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

[Ian Wilson]

Yes; but in the case where you have a fairly low-impedance antenna
system, and each receiver looks like a relatively high impedance,
adding a second, third, etc, receiver will make little difference to the
signal *voltage* presented to the first, second, etc, receiver.

For example, suppose you have a 50-ohm antenna and the input
reactance of each receiver is 40k. A single receiver will see 99.9%
of the available voltage from the antenna (40k/(40k+50)). If you add
a second receiver, each receiver will see 99.8% of the available
voltage (20k/(20k+50)).

Yes, I know I should resist replying to these posts.

73, ian K3IMW


On Thu, Feb 5, 2015 at 11:42 AM, AKLDGUY . <neilb0627 at gmail.com> wrote:

> Correct me if I'm wrong, but when multiple receivers are fitted,
> aren't all the antenna terminals connected together with short
> wires? So effectively all receiver inputs are in parallel?
>
> 73 de Neil ZL1ANM
>
>
> On Fri, Feb 6, 2015 at 7:43 AM, Kenneth G. Gordon <
> kgordon2006 at frontier.com>
> wrote:
>
> > On 4 Feb 2015 at 20:57, Dennis Monticelli wrote:
> >
> > >
> > > Guys,
> > >
> > > Maybe we're over-thinking this input matching "problem."
> >
> > Well, I haven't viewed it as a "problem": I am simply very curious
> whether
> > matching the input impedance could make any difference, either good or
> > bad.
> >
> > > If the input parallel tuned tank's parallel resistance at resonance is
> > in the
> > > neighborhood of 50K ohm (as Ian reports), then let's ask ourselves what
> > a 50 ohm
> > > antenna looks like after passing through the series 8.5pF input
> coupling
> > cap.
> > > The series circuit of 50 ohms and 8.5pF need only be transformed into
> its
> > > parallel equivalent circuit to answer that question.  Doing the
> > mathematical
> > > conversion at 400KHz results in 44K ohm in parallel with 8.5pF.  The
> > > capacitance will get absorbed by the antenna trimmer leaving 44K of
> > antenna
> > > source resistance.
> > >
> > > I just don't see a need to be fiddling with input matching networks.
> > And let's
> > > give the ARC-5 designers the credit they deserve.
> >
> > Well, as I have repeatedly stated here, as far as I am concerned, the ARC
> > engineers were pure geniuses. They appear to have thought of everything.
> >
> > Ken W7EKB
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