Ron, I have run a Stevenson screen and a Davis passive radiation screen
next to each other (i.e. about 3 m apart) in an open and unobstructed
site for the past 18 months (and previously at my former site, 1 km
away, for 9 months in 1997-98).

The SS tends to be a little warmer by day, particularly in strong
sunshine and light winds, but rarely by more than 0.8 degC or so even
in midsummer. I suspect a 3 degC difference owes more to one being in
deep shade and the other in open sunshine ...

Stephen