[Elecraft] K3S Preamp 2 Six Meter Noise Figure

[Wes Stewart]

Just because a "device" will do under 2 dB (or whatever) doesn't mean an 
amplifier necessarily will. Circuit losses, mismatch and bias settings will 
affect the circuit NF.

Additionally, something like an HP noise source is made to be a good source 
match, both on and off.  Those with lower excess noise, indicating the presence 
of an attenuator are better at this than higher noise sources. Nevertheless, 
there can be a difference.  If one of these sources is used to both tune and 
measure an amplifier, the NF can, and probably will be, different when connected 
to an antenna.  These differences are probably in the noise, excuse the pun, at 
6 meters but can be significant at higher frequencies.

Many years ago when GaAs FET were just arriving a friend designed and built a 
preamp for 1296 MHz.  I measured the NF at 0.6 dB, but after looking at the 
undercertainties the number was something like (going by memory) 0.6 dB +1, 
-0.6, in other words, it could be perfect.  This was all due to the 10:1 input 
SWR.  With a vector network analyzer these uncertainties can be resolved, but 
it's not trivial.

I had to chuckle after HP donated an HP8970 to the ARRL lab years ago and they 
reviewed an ARR preamp and listed the NF to two decimal places, presumably 
because that's what the instrument reported.

Wes  N7WS


On 5/1/2017 12:15 PM, John Stengrevics wrote:
> That was my reaction as well, but that is what the QST review states.
>
> John
> WA1EAZ
>
>> On May 1, 2017, at 3:02 PM, Charlie T, K3ICH <pincon at erols.com> wrote:
>>
>> I'll bet it’s a lot better than that.
>>
>> A 39¢ GG junction FET pre-amp will do under well 2dB on 6.
>>
>> Charlie k3ICH
>>
>> -----Original Message-----
>> From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of ab2tc
>> Sent: Monday, May 01, 2017 2:08 PM
>> To: elecraft at mailman.qth.net
>> Subject: Re: [Elecraft] K3S Preamp 2 Six Meter Noise Figure
>>
>> Hi,
>>
>> According to the ARRL review in November, 2dB! Deaf no more.
>>
>> AB2TC - Knut
>>
>>