[TMC] GPR-90 RXD Tuning

[jvendely at cfl.rr.com]

Hi Steve,

Back in those days, a high premium was placed on HF receiver sensitivity, so designers went for low noise figure.  This marketing-based obsession continues today in the ham radio market, to the detriment of some receivers.  Over most of the HF band, and particularly below 10 Mc or so, atmospheric noise is generally the limiting factor, so modern commercial and military receivers are usually optimized for dynamic range rather than noise figure.  So the old receivers usually had lower noise figures than the modern ones.  As such, the GPR-90 has plenty of "excess" sensitivity on 40M, so you might want to simply try adding attenuators ahead of the receiver to reduce the intermod.  6 to 10 dB of attenuation might do the trick, but you could probably add as much as 20 dB attenuation on 40M at night, if you have a reasonably efficient antenna, and still have adequate sensitivity, but drastically drop the intermod product levels. 

73,

John K9WT


---- wb9tow at egr.msu.edu wrote: 
> Excellent information on the TMC Receivers!
> Just a present day observation, I use a GPR-90 with
> GSB-1 on 40m CW about every day. The combination is
> not too bad depending on the "time of day". I live about 1 1/2
> miles from a 5KW MW BC station. Their station is off the air
> at night, which helps reduce the IM....
> I run a resonant antenna system and an ATU but still
> find the IM rejection quite poor.
> I've often wondered if there is a way to improve this design
> without destroying the value of the rx, but I think the best
> step is to acquire a GPR-90RXD or GPR-92....
> 73
> Steve w8tow
> >