Topband: BDR redux

[Bill Tippett]

Hi Jim,

N2EA wrote:
 >What does TenTec say?

         S9 is 50 uV in Orion.

 >Does the Orion happen to have its relative AGC indicator
calibrated in dBm?

         No, AGC Threshold is set in uV and ranges from 0.37 uV
to 192 uV.  AGC Threshold is the most unusual aspect of Orion
since we've never had anything like it before.  It determines
the point at which you set AGC action to take control.  If you
set it too low (BTW default has been changed from .5uV to 3uV)
the AGC boosts noise above that level making some think it
is a "noisy" radio.  In fact, what you want to do is set it
at a relatively high level so that big QRN or signals don't
blast your ears, yet leave weaker signals alone.  Sort of
like turning AGC off for weaker signals but on for stronger
ones in a classical radio.  The new 3uV default is OK for 10m,
which has relatively low atmospheric noise, but is still MUCH
too high for 160 meters.  About the lowest I ever set mine
on 160 is 20-30 uV, and often much higher.  For example, I
just checked it, and I had it set to 192 uV last night (a
relatively noisy night) while listening to A45XR who went
from 339 when I first heard him answer W1MK's CQ around 0030,
to a genuine S8 on the meter around his sunrise at 0245. BTW,
I copied his call one cycle before W1MK figured it out, even
though W1-3 definitely had the propagation advantage early on.


         Thanks to W8ZR's HP 8640B, below are S-meter readings for
Orion (preamp off).  I converted his measurements below S9
to dB relative to S9.  The dBm reference for S9 (50 uV into 50
ohms) is -73 dBm.  Remember that smaller negative dBm numbers
are higher and larger negative numbers are lower, so S1 is
-113.4 dBm and S9+40 is -31 dBm.

S-Meter Actual Signal    W8ZR's meas.  (in dBm)
          S1              S9 - 44.4 dB  (-117.4 dBm)
          S3              S9 - 30.5 dB  (-103 dBm)
          S5              S9 - 20 dB    (-93 dBm)
          S7              S9 - 11.7 dB  (-85 dBm)
          S9              50 uV         (-73 dBm)
          S9+10           S9 + 8 dB     (-65 dBm)
          S9+20           S9 + 18 dB    (-55 dBm)
          S9+30           S9 + 29 dB    (-44 dBm)
          S9+40           S9 + 42 dB    (-31 dBm)

Source:  http://dayton.akorn.net/pipermail/orion/2004-January/000191.html

dBu = 20 log (uV) (referenced to 1 uV),
dBm = dBu - 107 (dB above 1 uV),
so S9 = 20 log (50) = 34 dBu, and in dBm = (34 - 107) = -73 dBm

Assuming the typical 5 dB per S-unit, Orion's meter looks more linear
than most especially below S9.  It also seems stingier below S9 than my
old MP.  It seems stingier everywhere on 10 meters but that may be
because the MP adds an additional preamp for 10 & 12 meters only.

         Back to Orion's AGC Threshold settings, a 20 uv Threshold is about
S7+ and 192 uV is about S9+12.  So I was listening to A45XR in the
noise initially even though I had AGC Threshold set to S9+12.  For
weak signals near the noise floor, it also sometimes helps to reduce
RF Gain to find the S/N sweet spot where the signal seems to separate
from the noise.  This is only needed in those cases where the signal
is just barely above the noise level.  This is why Orion's AGC takes
some seat time to learn to use properly.  Once learned, it is fairly
easy to adjust and the AGC action is the best I've seen at keeping
a wide range of signals at comfortable levels.  In the CQ WW on 10
meters, it was a pleasure to set and forget it, almost never needing
to ride AF and RF Gain controls like I did with previous radios.

                                                 73,  Bill  W4ZV

P.S.  I forgot to mention I use Preamp ON on most bands including 160
so that should be taken into account for the my Threshold settings (i.e.
they are actually lower by whatever the Preamp's gain is).