identical in system performance at VHF.

At lower frequencies, it is another matter. Much more current 
flows through the inductor than the resistor. On ten meters, the 
nichrome suppressor would have much less Q and create more 
system loss. This is especially true because the suppressor handles 
a non-sinusoidal pulsed waveform, and the heating effect is greater 
than simple sine wave analysis of the fundamental frequency 
components would lead us to believe.

The main difference in philosophy between Rich and myself is 
simple.  I believe there is no reason to waste ten meter or 15 meter 
power when a PA oscillates near 200 MHz. If the PA oscillated  near 
ten meters, and the PA was not or could not be corrected by other 
means, the nichrome would be a good idea.

I also believe the designer should pay attention to what the 
suppressor does to the anode SYSTEM, not what it does sitting on a 
desk. See my example of the one ohm resistor and the fifty ohm 
resistor at the end of my long post about suppressor usage. Less Q in 
the suppressor does not translate into less Q in the anode system, it 
can actually indicate the anode system has more Q and less stability.

The reason most of this makes no difference and the argument is so 
unresolved is most PA's are very stable at VHF as manufactured, and 
a little bit of change one way or the other at VHF makes no 
difference at all in component or tube life.

The suppressor sales pitch is mostly harmless, except for the 
loss in ten meter performance and the occasional overheating failure 
of a small resistor or two that is added.

It's the "run all the grid current you want and throw away the 
grid overload protection system" stuff that twists at people's 
wallets.

73, Tom W8JI

--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm