From basic tests on the bench with a switched attenuator I think the S meter is about 6dB per S point.
I'm comparing a variety of aerials, using the Pluto rubber duck as a base (I know its not much good) then a PCB LPY, cantenna, double Bi-quad, 4x4 patch arrays (ex WiMax units) and finally 3 different dishes all fed with the nominally 6dBi LPY PCB aerial. (BSkyB, 24" FD 0.3 and a 26 " diameter FD 0.412
Any advice on getting the most meaningful readings from my receiver?
Pluto RD = S3
LPY= S4 (it has 6dBi 'average gain' apparently)
Cantenna S5 (just)
Dual Biquad (homemade from PCB & wire) S5.5 (an 'easy S5').
4x4 Patch S6 - just
BSB Offset dish fed with LPY, both used horizontally S6, ever slightly better than the 4x4 patch
2' diameter dishes with different F/Ds, hand aimed and optimised - an easy S6
All very unscientific but pretty much what I expected.
Had a quick play on 5.760 too while the gear was out in the garden.
Pluto RD - S6-7
BSB Dish and LPY S9+2dB
Gibeon 24HV flat plate aerial from yagi.pl S9+10
As you have found, the Langstone S meter is calibrated to about 6dB per S Point. It indicates the highest signal level inside the receiver bandwidth.
You are correct to turn off the Auto RX gain if you want to make measurements. That reduces the dynamic range of the receiver but stops the Pluto from adjusting its gain automatically.
Maybe another way of making the comparative measurements would be to adjust the TX attenuator to give the same reading on the S meter. The change in attenuator setting would then give a good indication of the relative antenna gain.
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73 John G7JTT
Of course multiplying is only good for CW or FM modes.