Rich Measures wrote:
(in reply to me)
>...snip...
>>Don't need to - I'm sure you're correct that those resonances exist, and
>>that they need to be suppressed.
>
>The grid and anode VHF resonances can Not be suppressed----slightly
>moved, yes.
>
"Suppress" has two meanings. I agree that the resonance cannot be
totally eliminated; I meant "suppress" in the sense of "restrain" or
"hold down", which is what the suppressor networks do.
>>What I'm still not convinced about is
>>the need to use nichrome suppressors.
>
> According to Wes' (N7WS) tests on otherwise-similar suppressors, a
>copper coil Ls produced about 2/3 more Rp at 100MHz than a Ni-Cr-Fe coil
>Ls. Since VHF voltage amplification is a pretty much Mu x VHF-Rp,
NO! This is where Rich still doesn't see it. He misses out the essential
parallel -> series circuit transformation - and that turns all his
conclusions upside-down.
Gary Coffman posted the analysis in r.r.a.homebrew several months ago.
I'll try for one last time.
Here is the VHF resonant circuit, pretty much as shown in all the
textbooks and Rich's own web page.
---X_Lp-- X_L_stray
-------- |------))))))--------
| |__Rp___| |
___ |
___ Anode etc capacitance Ca Pi-tank C1
| |
| |
-------------------------------------
The suppressor is represented by the equivalent parallel resistance Rp
and parallel inductive reactance X_Lp as actually measured at VHF using
a network analyser. (This puts all types of parasitic suppressor on the
same basis.)
Note that some components are connected in series, while others are in
parallel.
To determine the Q of this tuned circuit, we need to transform all the
series/parallel components into an equivalent combined form that is
either TOTALLY series or TOTALLY parallel. Let's go for the all-parallel
equivalent:
------------------
| | |
___ | )
___ Ce Re ) X_Le
| | )
| | |
------------------
At the VHF resonance, Q is going to be given by either
(Re / X_Ce) or (Re / X_Le). Higher Re means higher Q and higher voltage
gain.
If you want the detailed math, DejaNews your way back to Gary Coffman's
posting, or find it in the web archive of that marathon debate.
What comes out is that if Rp in the suppressor is high, Re in the
equivalent parallel network is LOW. There is an impedance *INVERSION*!
That means that higher Rp in the suppressor leads to a LOWER loaded VHF-
Q and LOWER VHF-gain - exactly the opposite to what Rich still persists
in saying.
Here's a reality check from Rich's own posting:
>those
>who want more VHF voltage amplification need a copper, or silver, Ls. If
>that does not produce enough, using no VHF suppressor at all in the anode
>circuit should increase Rp and VHF voltage amplification even more.
Let's separate out the two statements in that last sentence.
> using no VHF suppressor at all in the anode
>circuit should increase [...] VHF voltage amplification
Right - obviously.
>using no VHF suppressor at all in the anode
>circuit should increase Rp [...]
Wrong - that's completely self-contradictory!
No suppressor means using a piece of wire instead, so the Rp of a "no-
suppressor" has to be LOW.
For N7WS's own views on his network analyser measurements, check
http://www.azstarnet.com/~n7ws
Wes compared a conventional suppressor (R in parallel with silvered
copper inductor) and an AG6K suppressor (R and nichrome wire). Here are
the two key conclusions.
In the VHF region:
"As the chart shows, there are only slight
differences between the effective parameter values for the
two types of networks."
In the HF region:
"It can be noted that the conventional suppressor will have a greater
percentage of the total current flowing in the inductive (lower
loss) leg in the HF region than will the nichrome type."
I concur totally with Wes. His measurements show that the significant
differences are at *HF*, not at VHF.
Finally, let me say that I don't know whether Rich's parasitic
suppressors have any benefits over conventional ones or not.
I don't have any professional stake in this debate. My only interest is
to find a technically valid explanation of the confusing mess of
evidence.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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