[TenTec] OMNI-C sensitivity (and quietness!)

Clark Savage Turner csturner@falcon.csc.calpoly.edu
Sun, 18 Jun 2000 18:23:34 -0700 (PDT)

Well, I finally have the job I always wanted.  I have the summer to be
very flexible.  I can finally work on the radios in the shack.

I have some questions on the OMNI-C.  I see the sensitivity figure as 
2.0 uV on 160 ("tailored") to 0.3 uV on 10 meters for 10 db S+N/N.  OK,
good so far.  Don't really "need" the sensitivity on 160 where
atmospherics bring up the floor.  

Now, I have the Corsair II with sensitivity specs at 0.25 uV and the
OMNI-VI with 0.15 uV.  

For arguments sake, suppose the OMNI-C had specs at 40 meters of 1 uV.
The Corsair had 0.25 uV.  Is this four times better, or 6 db, overall
(assuming the filters same, etc.) ??

I ask because I got on OMNI-C and it is so "quiet" that I suspect it is
mostly deaf.  Of course, the S meter does not swing the same as on my
other receivers (and I admit, the S meters do somehow affect my thinking,
silly, isn't it?)

I would like to figure out if the OMNI-C has low sensitivity, and I have
no calibrated signal source to give me precise sensitivity readings.  I
CAN hear that the OMNI-C struggles to hear some weak stations that my
Corsair handles with ease.  I seem to hear them on the OMNI-C, but there
is not enough RF or AF gain to bring it out of the speaker.

I have bypassed the attenuator circuit (easy to do) and it is fine, no
change in signal strength.  I have not replaced the RF amp transistor as
it takes a little effort and I want to see if I am on the right track.

The OMNI-C receiver is truly "quiet" even when I throw in the 18 db RF
preamp (the cheap one from T-kit) in the line between the main IF filter
and the filter switchboard.  Sure makes the CW signals jump out with the
500 Hz filter engaged.  But the preamp does add noise for sure, especially
on a quiet band otherwise.  I don't think it is a solution.

Any suggestions as to checking sensitivity without fancy lab gear?  Is the
reduced sensitivity compared to other receivers (and on a mediocre
antenna) what others experience?

I do note that the OMNI-C roughly compares to my stock Argonaut 515,
though the 515 is a little more sensitive (it seems!) on 75 and 40, but
that might be expected?


P.S. Oh, by the way, checking out the main filter on the OMNI-C, it has
220 ohms Zin and 50 ohms Zout (or vice versa), it was before the standard
model 220 filter was used.  The filter board is easily reversed and this
will result in loss, as I have found.  Just a heads up for OMNI-C owners.

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