Mounting OTA antenna higher gets less channels?

[schwabrus]

Hello,

I ran the TV Signal Analysis results twice. On the first run, I put the height of the antenna at 10 ft. (for grins and giggles). It gave these results back:

http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d9fbe8762041bf7

I ran a second time at antenna height of 35 feet and it gave these results which are worse then at 10 feet (by looking at the "red" background" color. In addition I looked at the NM(DB) and those support what I'm saying (check out station WBFF):

http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d9fbe8e5810c62e

So this created some questions for me and not sure which is right, if anyone can clue me in, it would be appreciated:
- The results on 10 feet and 35 feet are not accurate and I should ignore the descrepency
- The results show that 35 feet are better and I need to learn how to read a chart
- There is a disadvantage to mount an antenna higher (which goes against my logic)
- it's automatically configuring cable length and taking that into consideration

Any ideas? I have my antenna currently mounted temporarily at 15 feet and was going to mount it on my roof at 35 feet but if I get a worse signal at that height then it makes no sense to do it.

-Chuck

[Tigerbangs]

I can only theorize that your location is such that you are shaded by different hills at 35' as opposed to 10'. It does seem illogical, but when you are dealing with 2-edge signals, you may be suffering from some cancellation effects that could cause the lower readings.

[ADTech]

Keep in mind that a mathematical model is only theoretical. Go with your logic and put the antenna up higher (if needed) unless your practical exercise demonstrates that the usually correct maxim of "higher is better" is wrong (as it sometimes will be in the real world).

[schwabrus]

Hmmm, I have a rather large hill (not a mountain, but a fair incline) 6 miles to my south where I want to pick up Baltimore, MD stations. That would be the only real obstruction. But I looked at the numbers to my east in Philadelphia and they are also showing that I have better signal strength at 10' then 35' as well. In this direction, there are no real hills or anything that would obstruct the signal.

So it seems like I'm not being a complete idiot here and that either some odd hill is throwing this off or the mathematical model is simply theory and creating odd numbers that don't support logic. If I find that higher is not better in my circumstance, I'll let you all know if interested.

[Dave Loudin]

Mystery solved. On either report, click on a Philadelphia station (I checked WPVI) in the table to get a path profile. Do the same for a Baltimore station (I checked WBFF). In both cases, you are heavily shadowed by terrain, and going to 35 feet does not get you out of the shadows. The propagation model is estimating that you will catch more energy closer to the ground, and only by 2 dB or so.

Note that WHTM comes in much better at 35 feet.

[mtownsend]

Just for completeness, there is a reason for some channels to be stronger closer to the ground.

It is a function of propagation physics. The interface between the ground and the air is an interface between two materials (earth and air) that have different dielectric constants. When radio waves encounter this interface, some of that RF energy will propagate along the surface of the interface before dissipating. Most of that energy stays very close to the surface (usually within about 1 wavelength). When the conditions are right (very low direct signal strength and shallow signal approach angle), the cumulative ground wave energy might be more than the energy in the direct signal itself. When that happens, you might see a little bit more energy close to the ground than you would higher up.

In the real world, this "ground gain" is usually not that helpful. In most cases, as you get closer to the ground, you get more random objects (e.g., other buildings, trees, walls, cars, roads, etc.) that affect your signal. The increased scattering effect will probably negate any gains you might see in signal strength. The ground wave might benefit you if all you have are open fields in front of the antenna, but for most people this is not the case.

If it's just a matter of a few dB, then you are probably better off going with a higher antenna that is above most of the surface clutter, even if it means losing a small amount of signal strength.

[kb2fzq]

Just for completeness, there is a reason for some channels to be stronger closer to the ground.

It is a function of propagation physics. The interface between the ground and the air is an interface between two materials (earth and air) that have different dielectric constants. When radio waves encounter this interface, some of that RF energy will propagate along the surface of the interface before dissipating. Most of that energy stays very close to the surface (usually within about 1 wavelength). When the conditions are right (very low direct signal strength and shallow signal approach angle), the cumulative ground wave energy might be more than the energy in the direct signal itself. When that happens, you might see a little bit more energy close to the ground than you would higher up.

In the real world, this "ground gain" is usually not that helpful. In most cases, as you get closer to the ground, you get more random objects (e.g., other buildings, trees, walls, cars, roads, etc.) that affect your signal. The increased scattering effect will probably negate any gains you might see in signal strength. The ground wave might benefit you if all you have are open fields in front of the antenna, but for most people this is not the case.

If it's just a matter of a few dB, then you are probably better off going with a higher antenna that is above most of the surface clutter, even if it means losing a small amount of signal strength.


Here is a person that knows and understands his RF theory...very well said, mtownsend!!!
I can only add a ground phenominon that I experience every day...my RF 6 only antenna is about 10 feet off the ground. When my girlfriend parks her car under the antenna array, channel 6 will show a 77% SS, when she leaves in her car, channel 6 will drop to 72%, until she returns, then I'm back at 77%. My suggestion would be for the OP to experiment with a temporary mounted antenna at both heights, recording the signal strengths of all stations, at different heights. This will provide real world SS to calculated SS conditions.
Just my 2 cents, but I've done just that myself in the past, with interesting results.....

[Dave Loudin]

There's not a lot of difference between the two predictions, so getting a conclusive answer could be tricky. The air density will also affect the net result. The truly interesting experiment would be to see if WHTM pops in at 35 feet.

The OP should experiment with antenna height within TVFool to see how far up he needs to go (easy to do in the maps option).

[schwabrus]

I'll make some notes about signal strenth at the current "temporary" spot of about 15' and then at the permenant spot at 30'ish'

One really stupid question. I went to get grounding wire today which was right next to the guy wire. My idiotic self grabbed galvanized steel guy wire without realizing it until after I got back to work. Is it possible to use the galvanized steel guy wire as a ground for my mast?

It's 4 strand, 20 gauge wire. Thought I could twist it to double-up the thickness but not sure if it's idiotic to bother with it.

[kb2fzq]

I'll make some notes about signal strenth at the current "temporary" spot of about 15' and then at the permenant spot at 30'ish'

One really stupid question. I went to get grounding wire today which was right next to the guy wire. My idiotic self grabbed galvanized steel guy wire without realizing it until after I got back to work. Is it possible to use the galvanized steel guy wire as a ground for my mast?

It's 4 strand, 20 gauge wire. Thought I could twist it to double-up the thickness but not sure if it's idiotic to bother with it.

I would return the guy wire and get solid copper ground wire, copper conducting an electrical charge much better then steel to a ground rod...