[TenTec] 6N2 observations

[Dr. Gerald N. Johnson]

On Sun, 2006-07-09 at 20:11 -0700, Duane - N9DG wrote:
> Not sure where the notion that a 2 dB NF is sufficient for 2M
> SSB/CW work comes from. I suspect though that it is a figure
> that has persisted for years in ARRL Handbooks. I would argue
> that a NF in the .6-.8 dB range is a more realistic minimum
> to shoot for. I run a transverter that is factory spec'd at
> .8 dB at the end of a coax run with a measured loss of .8 dB
> and still find occasions where I could use a lower NF. If you
> are running an antenna with a clean pattern, low loss
> feedline, and have quiet radio horizons at VHF a 2 dB NF in
> the RX is simply not low enough for 2M. 
> 
> Duane
> N9DG
> 
2 dB is a great deal better than 12 dB the 6N2 seems to provide. MDS 10
dB poorer than the anemic Ft-736. To get to 0.5 dB NF from 2 dB isn't
too hard, but from 12 dB its a real pain, with way too much gain in
front of the selectivity.

So far at CSVHF, the .2 dB NF preamps have not proven all that usable in
the hamshack because of out of band overload. Seems like with the best
of PHEMTs that the loss of the input matching circuit sets the NF, not
the device. And the HP NF meter much prefers a 10 MHz or so bandwidth to
have a smooth measurement. It down grades preamps that have only 250 KHz
bandwidth which is what is needed in the real world today even out near
Podunk. The single tuned input for one preamp I've used when tuned for
the best NF at 2m peaked at channel 13 TV for gain response. That makes
it hard on the radio to reject channel 13 when the tower is visible from
ground level. For that matter to reject all the highband TV channels.

I'm not sure what the best usable NF is. Depends on the RF selectivity
and the dynamic range. For sure there are situations where the better NF
hears better, such as when looking up at satellites or the moon. And
where any practical NF improvement improves S/N.

A speaker from Collins at the CSVHF conference in Cedar Rapids a few
years back opined that any NF better than 2 dB was a waste of effort and
not needed. I wanted to counter that with the argument that as hams we
don't insist on better than .01% reliability of a path, not 99.99% and
that since noise is a time varying phenomena its not good that the
receiver NF should be the limit during those rare times when the time
varying noise is lower than the normal peak. And unless the really low
noise RF stage compromises the system dynamic range excessively (so that
unwanted signals degrade reception) its not harmful when the weakest
discernible signal is determined by external noise rather than the
system NF.

I've worked lots of stuff over the years on 2m with a 2 or 3 dB NF. And
heard lots of line noise that I didn't hear with a 15 dB NF from a bad
front end. Its ludicrous that commercial equipment these days doesn't do
as well as the converter I made work in 1964.

-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer