[Orion] Orion S-Meter Test Results

["James C. Garland" <4cx250b@muohio.edu>]

Hi Gang,
I have been troubled by the ARRL expanded 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 2, 
and found it to be 50 ohms, within an ohm or two.

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 cable loss in the 15 ft of RG58U between the signal 
generator and thre receiver, and part 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.

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, I measured 
the 10 meter sensitivity, 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 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.

73,
Jim Garland W8ZR

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