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Lessons Learned: Downconverters & Preamps



I tested a couple of combinations of preamps and downconverters, and the results were not surprising (fortunately).  In summary, a low-gain preamp (DEM) feeding a high-gain downconverter (3733) seems to offer the "best" combination for my station.  It allows me to keep the rig's internal preamp off (low front end gain) and have no S-meter reading, but I can still hear the d/c is turned on.  Here's the hardware:

Rig:  Kenwood TS-2000 fed from a Hamtronics receive converter to the receive-only HF input.  Default RF gain is at max, but rig's preamp is off.  AGC is on, set about middle.  If bandwidth set at 1800 Hz (200-2000).

Preamps:  
 - DEM 13ULNA, 0.6 NF @ 18 dB gain (spec)
 - DB6NT , 0.7 NF @ 40 dB gain (spec & tested)
 - Conifer 1033, 1.15 NF @ 12 dB gain (tested)

Downconverters:
 - Drake 2880, 5.5 NF @ 22.5 dB gain (tested)
 - TSI 3733, 1.15 NF @ 32 dB gain (tested)
 - Norsat 2500, 4 NF @ 25 dB gain (guess)

DEM + Drake, good S/N, but weak audio (needs rig preamp)
DEM + 3733, best S/N, strong audio w/o rig preamp
DEM + Norsat, better S/N, but weak audio (needs rig preamp)

DB6NT + Drake, good S/N, with S2 background noise 
DB6NT + 3733, bad S/N, too much audio, S5-6 background w/o rig preamp
DB6NT + 2500, best S/N, just a little background noise 

Conifer + Drake, good S/N, audio a little weak
Conifer + 3733, not tested
Conifer + 2500, better S/N, just right with rig preamp

The tests were done with all the hardware sitting on the floor of the shack and a low-level signal source in a coffee can about 5 meters away :-)

It is not surprising the high gain preamp (DB6NT) feeding the high-gain downconverter (3733) was a poor combination.  However, the DB6NT looks like a good choice to perk up your simple-mods Drake or Norsat 2500 and get the most from the system.  Unfortunately, the basic unit is not WX-proof.  

Even the weakest of these preamps and d/c combinations will allow you to work lots of weak signals.  I started out with a Drake and Conifer and the difference between that pair and the ones I am using now is only incremental--it is not night and day.
73,
Jerry, K5OE

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