[Elecraft] K2 receive sensitivity

[Ron D'Eau Claire]

Paul wrote:
hank you for your response, Ron. (and also Don).
I am aware that the test I described is only a quick check to see if the 
ability to hear weak signals is being masked by internal receiver 
noise.   However, the K2 is the only  receiver (that I have owned) that 
has ever "failed" this test on 10 meters (meaning that more gain was 
needed so the preamp should be turned on) and I am wondering if others 
experience the same thing or there is something not right with my 
particular K2.  So far, I have not been able to get an answer to that 
question.
I do own the XG2 and have performed MDS measurements on my K2 using the 
XG2 and also a milspec signal generator I have.  The numbers agree very 
closely with published specs ON 20 METERS.  On ten meters I measured an 
MDS of -124 dbm (preamp off) and -130dbm (preamp on).  IMHO, an MDS less 
than -130 dbm is not acceptable for weak signal reception.  Which means 
to me that on 10 meters the preamp is absolutely required. 

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I think "failed" is a misnomer. It sounds as if the only receiver in your
comparative tests that is behaving ideally is your K2. 

The last thing you want is excess "front end" gain. All you want is just
enough to bring signal levels up above the internal receiver noise and then
have the bulk of the gain *after* the i.f. filters where only the signal of
interest is being amplified. 

The K2 exhibits that characteristic, as you observed. On the lower
frequencies no pre-amp is needed. The band noise easily over-rides any
internal receiver noise. But on 10 meters it does not until you turn on the
preamp. That means the K2 does not have excess front end gain on the lower
bands that will limit its dynamic range. 

It sounds like the receivers you're using to compare the K2 with have a
less-than-ideal gain distribution. 

To the "heart" of your question, yes, on my K2 at least, the preamp is
needed on 10 meters. I've not tested the MDS on that band, but at the
narrowest bandwidth my receiver's ability to "hear" is limited by the
external noise under the quietest band conditions, as indicated by the
"DeMaw test": removing the external antenna and noting a significant drop on
QRN. That means that seeking a better MDS is a waste of effort for me with
my antenna system. 

Ron AC7AC