Topband: Re: 160m RX

[i4jmy@iol.it]

> 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...

Not necessarily everyone uses efficient antennas on reception, 
expecially on 160m. Loops or Pennants, not counting beverages with long 
runs of lossy cable will deliver to receiver very attenuated signals 
(often 30 dB or even more). In such cases the 10uV level has to be 
accordingly reduced to satisfy own requirements.
If receiver has low noise and good dynamic range there's no need of hi-
performing remote preamps (with low NF and bad IMD, or vice versa).

As everyone (should) know, mixer is inherently a non linear circuit 
otherwise it wouldn't mix anything. (mixer is an amplitude modulating 
device) 
Moreover best Hi-level mixers have not so good NF as generally is 
thought and the best receiver is that were gain is properly 
distributed, not strightly reduced to minimum possible.
I would check carefully the NF before using a power mosfet in a low 
band receiver as mixers or preamps, not counting that field effect 
semiconductors tend to be noisier than bipolar at lower frequencies.
 
> 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...

Saturation of other circuits has nothing to do with phase noise. 
Phase noise of a local oscillator is a sort of imposed noise that will 
cause the weak mixed signal to drop under noise if the signal ratio 
between incoming signal and local oscillator is not favourable.
In other words, the ideal is to have a real carrier as a local 
oscillator instead of a phase modulated signal like it happens with the 
VCO of the PLL circutry.
(Note: phase noise can be reduced at price of frequency accuracy if 
increasing the VCO swinging period) 


> Modern VHF semiconductors technology can't help us on 160m.  Nice 
example is
> latest FT-847 with intergrated quad FET first mixer operating on 3V 
DC!  This IC
> probably comes straight from the mobile telephone...

Supply voltage has little to do with performances, the matter is to 
match impedances in the proper way to avoid the incoming signals 
approaches more than a fraction of voltage supply.
If low voltage was an inherent limit we should supply everything at 
several KV and small currents whose fact is luckily not real.

> Unfortunately, HF is commercially obsolete band and we are getting 
oversensitive
> RX
> intended for dummy load antennas.  They can hardly withstand strong 
BC signals
> on 40 and 160m.

There's no need of particular experience to see what the HF spectrum 
is, any RF engineer in charge of designing an HF receivers is able to 
understand what's needed, but in spite of what you think he has 
marketing inputs to satisfy.
Personally I'm convinced modern receivers (HF VHF UHF) are designed to 
be cheap and to work with marginal (very poor) antennas, like mobile 
antennas or idoors.
This happens because of hundred reasons (no space, legal problems, 
advertisment of "miracle radiators", aging of Ham populations, etc) and 
amateurs with real antennas are now a small minority compared to the 
total.
If someone with a real antenna farm wants to have a good receiver, in 
most cases has to perform internal mods to what is bought.

>  I wonder when would BBC, VOA, DLW stop?

Hopefully never, otherwise it is the sign "big brother" won and also 
amateur will disappear because it will be illegal to contact one to one 
without a third that's supervising (and catching money from both).

73,
Mauri I4JMY



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