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Meteor head echo altitude distributions have been derived from datacollected with the EISCAT VHF (224MHz) and UHF (930MHz) high-power,large-aperture (HPLA) radars. At the high-altitude end, the distributionscut off abruptly in a manner reminiscent of the trail echo height ceilingeffect observed with classical meteor radars. The target dimensionsare shown to be much smaller than both the VHF and the UHF probingwavelengths, but the cutoff heights for the two systems are stillclearly different, the VHF cutoff being located several km above theUHF one. A single-collision meteor-atmosphere interaction model is usedto demonstrate that meteors in the (1.3–7.2)<i>µ</i>g mass range will ionisesuch that critical electron density at 224MHz is first reached at oraround the VHF cutoff altitude and critical density at 930MHz will bereached at the UHF cutoff altitude. The observed seasonal variation inthe cutoff altitudes is shown to be a function of the seasonal variationof atmospheric density with altitude. Assuming that the electron densityrequired for detection is in the order of the critical density, the abruptaltitude cutoffs can be explained as a consequence of the micrometeoroidjoint size-speed distribution dropping off so fast at the large-mass,high-velocity end that above a certain altitude the number of detectableevents becomes vanishingly small. Conversely, meteors at the low-mass endof the distribution will be gradually retarded such that the ionisationthey generate never reaches critical density. These particles willremain unobservable.  <b>Key words.</b> Radio science (instruments and techniques) –Interplatery physics (interplanetary dust) – General or miscellaneous(new fields

Meteor head echo altitude distributions and the height cutoff effect studied with the EISCAT HPLA UHF and VHF radars