[ARC5] Input impedance - BC-453-B - un-un

[Ian Wilson]

I wouldn't think there's much point trying to match the input: looking
at the input circuit as a potential divider, most of the input *voltage*
is going to appear across the resonant input circuit, anyway. There is
a large *power* mismatch, but at these frequencies you can tolerate
this easily (since the band noise will dominate).

Think of this as one of those amplified short antennas that people use
on VLF receivers.

73, ian K3IMW


On Fri, Jan 30, 2015 at 12:32 PM, Kenneth G. Gordon <
kgordon2006 at frontier.com> wrote:

> On 30 Jan 2015 at 10:03, Ian Wilson wrote:
>
> > Reposting my approximate calculations. I have added a note ([1] below).
> >
> > Robert is correct about the input trimmer: this is in parallel with the
> tuned
> > circuit, and the antenna connects to that through a small fixed
> capacitor. My
> > bad. The impedance of the input at resonance is not determined by the
> grid leak
> > resistor of the first tube, but by the effective parallel resistance of
> > the coil, namely 2pi * L * Q [1]. Guessing that the tuning capacitor is
> around
> > 300pF at 500kHz, then the coil inductance will be on the order of 350uH.
> With a
> > Q of 50 the equivalent parallel resistance will be around 55k ohm.
> >
> > So the effective input circuit will look like 8.5pF in series with about
> 55k ohm
> > (to ground).
>
> OK. I was off by at least a factor of 10: 4K vs ~40K. My memory is
> defective.
>
> > [1] at the frequency of operation of the BC-453, we can ignore the real
> > component of the complex input admittance of the RF amplifier pentode. At
> > higher frequencies, various losses and effects such as transit time and
> lead
> > reduce the effective input resistance of tubes radically.
>
> Yes. That all makes perfect sense. I firmly believe in ignoring those
> factors
> which cannot influence data beyond a few percent. It makes calculations
> easier, although not more accurate. We really don't NEED accuracy beyond
> 10, or even 20% in the circuits with which we are dealing.
>
> Now, my next question:
>
> As far as the input impedance of our receivers is concerned, in my opinion,
> any thing between 40K and 60K ohms is close enough.
>
> So, my next question: if we routinely want to connect our receivers to 50
> ohm
> systems, what sort of un-un would we have to make?
>
> 7 turns on the 50 ohm side to ~200+ turns on the 40K ohm side...but on what
> sort of form? Toroid? What "mix"? What size wire?
>
> Expectations of improvement? Any? None?
>
> Ken W7EKB
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