the lower the better?
 

I live at Pleasanton, CA, zip code 94588. I found from TVFOOL that if I mount the antenna lower, e.g., 2ft above ground, the signal I suppose to receive has much higher NM (4db) than I mount 25 ft high. This is puzzling. Anybody to suggest why?

Notice that it's the "edge-path" signals that are effected this way... not uncommon that signals bending over terrain will experience this phenomena. Something like a mirror effect.

The underlying program (Longley-Rice) that calculates the signal strength often reports errors when very close to the ground or very close to a hill. TVfool ignores those errors.

In order to find the optimum height for your antenna use trial and error and keep these guidelines in the back of your head: http://www.hdtvprimer.com/ANTENNAS/siting.html

Thank you both, sir.

The Tvfool shows some 2edge channels with reasonably high NM, e.g., KTSF 14-18db. I previously used a RCA751 antenna which could not bring in this channel. So I replaced it with 91xg, the highly recommended antenna, but still could not receive the signal from this channel. By the way, I had a RCA preamplifier installed with the antenna. So why is this? I suppose to bring in this channel even with 0 db gain antenna. But no matter how much I tried, I had no return for this channel.

Did you actually try the 91 XG without the amplifier? By without I mean totally disconnected, not simply unplugged.

I managed to receive the KTSF at 30% level through trying. I have a cable run of about 60ft, so I probably need the preamplifier. I tried once to mount the antenna on the roof, unfortunately, cannot receive strong signal, mostly due to the redwood tree just blocking the signal path. mounting it on the ground, the signal is much better.

Moderator note
 

Thread has been moved to the Help With Reception area.

Tv Antennas and Reception
 

Please post the tvfool radar report like the other question askers do. http://forum.tvfool.com/showthread.php?t=4 Can you move the antenna to shoot around the tree?? Can you move the antenna to the other side of the tree??

I suspect preamplifier overload. If a pre-amp is truly needed I would only consider the Winegard HDP-269. Make sure the system is well grounded as the 269 is sensitive to nearby lightning strikes.

The 91-XG can pull in stations down to around 0db with no amplification in most instances. This is even possible with 100 feet of coax & no splits. This may require a bit of height tweaking (in 6 inch increments) if the antenna is pointed directly into any obstacles.

Every antenna I have tried at my location requires a rotor even though the desired stations are in the same direction. Any trees, houses or hills in front of an antenna can create a tremendous amount of signal reflections (mutipath). An off axis aim may actually work better under these conditions with affected channels.

I have several channels with 100% strength before I used the preamplifier. But even I used the preamplifier later, they were still working perfectly. So I do not think it was due to overload. Several channels are in the same direction as KTSF with higher NM, and are around 50-90% signal strength. I tried to unplug the preamplifier and just used bare cable, no help for KTSF, but some stations became unstable. Maybe the trees that blocked the LOS is the most important factor. I will try to move my antenna to the other locations and report back later.

Unplugging the power supply does not bypass the amplifier.

I unplugged both the power and preamp, and connected directly from antenna to TV.