[Elecraft] Trade off PR6 vs PR6-10 pre-amp

[Joe Subich, W4TV]

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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