Rich Measures wrote:
>During the grate suppressor debate, I repeated asked Mr. Rauch to design
>a copper-wire parasitic suppressor that equaled the performance of the
>resistance-wire suppressor. I repeatedly asked Ian White and Wes to
>design one. ......... So far, nothing.
Not true, as DejaNews would show, but I'll bite again.
The N7WS measured Rp and Lp values for Rich's suppressor at 100MHz are
Rp = 103.2 ohms, Lp = 123.9nH. With ideal components, those are the R
and L values you'd connect in parallel to EXACTLY reproduce the
behaviour of Rich's suppressor at 100MHz.
But that simply begs two more questions:
1. What about real-life components?
2. What happens at other frequencies?
You can't "design" a real-life suppressor entirely on paper, because of
the stray properties of the components at VHF: the copper-wire inductor
has very significant self-capacitance and some series resistance, and
the resistor has some self-inductance.
What you CAN do is sit at an impedance/network analyser with a bunch of
real resistors and some copper wire. Using the idealized values as a
starting-point, you can cut-and-try to find a combination of values that
will EXACTLY reproduce the measured parameters (Rp and Lp) of Rich's
R/NiCr suppressor. Note that the analyser impartially measures the R-X
properties of whatever is connected to its terminals - it doesn't know
or care how the network is constructed.
With a wide enough choice of R and L values, it is ALWAYS possible to
reproduce Rp, Lp and therefore Q, at ONE frequency. There is absolutely
no doubt about that. Naturally you would choose the frequency of the VHF
parasitic resonance in the PA that you're trying to suppress.
The second question is: what is the difference between the two types of
suppressor at other frequencies?
Plotting the N7WS measurements on a (log Q) versus frequency scale
showed that the Q values of the two different types of suppressor track
quite closely across the VHF range. The measured Q values were about 40%
different at 100MHz, but the two curves run pretty much parallel across
the VHF range. On a log scale, that means that they pretty much
maintained a 40% ratio between them.
So, if you make a conventional R/L suppressor that mimics the Rp-Lp
behaviour of the R/NiCr suppressor EXACTLY at one chosen VHF frequency,
you'd then find that the two curves remain very, very close across the
VHF band.
But N7WS's measurements show that the Q of the conventional suppressor
rises FASTER at HF - in other words, at HF it looks less like a resistor
and more like a small inductor, which is exactly what it's supposed to
do. The Q of the R/NiCr suppressor also rises at lower frequencies, but
more slowly. In other words, the significant difference between the two
types of suppressor is not at VHF, where their performance can be made
almost identical. The main difference is at HF.
OK, that's it for a second time, in a different newsgroup and hopefully
explained even more clearly.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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