On Mon, 23 Oct 2006 13:42:45 +0000 (UTC),
wrote:

I recall reading about false lobes on the glideslope, and have seen it
firsthand doing practice approaches in VMC. I thought it was due to antenna
sidelobes, and thus the localizer might be subject to a similar phenomenon. I have
yet to see anyone reference this, so is there a technical reason why they don't exist?
One of the localizer transmitter frequencies have a different antenna pattern to
interleave the sidelobes perhaps?

I've observed these routinely while being vectored to the IAF. Unless
you're approximately on the approach course, the CDI may be pointing
anywhere. While being vectored around the airport toward the IAF, you
can expect the CDI to be flipping back and forth like a windshield
wiper. I don't notice this while flying the full ILS (ADF to LOM,
etc.) I suppose it's because in that case I'm fairly close to the
localizer. When ATC is vectoring, they usually keep me far from the
airport. That far out, anything can happen. I've always assumed it was
because the localizer is formed by an array of antennas and a stack of
antennas is always going to have multiple nulls, especially at extreme
angles. Another factor might be the mountain ridges surrounding the
airport.

The point is, you have to be at the specified point in space before
you can trust either localizer or GS.

RK Henry