[Lowfer] EAR and 137 kHz monitoring with modular DBM rx converter, no preamp

[Michael Sapp]

High All:  I decided to try something different and cobble an LF rx 
converter together from an LO, a Mini-Circuits ZLW-1 DBM with the RF/IF 
ports reversed, some

filters and 50 ohm diplexers.  Isotemp 3.0 MHz LO does not have the oven 
running and output slightly reduced on 12V battery.

https://www.dropbox.com/s/3dgca4zlduc3utd/20180119_213719.jpg?dl=0


EAR at mid to late day, slightly darker background to the right side of 
waterfall after 2133 UTC is when I installed the 137 kHz diplexer.  137 kHz 
Diplexer lowers the 188.83 kHz signal about a half dB but the background 
noise went down a few dB.  That got be looking more closely and I was 
overdriving the mixer somewhat with LO energy, so I had to insert a pad in 
line to get closer to +7 dBM LO spec...

https://www.dropbox.com/s/m7gb3vi29nz0kff/EARQ30B.JPG?dl=0



137 kHz on my NW EWE antenna. Typical evening local QRM present

https://www.dropbox.com/s/mji6853ievxywb6/20180119_213730.jpg?dl=0


With two 50kw  AM BC stations in my back yard maybe not having a preamp is 
not that much of a deterrent,  7 dB or so NF may be good enough and it is 
easier to spot improvements with diplexer and filter placement experiments.

I built the 2 MHz 11th order Butterworth filter awhile back to use with the 
RTL SDR dongles to help with AM BC overload.  When I tried it on the LO port 
on this rx converter setup I noticed K2ORS signal trace came up a few db. 
Perhaps reflecting some of the LF signal back into the mixer from the LO 
port and increasing efficiency?  Then again, OTA tests don't control the LF 
RF signal at the input port, so it could have been a coincidence change in 
short term propagation (I've been fooled B4)

RF connectors are a bit of a pain to keep really clean and low loss, but 
they make it easy for modular changes.

I might just box this up, get good B+ filter on the LO and oven DC power, 
and use it for awhile....

73 Mike wa3tts