Topband: Receivers for 160M

[Tom Rauch]

> Jerry,  The reason, I believe that made the 75A4, with 500hz Collins
> mechanical Filters, such a fine 160 meter receiver in its time was the
> fact that on that band the design used single conversion rather than the
> double conversion it used on the other bands.  I think this was the key to
> lower IMD.  There was one drawback.  On 160, if I remember 

Hi Herb,

There is no inherent disadvantage in multiple-conversion schemes if 
gain distribution is proper, the filters are of good design, and mixers 
are properly designed. 

The shortfall with the Drake R4C isn't rooted in the triple conversion 
scheme, but how they engineered things. Narrow filtering is placed 
behind two poorly designed mixers, and shielding around the filters 
is somewhat poor. They probably had a target cost and size to 
work with, and did what they could do with the components they 
had available. 

That's the reason an R4C changes from a mediocre receiver to one 
of the best receivers available when narrow filters are added after 
the first mixer. (This excludes R4C's under S/N 18700 or so, which 
are unrepairable without converting the MOSFET second mixer to a 
better design.)

I have an R4C modified with balanced mixers and solid-state IF 
amplifiers that overloads at 23 dBm (2-1/4 volts) input, yet has -145 
dBm noise floor (12.6 nanovolts). It's still triple conversion. The 
design was just converted using modern mixer and amplifier 
technology.

73, Tom W8JI
w8ji@contesting.com



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