Antenna height question

[MetHerb]

My property slopes up hill - my house is at the lowest part and my land slopes up hill about 30' from there. I currently have an HD9032 on a 25' mast on top of this hill. Based on advice to another post of mine, I'm going to upgrade to a 91XG and a YA1713 and combine them with a CM7777 preamp.

I was going to upgrade the mast to a 40' pole, but I was playing around with the mapping feature and I'm wondering if I really should go shorter. Here's my current radar:

http://www.tvfool.com/?option=com_wr...e07c16798dbdde

The most distant station that I can get a signal from is WSBK which has a NM of -14.9 but it's not watchable. Under the right conditions, I can sometimes get WGBX which has an NM of -9.8. The "weakest" station that I can regularly get is WGBH which has an NM of -5.5. That always comes in.

What I noticed on the maps is if I lowered the antenna to only 10', the signals seem stronger:

http://www.tvfool.com/?option=com_wr...e07c16a39d74fb

WSBK is now -11.9, WGBX is now -6.3 and WGBH is now -1.6.

I'm wondering then if that is the way to go - go with a shorter antenna? I don't understand the difference, but I'm sure someone with more knowledge can help me. FYI, I compensated for the fact that my land slopes uphill by moving the marker on the map to where the mast would actually be and changed the address to reflect that as well for the radar.

Thanks - Dave

[Tower Guy]

Originally Posted by MetHerb

I'm wondering then if that is the way to go - go with a shorter antenna?

The way to know for sure is to try it. With downsloping land in front of the antenna, there will be optimum heights that are 6 db more or less signal than the TVfool prediction. The difficulty is that the height will be different for high vs. low channels and also different for varying azimuths.

http://www.hdtvprimer.com/ANTENNAS/siting.html

[ADTech]

Playing with the heights in the interactive tools may be entertaining, but it rarely corresponds to what I've observed when I actually put an antenna up in the air unless here is a clearly defined knife-edge diffractor. Since the modeling software in use here averages terrain in 100 meter square chunks, any expectation of actual accuracy based on a couple of feet difference in the entered elevation of the antenna is unfounded. The averaging of the terrain becomes the limiting factor for accuracy.

I usually recommend putting the antenna up as high as possible, then adjusting if needed. When folks ask me how high an antenna should be installed, I tell them "As high as it takes, preferably above any trees."

[mtownsend]

Originally Posted by MetHerb

What I noticed on the maps is if I lowered the antenna to only 10', the signals seem stronger:

WSBK is now -11.9, WGBX is now -6.3 and WGBH is now -1.6.

When you get closer to the ground and the signals are coming in at a very shallow angle, you DO get a slight increase in signal strength due to ground conductivity. A small amount of the signal hitting the Earth-air interface will travel along that surface, and if your antenna is close enough to the ground, there is a bit more signal to be picked up.

HOWEVER, this only works if the ground along that signal path is wide open. If there are any buildings, trees, or other obstacles in the way, then the ground-wave gets disrupted and you won't see that much of a signal gain.

This ground effect is generally pretty weak, so you might notice a big change in signals that are weak to begin with (down in the red or gray zones on your list), but the stronger signals will not see that much gain. A few of your channels, like WWLP, lose a lot of signal when you go with a lower antenna (dropping from 33.1 to 22.3 dB NM), so the trade-off probably isn't worth it. In most neighborhoods, there are too many surrounding structures to make this "ground gain" work effectively, so it's not something I would count on, although in certain situations (open farm lands or fields surrounding the home) it has been shown to work.