[TenTec] Orion S-Meter Evaluation

[James C. Garland]

Hi Gang,
I'm an Orion newbie, so pse excuse me if I'm covering ground that has been 
trod previously. I have been troubled by the ARRL test results of the Orion 
S-Meter sensitivity, which found the Orion's main receiver S-meter at 14.2 
MHz (preamp off) to have S9=135 uV. This result is far off the accepted 
standard of S9=50 uV.  My Yaesu MP - Mark V' s S-meter is very close to 
meeting the 50 uV standard, and in comparing S-meter signals in A-B tests 
between it and my new Orion, I hadn't noticed much subjective difference. 
Therefore, I decided to verify the ARRL data by doing a careful measurement 
of the Orion's S-meter sensitivity. Here's the description and results of 
my measurements:

Methodology: I used an HP8640B signal generator, with an HP3406A broadband 
(DC-1 GHz) RF voltmeter to measure the signal generator output voltages. 
The HP8640B has a 50 ohm output impedance, which means that its open 
circuit voltage (no load connected) is twice the voltage as when its output 
is terminated in 50 ohm. (This is because half the generator's output 
voltage is dropped across its own internal 50 ohm resistance, when an 
external 50 ohm load is connected.) I set up the circuit so I could measure 
the rf voltage at the antenna connector of the Orion, and also at the 
output connector of the HP8640B. This was to rule out the possible 
influence from reflections and standing waves on the interconnecting coax 
cable.

Step 1: I measured the input impedance of the Orion main receiver at Ant 
Port 2, and found it to be 50 ohms, within a few ohms. It is important to 
know this, because if the input Z is not matched to the 50 ohm antenna 
impedance, the S-meter reading will not reflect true signal strength. (For 
example, if the input Z were, e.g., 1000 ohms, the S-meter would read high, 
by about a factor of two (6db).)

Step 2: I made detailed measurements of the S-meter sensitivity at 14.05 
MHz, main receiver, Antenna 2, BW=2800 Hz, preamp OFF. Here are the results:

S-Meter Actual Signal
S1      0.3 uV
S3      1.5 uV
S5      5 uV
S7      13 uV
S9      50 uV
S9+10 50 uV + 8 db
S9+20 50 uV + 18 db
S9+30 50 uV + 29 db
S9 +40 50 uV + 42 db

Although the Orion meter goes to S9 +60 db, I was unable to get it to move 
much past S9 +40. Up to that point, however, the Orion S-meter was right on 
target. In fact, it seems about as accurately calibrated as the S-meter on 
any receiver I've ever tested. The ARRL results are clearly wrong.

Step 3: I measured the band-to-band variations in S-meter sensitivity.Here 
I found some modest variations in the signal required to give an S9 
reading. Here are the results:
         Freq Signal to give S9
         1.85   64 uV
         3.80   50 uV
         7.20   61 uV
         10.15   50 uV
         14.20  50 uV
         18.10  70 uV
         21.20  70 uV
         24.90  80 uV
         28.5    80 uV
Evidently the S-meter sensitivity decreases by about 4 db above 20 meters. 
Part of this might be band-to-band calibration error in the HP8640B signal 
generator. For these measurements, I just set the gernator output at 50 uV 
and assumed it stayed there as I varied the generator frequency. In any 
case, this is very good band-to-band consistency for the Orion.

Step 4. I measured the effect on S-meter sensitivity of changing the 
bandwidth (including the roofing filters). There was no change down to 
about 1.00 kHz bandwidth, and about a 1 S unit decrease in sensitivity at 
narrower bandwidths. I'm guessing this is a consequence of slight 
additional attenuation in the narrower (stock) roofing filter.

Step 5. I measured the effect on S-meter sensitivity of changing reception 
modes. I found no difference between CW and SSB. In the AM mode, however, 
the S-meter sensitivity decreased about 10 db. This is a signiicant change.

Conclusion: Other than the decrease in S-meter sensitivity in the AM mode, 
the Orion has a remarkably well-calibrated.S-meter.

While I had the radio on my bench, I did two other tests. First, out of 
curiousitym I measured the 10 meter minimum detectable signal, and found 
that I could copy (barely) a CW signal of 0.2 uV with the preamp off. With 
the preamp on, I could copy a .02 uV (20 nanovolt!) CW signal.

Also, I measured the isolation between the two antenna ports. On 10m, the 
isolation was about 40 db, and it increased to 45 db on 160 m. This is 
acceptable, but not impressive isolation. It means that an S9+40db signal 
on one antenna will still read S9 if the other antenna port is selected. 
Normally, this would not be a problem, since to be troublesome the two 
connected antennas would have to have greater than 40 db differentiation 
when tuned to the same signal. (Who has a beam  with a 40db front-to-back 
ratio??)

All in all, I was very pleased with these results.

73,
Jim Garland W8ZR