Topband: Re: 160m RX

[Tom Rauch]

I can directly measure signal level on accurate selective voltmeters.

I'll give measured data, rather than pulling numbers from thin air. 

> I wonder if God has spared Georgia of galactic noise?  If not, 10 uV
> sensitivity is more then enough for 160m.  This leaves plenty of room for
> a good input filtering permitting much greater input voltages close to
> 100V.  We only need high level input mixers to sustain such a signals. 
> Pair of power MOSFET would do but they are not commercially offered as a
> mixer...

Galactic noise is a non-issue on 160 meters, because the 
ionosphere shields the earth from galactic noises. The bulk of 
noise on 160 meters is locally generated or propagated in from 
distant terrestrial sources. Because of that... noise varies greatly 
with propagation, location, season, and time-of-day. 

10uV is probably not enough sensitivity in any area, let alone a 
rural area. The example below shows why:

At 9AM today the beacon (500 miles or more distant) on 1791kHz  
easures -96dBm (75 ohm system) on a southwest 500 foot long 
Beverage. Noise is -128dBm on the same antenna using a 3 kHz 
filter. ~32dB S/N ratio @ 3 kHz BW.

That same signal is -94dBm on my 200 ft omni-tower, and noise is -
107 dBm (three kHz BW). ~13 dB S/N at 3 kHz BW.

10uV sensitivity is  -87dBm into 50 ohms, or -88.75 dBm into 75 
hms.

A 10 uV sensitivity receiver would be over 20 dB SHORT on 
sensitivity on my omni-vertical, and 40 dB short on that Beverage.

0uV is about -88 dBm, my FT-1000 is -138 dBm. My FT-1000 
would work poorly with a 50 dB attenuator, especially on a 
Beverage!

I suspect most other FT-1000's at most other locations would work 
poorly with a 50 dB pad.

> Yes.  Good engineering would make RX VFO clean enough for other "linear"
> circuits to saturate first.  Modern gadgets one buys these days are DSP,
 color displays etc...

That's true. Many receivers have such poor attention paid to 
dynamic range that the noise in synthesizers is unimportant. That's 
why the R4C is an excellent platform to do mods from, if you want 
a high performance receiver. The TenTec also has a lot of potential 
because of it's quiet LO, although I've never gotten deep into one.

Stock, the R4C is not nearly as good as people seem to think. 
Modified (new mixers, better amplifiers) it can be better than 
anything available.


3, Tom W8JI
w8ji@contesting.com



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