Hmm. I see.
I'm planning a 'passive' grinder. That is, I don't need to baby-sit it. I can go about my day to day activities and every now and then a monster will be grinded (sic) and I can pick up the stuff as I travel to/from my main projects. A water delivery system can plonk drops on a pressure plate that can light a redstone torch or similar so I'll know something is waiting for me.
For this reason, if I understand correct, a high platform is not what I'm after - but that does demonstrate the mechanics. It seems that, because I'm working close to the ground, the spawner needs to be close to the ground, and that is just one of those things I need to live with - that the mobs may not actually spawn in my spawner! Though, I must add, that my ground level enclosed spawner will work brilliantly during the day - yes? Evidence suggests that monsters only spawn, or spawn more often, during the night phase.
If I'm running around the surface, to what depth will mobs spawn? If I put my spawner 40 blocks underground will the mobs still spawn as I feed my chickens far above?
I'm not after a high volume uber overpowered OMG machine. Just something that will trickle feed a few mobs a day. Thx.