I agree with Gerald K5SDR.
Taking reasonable numbers for a 2 meter system with tower-mounted preamp
[which is a must for serious 2 meter weak signal work]:
F = F1 + (F2 -1) / G1
NF = 10 log F
F = 10**(NF/10)
So:
If NF1 = 0.5 then F1 = 1.1220
if NF2 = 1 then F2 = 1.2589
if G1dB = 25 dB then G1 = 316.2
F = 1.1220 + (0.2589/316.2) = 1.1220 + .0008 = 1.1228
NF = 0.503 dB
if NF still = 0.5 then F1 still = 1.1220
if G1dB still = 25 dB then G1 still = 316.2
but if NF2 = 4 then F1 = 2.5119
then F = 1.1220 + (1.5119/316.2) = 1.1220 + 0.0047 = 1.1267
and NF = 0.518 dB
Degradation of system noise figure in going from 1 to 4 dB for NF2 is
only 0.015 dB
So, I think I will take the 4 dB noise figure for the 6000 series and
the nice associated increase in dynamic range and be perfectly happy for
2 meter weak signal work. ;)
And when the 6000 series is used as a 2 meter IF rig for microwave work,
NF2/F2/G2 in the Friis equation are for the transverter, and the 6000
series term becomes [+ (F3 -1) / (G1 * G2)], so I am even less concerned
about F3.
On 144 MHz and up the key to optimal system performance is achieving
adequate gain and NF of the tower-mounted preamp, and maintaining
appropriate gain balance throughout the receive chain. Failure to
achieve optimal performance is in my experience most commonly due to
inadequacy of the tower-mounted preamp, as the Friis equation would
suggest. Either gain, or noise figure, [or dynamic range] problems at
the first stage are impossible to erase by adjusting the performance of
following stages. As long as adequate first stage gain is achieved,
variations in the noise figure of the following stages have only a very
small effect on overall system performance, as is shown above. But the
dynamic range of each stage IS important and a failure to achieve
adequate dynamic range at ANY ONE stage will seriously impact system
performance. So best practice dictates optimizing the dynamic range of
later stages in preference to noise figure, and optimizing gain, noise
figure, and dynamic range of the first stage [tower-mounted preamp] as
the most important consideration of all. P.S. the antenna is extremely
important as well. It is "really" the first stage...
73,
Roger Rehr
W3SZ
http://www.nitehawk.com/w3sz
On 5/24/2012 8:45 PM, Lee Mushel wrote:
Gerald,
I think we have to start over. My concern with noise figure had to do only
with 2 meter DX. I had no question about any performance on the HF bands or 6
meters. And I certainly know about putting the low noise preamp at the feed
point of the antenna and I know about the switching associated with a practical
system and I do have a sequencer.
Now, I insist that we go back some 50 years to find me growing up in northern
Wisconsin where there were a total of some four teenagers routinely on the
cutting edge of ham radio: two meters. And this is over nearly one half of
the state. Finding someone to talk to was a very big deal. And I think I can
still find low noise preamps using the Western Electric 416b and 417a tubes in
my shed that I built at that time. Moonbounce was only on 1296 and there was a
lot of discussion on what was involved in calculating total system performance.
Now let's go back a few days. I don't think I ever mentioned it to you but I
do have the V/U upgrade and sitting on top of my 5000A are the Elecraft 144 and
432 transverters. I feed those into the RX2.
If I can believe my 5000A my local noise floor a few days ago was a little
better than -140 dBm. I would think most folks would expect that I would be
interested in comparing the performance of the Elecraft and Flex front ends.
And I do this frequently and I can safely say that with the Flex I can hear
weaker signals and that never surprised me because the specs of my several year
old Elecraft.
I sincerely regret having commented on what I'm fairly sure is still an
encouragement of mediocrity and I will not do that again.
But I appreciate your taking the time to write me.
73
Lee K9WRU
----- Original Message -----
From: Gerald Youngblood
To: Lee Mushel
Cc: Brian Lloyd ; Ron Stockton ; [email protected]
Sent: Thursday, May 24, 2012 3:49 PM
Subject: Re: [FlexEdge] Flex 6000 MDS
Lee,
It is a very common misconception that lower noise figure is always better. A good
article on the subject was in the June 2010 issue of QST. The article written by Joel
Hallis is titled, "Receiver Sensitivity - Can you have too much?" The answer
is yes. All you get is more noise and lower total dynamic range. What you want is for
the gain to be set optimally for the band noise floor at your specific location.
In fact a low noise figure may actually reduce total dynamic range for a given
band and conditions. We could easily have put a <1 dB preamp for the same cost
in the radio but that would have degraded total total IMD dynamic range. If you
really care about a 0.1 dB NF preamp, it would be a total waste to put that inside
the radio because it would ruin gain distribution and it would be swamped by the
coax loss.
The FLEX-6700 can give you a 4 dB NF on 20m but that would would ridiculous
since the atmospheric noise figure equivalent in a rural area is probably
greater than 35 dB above kTb. All you would be doing is to reduce the total
dynamic range because you have too much gain. MDS of around -120 dBm is
probably appropriate for most locations on 20m On 10m you probably need an MDS
of around -130 dBm in rural areas and -122 dBm in residential areas. On 10m
you can probably use -137 dBm (10 dB NF) only if you are in the quietest rural
areas.
The bottom line is if you want lowest noise figure on 2m, put the low noise
preamp at the antenna and turn off the preamp in the radio. That will give you
better gain distribution and will overcome the coaxial line loss.
Regards,
Gerald
Gerald Youngblood, K5SDR
President and CEO
FlexRadio Systems(TM)
Email: [email protected]
Web: www.flexradio.com
Tune In Excitement (TM)
PowerSDR(TM) is a trademark of FlexRadio Systems
On Wed, May 23, 2012 at 9:39 AM, Lee Mushel <[email protected]> wrote:
I sure hope you are wrong about that number. You know, in 1958 I had a
Techraft Converter that had a noise figure of 7dB and my quite excellent V/U
upgrade is somewhere around 1 dB and my dedicated pre-amp is somewhere close
to 0.4 dB. A noise figure of 4 in 2012 would rate somewhere between wretched
and miserable!
Lee K9WRU
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