[ARC5] Receiver input impedance.

[WA5CAB at cs.com]

This is I think at least the second post to say that the antenna post on 
the Command Sets is connected to the tank circuit (antenna coil in parallel 
with one section of the tuning capacitor) by the front panel mounted trimmer 
capacitor.  But it isn't.  The trimmer capacitor is in parallel with the tank 
circuit, one side of all three components going to chassis.  The antenna 
post connects to the three parallel components through a fixed 8.5 mmf ceramic 
capacitor (at least in the BC-453-(*) ).  And that point is also connected 
to a fixed 100 mmf capacitor which connects to the 12SK7 grid and the 2 meg 
grid leak resistor.

So the imput impedance is about 2 meg plus the reactance of the two series 
fixed capacitors, less any MFP leakage.

In a message dated 10/31/2012 00:20:28 AM Central Daylight Time, 
vk2bcu at operamail.com writes: 
> Looking at the circuit for the BC-946 (similar to R23 etc) I see the
> tank circuit is connected to the antenna (or the aerial if you live in
> Australia) by a trimmer cap.  According to the theory (and working in
> the ideal) the impedance of a parallel tank  circuit at resonance is
> very high (infinite, for the idealist) and so the advantage in matching
> an antenna to THIS load comes from stepping-up Z from 50 ohms to a
> higher Z.  However, if the loop is also tuned to resonance, it will have
> a similar high impedance (infinite for the idealist) and so there is no
> need for a matching device.
> 
> This is why the signal "peaks" when the trimmer is adjusted.
> 
> That's my "teary" of how "de aerial" woiks.
> 
> 

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480