It is likely the BUK TELAR which shot down was positioned in a field such that it could only target high flying aircraft.
It was positioned close to a series of trees. It could be the centimeter-wave radar of the BUK TELAR just can’t work through trees. (needs confirmation)
Each SA-11 transporter erector launcher and radar (TELAR) was equipped with a 9S35 Fire Dome X-band multi-mode engagement radar under a radome on the front of the rotating launch platform, which provided tracking and CW illumination for the missile seekers. The radar, which has search, track and illuminator functions, can scan through a 120-degree arc, independent of the movement of the launch platform. (GlobalSecurity)
The intended target likely was an Ukraine Air Force transport aircraft flying in the same airway, same direction as MH17.
Photo by Jeroen Akkermans
The image below shows the line of sight of the BUK TELAR from the launch location determined by JIT towards the location where the missile exploded. Distance from the radar towards the trees is about 30 to 40 meters.
The image below shows the radar beam of the BUK TELAR. It is possible due to the trees, the intended target being the Antonov, was shown on radar only for a short time. Likely the BUK crew switched on radar at the latest moment to prevent detection by SU-25’s.