Can there be false lobes on a *localizer*?

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

: 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

I've definately seen it with the GS, just never with localizer.

The reason it came up is I was doing some safety-pilot with a friend. They'd
never heard of "false lobes." Of course, we're non-radar, VFR, not talking to anyone.
While flying a VOR radial towards intercepting the localizer, I said it's best to get
another reliable means (e.g. DME or another VOR radial) to get in the ballpark of the
IAF on the LOC. After getting close (but not TOO close), flipping to the LOC and
waiting for it to come in was the way to get accurately on the LOC.

In other words, two "extremes:
1. If one waits for the cross VOR radial to come it, they can be so inaccurate that
one could blow through the LOC before knowing it.
2. If one drives down a radial (possibly a number of miles) with a LOC tuned hoping to
cross it, one of the sidelobes might make you think you're establish... but you're
actually not there yet.

Reasonable summary?

-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
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