Topband: R4C RF stage modifications

[Tom Rauch]

> The R4C's performance on 160m CW is also excellent, but it lacks the
really
> sharp edge the R4C has on 40m over the MP.  Tbinking about why this might
> be so, we have become increasingly of the belief that this shortcoming
lies
> in the permability tuning of the 6BA6 RF stage.

I wouldn't know what that would be true.

If the receiver limits on antenna  noise, rather than internal noise, that's
all you need in the front end gain or loss. The only other requirement of
the front end is that it suppress out of band signals enough that nothing
further down the chain overloads from out of band signals, and that the
selectivity rejects signals that would otherwise be heard through spurious
mixer responses (like IF images). The R4C front end is one of the most
selective around on 160 meters, as evidenced by having to retune every 5 or
10kHz  (not that it needs to be anywhere near that selective).

> reflector as to what they think of the idea of swapping the R4C
permability
> tuning for a three-section bandpass filter.

If you want broad bandwidth without retuning, go for it. Other than that, it
will make no difference. There is nothing wrong with the R4C RF stage tuning
system. It doesn't help performance, it doesn't hurt performance.

You can get a bit better dynamic range and more sensitivity by increasing
the RF amplifier screen grid voltage, but after that the first mixer becomes
the limitation. The first mixer system (as designed by Drake) is somewhat
starved for injection, and so will have more conversion gain and less IM
response when the injection level is increased significantly.

73 Tom