>
>Something I don't understand - maybe Carl or Rich or Jon can explain.
>
>If I take an inductor (start with a 'perfect' inductor) and put a resistor
>in series with it, at LF it looks like the inductor isn't there.
? The inductance and the resistance are always there, whether it's LF or
UHF.
> If I make
>the inductor out of resistance wire, it still looks like a resistor.
>
? If one makes an inductor out of resistance wire, it looks like an
inductance in series with a resistance, however, there is a parallel
equivalent that is an electrical twin. .
>Now increase the frequency. Inductive reactance rises, and the Q, given by
>2*pi*F*L /R increases, eventually becoming infinite at infinite frequency
>(OK, 3-500s don't have any gain there!) So I have a series inductive reactance
>with Q increasing with frequency.
>
>Now take the coil, wind it out of silver tape if you like, and put a 33 ohm
>resistor across it. At this stage, it's a perfect resistor. At LF, the low
>reactance of the coil shorts out the resistor, but as the frequency rises,
>so does the coil reactance. Because it's a parallel circuit, the impedance
>tends asymptotically to 33 + j0. Even a practical resistor will tend that
>way. As Q is Rp/1*pi*F*L, the Q is falling with frequency - in fact, it will
>have a maximum value at VLF.
>
? ok
>If I understand Rich's argument, the Q of a parasitic suppressor should fall
>with increasing frequency.
? Getting there is the sticky wicket. The Q of resistance-wire falls
with increasing f.
>So there seems to me to be an anomaly - from the
>above argument, an L-R shunt circuit is indicated.
>
>can someone explain?
>
? Every L-R series circuit has a L-R parallel-equivalent circuit. Every
L-R parallel circuit has a L-R series equivalent. Thus, the two are
interchangable - after a bit of math.
. It is true that the HF amplifier's anode circuit VHF Q can be
reduced by simply increasing L-supp. However, the gotcha is that
haphazzardly increasing L-supp can result in the destruction of R-sup
during 10m operation. Cutting to the chase, using resistance-wire
instead of copper-wire for L-sup typically reduces 100MHz-Q by about 40%
Without increasing the 10m dissipative burden on R-supp.
In other words, vhf suppressor design is unavoidably linked to the
ability of R-sup to dissipate energy at 10m. Resistance-wire helps a
bit. Using more than one vhf L/R suppressor helps a bit. // Another
issue is the division of vhf currents through L-supp and R-supp.
- (see "Calculating Dissipation..." in March, 1989 *QST*)
? ...... "everything is more complicated than it looks"......Murphy
- cheers, Peter
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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