[Elecraft] K3 Noise Floor 160m - 75m

[Paul Christensen]

I was recently looking at the noise floor of my K3 with SDR-IQ with antenna 
disconnected.  As I went from the upper bands down to 75m and 160m, I 
noticed that the noise floor was not particularly clean on some portions of 
the low bands.  I have attached links to two print-screen shots of what I am 
referring to.  In both cases, the antenna is disconnected and the pre-amp is 
engaged.  I can hear the high noise floor as well as see it on the SDR-IQ 
screen, even with the SDR-IQ disconnected.  A few more important points:

1) The radio is not being coupled to a nearby magnetic field.  The shots 
taken are isolated from the radio shack and the rig is battery-powered as is 
the laptop PC.  Same result when the rig is in the shack and connected to a 
desktop PC;

2) The result is the same if I toggle the ANT2 port with no antenna 
attached.;

3) Although I am sampling the IF with the SDR-IQ, the noise floor remains 
with the SDR-IQ completely disconnected from the K3.  The only relevant mod 
is the change to the IF voltage divider as described by N8LP; and

4) With the pre-amp disengaged, the noise floor drops as expected, but the 
noise floor rise as shown in the screen capture is still evident.

In the first link, I have captured the effect on the upper end of 75m. 
Notice that the broadband noise bump is generally just outside the band but 
even so, the noise floor is noticeably higher than when measured down toward 
80m as shown in the second link:

http://tinyurl.com/ltyc2o

Down toward 80m...

http://tinyurl.com/kpxhgb

I can replicate this on 160m as well.  All other bands are clean.  Can 
someone confirm this effect by using their K3 (with N8LP buffer mod) and 
SDR-IQ unit and get back to me by private E-mail?  Thanks!

Paul, W9AC