To: <tentec@contesting.com>
Date: Sun, 18 Jun 2000 18:23:34 -0700 (PDT)
From: Clark Savage Turner <csturner@falcon.csc.calpoly.edu>
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.
My reply:
Clark,
Although you say that you find your Omni-C to have less sensitivity
than your Corsair, I've found just the opposite to be true.
I have both an Omni-D, version B (same rig as your's except
minus WARC bands and a different xtal filter switch) and a
Corsair II.
I feel that sensitivity in terms of microvolts doesn't mean much.
In ARRL test reports, the MDS spec is emphasized (minimum discernible
signal). According to the League, the Omni-D in their 1980 report
came in with an MDS of -128 Dbm on 80 mtrs and -139 DBm on 20 mtrs.
The Corsair II in the League's 1987 report came in with an MDS of
-127 Dbm on 80 mtrs and -124 DBm on 20 mtrs.
Thus in the laboratory, the Omni-D was slightly BETTER than the
Corsair II in terms of real, useable sensitivity.
BTW, if your web browser accepts LONG email file attachments,
I can send you a copy of these test reports in .pdf format.
I suspect that if you've found otherwise, your Omni-C may have
a problem, or your ears are being faked out by some factor.
Your filter alignment problem also puzzles me.
According to a spec sheet I have, the model 220 filter was
installed in the Omni-C, and perhaps even before that in
the Omni-D. Per Ten-Tec, this filter is bi-directional
with no input/output impedance difference.
Several years ago, I did A/B comparisons between my Ten-Tec Triton IV
(similar receiver to the early Omni's and an Icom 735 and 751.
The useable sensitivity and weak signal work was much better on the
Ten-Tec under identical conditions (same antenna).
73,
Doug/WA1TUT
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