Originally Posted by Larry Kenney
Tilt is a new subject that I'm not familiar with. What causes the signal to tilt... is it something in the transmitter or is the atmosphere doing it? How does tilt affect the picture?
I suspect it has to do with differing attenuation over the terrain the signal has to refract over. It shows up in all sorts of ways; tilts, dips, or a ragged looking trace. As far as I can tell the SNR is reduced to whatever the weakest part of the signal is but it doesn't cause multipath.
It occurred to me that the physical placement of the antennas on Sutro Tower seem to affect whether or not you see a station's signal. 7 and 44 are on the southeast tower. 9, 32 and 60 are on the northeast tower. The stations that you don't receive... 2, 4, 5 and 20 are all on the west tower.
It's true that 2, 4 and 5 are poor here, although 5 is somewhat better than 2 and 4. 60 is the worst, even weaker than 34. But I cannot receive 34 because of co-channel analog KACA.
20 is actually about equal to 9 but very hard to receive because of adjacent channel KUVS which is often 50 dB stronger even in an antenna null. See the attached image of KOFY and KQED and you'll see the problem.
I wonder why you can't get 38 (RF 39). It's running 1 MW ERP and is on the same tower at 9, 32 and 60, which are all running less power. There's nothing transmitting on 39 over your way that I know of.
I receive KCNS on occasion but it's pretty weak. One would think that over a 110 mile path that there would be no detectable difference between any station from the different locations on Sutro Tower but there is. I do have KMMD-LD on Mt. Toro to contend with on RF 39 but it's usually very weak when pointed to SF.
You can see on the KOFY/KQED image that the signals are not flat but they're not that bad.
Another problem is not being in the main lobe of the antennas. The ERP often changes with frequency which is another cause of non flat signals. I see this on KMMW, KCBA and KSMS. The grossest example of this is KCSO which I've posted another image of. There's a 20 dB swing in it's ERP across the its signal. That ugly signal still produced an SNR of 25 dB when I took the image using an FM antenna. I think that image is proof that you cannot trust the published FCC antenna patterns at least if you're not in the main lobe of the antenna.