"Peter R." wrote in message
...

And conversely, if ILS hold short lines are needed at most towered
airports, why are they not needed everywhere? What makes certain ILS
arrangements prone to interference and others not?


Towered fields tend to have more operators to accommodate so they have more
ramp space and taxiways, thus a greater opportunity to enter critical areas.



It seems to me that any ILS antenna that can be passed by an aircraft on
a taxiway would be prone to interference.


And if it can't be passed by an aircraft on a taxiway it isn't a problem.
If access to the runway is from just one side and the glideslope transmitter
is on the far side of the runway it can't be passed by an aircraft on a
taxiway. That's the situation you find at most uncontrolled fields.