I think it's generally true that if you connect the antenna and the
background noise increases, you have adaquate sensitivity. I've had many
rigs and even without an antenna, they roar like a freight train. I've found
the Omni C without an antenna is almost silent and adding the antenna brings
up the noise level. If the signal is below the noise level then chances are
you won't be able to copy it anyway. It's all a ratio thing. Using the audio
filter tends to peak the response to a single tone. I usually leave my audio
selectivity set on position 1 and the 1.8khz filter seems just right for
enhancing cw just the right amount.
Steve
N4LQ
----- Original Message -----
From: "Clark Savage Turner" <csturner@falcon.csc.calpoly.edu>
To: <tentec@contesting.com>
Sent: Sunday, June 18, 2000 9:23 PM
Subject: [TenTec] OMNI-C sensitivity (and quietness!)
>
> 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?
>
> Clark
> WA3JPG
>
> 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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