Don,
> I agree if the frequency is higher than 6 meters, but at 50 MHz and
> down, the cable loss does not appreciable degrade the noise figure
> (unless the feedline is exceptionally long).
Cable loss *always* degrades noise figure because it increases the
system noise figure on a dB for dB basis just like *any* attenuator
present before the first amplifier. The question is whether that
increased noise figure is significant in the upper HF (20 - 30 MHz)
range ... and I would argue "it can be".
ITU/CCIR studies show man made noise levels between 15 and 20 dB
higher, on average, than thermal noise in that range - meaning
background noise will run from -127 to -132 dBm in 500 Hz or -120 to
-125 dBm in 2.4KHz. Sherwood measured the K3's MDS at -130 dBm
(presumably at 400 Hz) with preamp off which equates to a noise
figure of 17 dB. 3dB of feedline loss (200 feet of "RG8X" - not
"exceptionally long") ahead of that would directly impact performance.
Even with the K3's stock internal preamp (NF= 7.5 dB, gain 12 dB),
3 dB added to the 9.5 dB system noise figure (MDS= -138 dBm @ 500 Hz)
could reduce system sensitivity to a point that it was insufficient
in an exceptionally quiet location.
73,
... Joe, W4TV
On 7/13/2013 8:14 PM, Don Wilhelm wrote:
Olli,
I agree if the frequency is higher than 6 meters, but at 50 MHz and
down, the cable loss does not appreciable degrade the noise figure
(unless the feedline is exceptionally long). That does not equate to
those frequencies above the 50 MHz band.
The real answer depends on your particular situation (antenna gain and
feedline loss).
73,
Don W3FPR
On 7/13/2013 8:03 PM, Oliver Dröse wrote:
Am 13.07.2013 21:09, schrieb John Buck:
The slightly higher gain of the PR6-10 with the same noise figure
then might be an advantage by helping overcome cable loss.
How should that work if the preamp is mounted at the K3 between RXANT
in & out? It's after the cable then, isn't it? ;-) If you want to
overcome cable loss buy a mast-mounted preamp.
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