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

[AKLDGUY .]

Why not simply connect a standard antenna coupler backwards?
If placed in the antenna line, that also takes care of matching the
transmitter to the antenna, whether it be an ARC-5 or BC-230 or
some other MIL transmitter. If the transmitter can be successfully
matched that way, you can bet the receiver will be matched pretty
well too.

Of course, any built-in SWR meter will read backwards - forward
power will indicate as reflected power and vice versa, but the SWR
reading will still indicate the actual ratio.

73 de Neil ZL1ANM


On Sat, Jan 31, 2015 at 9:32 AM, 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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