[ARC5] A simple receiver improvement

[Kenneth G. Gordon]

On 14 Jan 2014 at 12:15, Ian Wilson wrote:

> Ken,
> 
> The input circuitry of the receivers has a small capacitor from the
> antenna terminal to the top of the input tuned circuit (with a small
> variable capacitor to ground to provide adjustment for different
> antenna capacitances).

Yes. Exactly.

> A couple of schematics - sorry, didn't have time to check for the R-26
> specifically

For the R-26, series cap is 11 pfd, parallel variable is 15 pfd.

> - show values for C1 of 8.5pF and 11pF. At 3.5MHz, 11pF
> looks like 4.1k of reactance.

Again, ah ha! Quite high.

> So, your balun is effectively working as a voltage stepup transformer.
> I would expect this to help significantly. It will not only raise the
> input level, it will probably improve the S/N ratio of the receiver

It most certainly did.

> (since you now have almost noiseless gain ahead of a somewhat noisy
> pentode RF stage).

Indeed...to both.

> I would guess that the input circuitry of these radios was designed to
> work with 'almost any' antenna. There is no attempt made to match the
> radiation resistance of the antenna. In return for some power gain,
> you get a receiver that will work pretty well with many different
> types of antenna. Another good piece of design, methinks.

Oh, I heartily agree ! :-)

Ken W7EKB