> From: Peter Chadwick <Peter.Chadwick@gpsemi.com>
To: <amps@contesting.com>
> Date: Mon, 8 Sep 97 14:18:09 +0000
Measures wrote:
> >- The advantage of resistance-wire suppressors is that, at 100MHz,
> they
> >exhibit about 40% less Rp than do otherwise similar, conventional
> >suppressors made from copper-wire.
You replied:
> Is this 40% less Rp than the 'conventional' parasitic suppressor i.e.
> coil and shunt resistor, or just 40% less Rp than the coil alone? If you
> are going to compare a nichrome parasitic suppressor against a straight
> inductor wound with copper and no shunt resistor, then I don't believe
> the copper suppressor has a hope in hell - which I think is what you've
> been saying. Even if you use a parallel R-L circuit, you need to use the
> right sort of resistor in shunt - a 47 ohm 0.3 microhenry job would be
> as much use as teats on a bull!!
If you look at N7WS's data, and his conclusions, the stock AL-80B
suppressor is essentially equal to the nichrome model near
the frequency where stability is a problem, although the stock
suppressor does lower SYSTEM Q at VHF slightly more than the
nichrome replacement.
The "advantage" of the nichrome really kicks in at HF, as evidenced
by the heat produced and efficiency lost on upper HF when the
nichrome suppressor is added.
As measured by N7WS, the nichrome is about 103.3 ohms at 200 MHz with
a resistance value of 78.7 ohms and an inductive reactance of 35.3
ohms. The stock AL-80B suppressor, on the other hand, has a Rp of
169.49 ohms at the same frequency and a resistance value of about 110
ohms!
Follow this simple series of facts........
1.) Rich claims a "low Rp" is desirable, and we both agree his
suppressor has lower VHF Rp than a stock suppressor.
2.) A direct short has an Rp of zero ohms.
3.) Removing turns from a suppressor LOWERS the suppressor's Rp
value, because it makes the suppressor look more and more like a
direct short.
Now with that all in mind, ask yourself this very simple question.
When you want to increase VHF stability, do you REDUCE the
turns in the suppressor in the anode lead, or do you increase the
turns in the suppressor??
Is the best suppressor to use with an unstable tube a zero ohm
resistor? Pretty obvious about the "low Rp" always being desirable,
isn't it.
73, Tom W8JI
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