W8JI wrote:
> > >My 160 meter noise floor, after pre-
> > >amplifiers, on a quiet winter night when the band is open to Europe is
> > >about -117 dBm measured at 3.5 kHz bandwidth. With a 350 Hz bandwidth,
> > >the "off-the-air noise floor" of my system is -127 dBm. With a 35 Hz
> > >filter it is -137 dBm.
Mario replies:
> I stated long ago that the claims of nV sensitivity for 160m RX is excessive.
> We can now all see that 20 dB attenuation is optimal on any 160m SSB RX.
> >I know you keep saying that Mario, but the numbers indicate otherwise.
> > My noise floor at night through a low-gain preamplifier that just
> > offsets feedline loss is about -119 dB with a SSB filter bandwidth.
> > That is 0.25 uV!
There were two W8JI claims in Aug. 2000 worth remembering:
>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.
20 dB input attenuator is very welcome with the above RX for 250 nV noise!
Average ham with 160 m dipole may have the following situation (W8JI data):
> At 9AM today the beacon (500 miles or more distant) on 1791kHz
> measures -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.
Noise level on the second antenna is 21 dB higher although signal is weaker!?
> An FT1000D with preamplifier ON barely makes that sensitivity.
> With a CW filter, even more sensitivity is required.
FT-1000D was tested by ARRL in March 1991 and RX sensitivity in SSB mode without
preamp was 0,25 uV in 1,8-30 MHz range. Latest MARK-V is some 8 dB better. This
makes FT-1000D an ideal RX for the beverage while 20 dB attenuator might be
recommended for more efficient RX antennas.
> That is why **virtually everyone** with Beverages uses a
> preamplifier. There is a very clear need for them!
W8JI properly designed outside preamp just compensates beverage feed line
losses.
That much extra gain is always available in all RX built-in preamps on 160 m.
> The poor close-spaced blocking and third-order intercept of
> receivers also is a problem, and the HF bands are often very
> crowded with strong signals next to weak signals. Everyone who
> works 160-40 meters knows what a problem it can be to hear noise
> floor signals next to strong local signals.
My experience on EU 40 m dates back from 1962 while 160 m YU license was issued
in 1972.
We used Collins 51J4 with additional 40 m crystal filters for the weak ones in
Peking QRM.
Top band sounded like "magic band" (50 MHz) and my DX-ing was done with AM radio
with BFO.
Last year serious CQ WW 160 m contest operation with vertical by the sea and two
beverages
was done with IC-775 using input attenuator most of the time.
> Despite what you claim, few receivers are good enough for the
> anything but the most casual local 160 operations.
Good RX are statistically optimised to receive largest number of signals under
real HF conditions. The case of noise level signal near much stronger ones is
very rare and nobody can economically justify such a HF RX design these days.
Especially not with CW trends...
Recent MARK-V improvements in strong signals handling with tuned preselector
together with cheap IC-718 significant VCO noise reduction makes me happy of the
general trends.
I also enjoy playing with Atlas-350 RX improvements but I do not expect anybody
to reproduce it. One can buy K2 instead...
Disclaimer: I have no commercial relations with any radio manufacturer
mentioned.
73 de Mario, S56A, N1YU
P.S. Tom, W8JI contribution to hamradio reflectors is great thanks to all the
real data he provides.
I almost blew my SB-220 once while disputing his correct claim on arcing due to
high voltage overshoots under light PA load conditions :-)
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