Help understanding antenna height (lower=better?)

[cycomiko]

To test my HD8200u and Channel Master 7777 preamp, I hooked it up to a tivo roamio at a height of 6' and am receiving multiple markets at 80-100 miles away. Aimed towards 129 degrees, I receive ABC 27.1 from Harrisburg, PA, ABC 7.1 and CBS 9.1 from DC, and some closer stations. At times, it'll pull in the big 3 networks from Baltimore as well. So I was looking forward to sticking the antenna on my roof, and was shocked to see TvFool reports better signal at the 6' level. The DC stations appear to 2edge at 6', and in the Troposphere at 30'. I've read all I can find, and don't understand this. Can someone shed some light on this for me? Thx

report at 6 ft
http://www.tvfool.com/?option=com_wr...d6dbcf49770212

report at 30 ft
http://www.tvfool.com/?option=com_wr...d6db1ee4f2cf2c

[rabbit73]

Welcome, cycomiko

Keep in mind that the tvfool report is only a computer simulation, and is known to be less accurate for 2Edge and Tropo signals than for LOS signals.

If you look at the terrain profiles for ABC and CBS from DC you will see that there aren't any direct signals from them because of terrain interference. The signals arrive at your location with the benefit of bending from atmospheric refraction and diffraction as they pass over the peaks.

The transmitter is at the left; your location is at the right.
WJLA
http://www.tvfool.com/?option=com_wr...ALLTV%26n%3d13
WUSA
http://www.tvfool.com/?option=com_wr...ALLTV%26n%3d13

In addition to the terrain interference, signals 82.9 miles away are blocked by the curvature of the earth. Reliable reception of signals that far away is not to be expected.

Your 7777 preamp is probably being overloaded by your strong local signals, which makes reception of weak signals more difficult. CM cautions against its use in strong signal areas.
http://www.channelmaster.com/TV_Ante..._p/cm-7777.htm

[cycomiko]

Keep in mind that the tvfool report is only a computer simulation, and is known to be less accurate for 2Edge and Tropo signals than for LOS signals.

Okay, that sheds some light on why I can pick up the weakest station on the TvFool list with a cheap "150 mile" $35 antenna at 25ft up with 80% reliability. Helping a friend analyze his area, I pointed out that my 99% reliable channel, although the Google Earth didn't show any signal within a few miles.

If you look at the terrain profiles for ABC and CBS from DC you will see that there aren't any direct signals from them because of terrain interference. The signals arrive at your location with the benefit of bending from atmospheric refraction and diffraction as they pass over the peaks.

The transmitter is at the left; your location is at the right.
WJLA
http://www.tvfool.com/?option=com_wr...ALLTV%26n%3d13
WUSA
http://www.tvfool.com/?option=com_wr...ALLTV%26n%3d13

Thanks for referencing these. I actually pull the two DC hi vhf stations at 99% reliability. Major fog, heavy rain, clear skies. And that was at 6 feet, where it's staying until spring, likely. Seeing the above images, I can't see how elevating will hurt, and should help greatly, as I though it would.

In addition to the terrain interference, signals 82.9 miles away are blocked by the curvature of the earth. Reliable reception of signals that far away is not to be expected.

Your 7777 preamp is probably being overloaded by your strong local signals, which makes reception of weak signals more difficult. CM cautions against its use in strong signal areas.
http://www.channelmaster.com/TV_Ante..._p/cm-7777.htm

[/QUOTE]

When I tried the antenna out in this location, I did so without a preamp, and had a laptop with a Hauppauge 955q USB tuner card. The channel results with a 6ft rg6 cable into that setup yielded the same channels and reliability. I knew I had to maintain a strong signal, as it was going into a Tivo Roamio. So I placed the DVR in the bedroom, to shorten the run, and in the living room have a Tivo Mini over ethernet to stream from DVR. This has seemed to work out for me. I'm not looking at getting the most channels I can, I just want to have signal reliability. Our schedules in this house are crazy, so we rely on a DVR more than livetv.

Thank you, Rabbit73, for giving me some understanding on this subject.

[rabbit73]

Glad that I was able to give you some additional insight.

There are cases where a stronger signal is available at a lower height, just as the report indicates. This is when the signal reflected from the ground or other horizontal surface adds to the direct signal in phase. This is called "ground bounce."

or ground gain

http://www.qsl.net/oz1rh/gndgain/gnd_gain_eme_2002.htm

http://www.hdtvexpert.com/classic-pe...fringe-082009/

[rabbit73]

Here is another case of lower = better:

http://www.digitalhome.ca/forum/186-...ml#post1289804

Originally Posted by balm

majortom,

my only chance at decent reception for my 2nd edge channels with any of my antennas (now 4), is at 12-15 ft high, go higher and i lose all reception,

TVFool actually predicted this, but i didnt beleive it at first

http://www.digitalhome.ca/forum/186-...ml#post1289804

Originally Posted by rabbit73

We have been conditioned to think that "higher is better" for antennas, but in some cases lower is better.
Based on your comments about your location, I found one that seems to fit which gave the following NM values (using the interactive map feature) for different heights:

WVNY NM:
5 ft, +0.6
6 ft, +6.4
7 ft, +6.1
8 ft, +5.8
9 ft, +5.6
10 ft, +5.3
13 ft, +4.8
15 ft, +4.5
20 ft, +4.0
30 ft, +3.2
40 ft, +2.8
50 ft, +2.4
100 ft, +1.7

tvfool report now:
http://www.tvfool.com/?option=com_wr...dfaf56cb924fc9

CJOH analog 8 is now digital on 13.

WVNY terrain profile:
http://www.tvfool.com/?option=com_wr...dALLTV%26n%3d9

He had a lot of factors working against him:

1. Weak WVNY signal in the presence of another local VHF-High channel (analog 8) that was much stronger
2. Co-channel interference from a local channel also on 13
3. Reduced ERP from WVNY because of its directional transmitting antenna
4. Curvature of the earth blocking the WVNY signal 88.4 miles away
5. Terrain interference from hills