Can there be false lobes on a *localizer*?

Tauno Voipio wrote:

The localizer signal is a combination of three signals

Whoah. Hang on. I've learned (from appropriate FAA publications) that
the localizer was made up of two signals, one modulated at 150 Hz and
the other at 90 Hz, each sent in lobes on either side of the centerline
and the localizer reciever just compares the relative strength of each
signal. Is that right, wrong, or overly simplified?

thanks,
Peter

On 23 Oct 2006 22:11:08 -0700, wrote:


Tauno Voipio wrote:

The localizer signal is a combination of three signals

Whoah. Hang on. I've learned (from appropriate FAA publications) that
the localizer was made up of two signals, one modulated at 150 Hz and
the other at 90 Hz, each sent in lobes on either side of the centerline
and the localizer reciever just compares the relative strength of each
signal. Is that right, wrong, or overly simplified?

thanks,
Peter


You are exactly right. If you want to be picky, there is also a 1020
Hertz AM modulated Morse code identifier, but it does no add to the
operation of the needle.

Yes, it happens. We have that problem here in Billings. It is for that
reason that our RY28R ILS has no procedure turn. It's also why I have
two marks on my radar scope, one north and one south. The aircraft must
be at or inside these marks beore I can turn an aircraft on the
localizer and issue the clearance. If I do it too soon the aircraft may
grab the wrong localizer.



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?

-Cory

wrote:
Tauno Voipio wrote:

The localizer signal is a combination of three signals


Whoah. Hang on. I've learned (from appropriate FAA publications) that
the localizer was made up of two signals, one modulated at 150 Hz and
the other at 90 Hz, each sent in lobes on either side of the centerline
and the localizer reciever just compares the relative strength of each
signal. Is that right, wrong, or overly simplified?

That's right - after the three components are combined in
the receiver antenna.

All the three components carry both 90 Hz and 150 Hz modulation,
but they are all different in such a way that being off the
centerline causes an increase of the modulation depth (sound
stregth) one modulation frequency and a decrease of the other.
This is where the phase differences are needed.

--

Tauno Voipio
tauno voipio (at) iki fi

On Tue, 24 Oct 2006 17:35:41 GMT, Tauno Voipio
wrote:

wrote:
Tauno Voipio wrote:

The localizer signal is a combination of three signals


Whoah. Hang on. I've learned (from appropriate FAA publications) that
the localizer was made up of two signals, one modulated at 150 Hz and
the other at 90 Hz, each sent in lobes on either side of the centerline
and the localizer reciever just compares the relative strength of each
signal. Is that right, wrong, or overly simplified?

That's right - after the three components are combined in
the receiver antenna.

All the three components carry both 90 Hz and 150 Hz modulation,
but they are all different in such a way that being off the
centerline causes an increase of the modulation depth (sound
stregth) one modulation frequency and a decrease of the other.
This is where the phase differences are needed.

A very interesting discussion and nice to have a technical
explanation. It makes more sense as a simple signal strength change
would be likely to be unreliable.

Thanks for the explanation of the glideslope antenna. I always thought
it was the ground reflection that caused the unwanted lobe and had not
appreciated the ground was deliberately used to simulate the
underground antenna as well as introducing the extra unwanted lobe(s).