How far in from the trees does the signal diffract back in?

[hillpc]

...or maybe reflect back up off the ground?

My large Radio Shack VU-210XR combo antenna at 25 feet up is pointing directly at my desired station (FOX, Philadelphia channel 29, actual channel 42, and a number of others; see attached tvfool signal analysis), but the foliage of a 70 foot tall oak tree is literally inches away from the tip of the antenna. It's up against the tree leaves and pointing into the tree. Channel Master 7777 preamp to deal with the 80 feet of RG-6 and splitter, though right now I've removed the splitter to try to help. No trace of a signal, either on my hdtv TV or a cheap digital/analog converter box, though I know I was able to receive this channel in the winter. I do get others from the same direction at this time, those ranked (by Noise Margin) numbers 2, 3, 4,and 5 in the tvfool analysis. In the winter I get a good bit more.

With much effort (it's a huge antenna, with rotator ) I could move the antenna to the chimney area near the center of the roof, which would get it out of the leaves, but then there's still a whole forest of 70 foot tall trees about 60 feet away, in the desired direction. (There's a treeline of 70 foot tall trees in all directions, some closer than others.)

I'm wondering if the channel 42 signal coming over the trees gets diffracted enough to fill in the pattern in the 60 feet of distance from the treetops to my chimney, enabling reception. The cartoons of "UHF diffraction" and then also "Average power diagram" under the heading "Trees and UHF", at http://www.hdtvprimer.com/ANTENNAS/siting.html does not look promising, if it's to scale, though I don't know where the strong center beam comes from in the tree cartoon. Channel 42 should do a little worse than channel 35 used in that simulation, if I understand correrctly. Doesn't it also help that the transmitting tower is (presumably) much higher in elevation than my chimney? The cartoons show the waves coming in horizontally.

I could see myself wasting a bunch of effort if the trees 60 feet away kill the signal anyway. (Also, the hdtvprimer website poo-poos the use of cheap unspecialized field strength meters with today's digital signals.) If the trees'll kill it anyway I'd have to wait for the leaves to fall (we'll be halfway through footbal season by then) or put up a monster tower to get above them.

[No static at all]

Your channel 42 issue may be due to the MPT station in Annapolis Maryland on the same frequency. Unfortunately the TV Fool radar plot isn't showing.

[MisterMe]

...

I could see myself wasting a bunch of effort if the trees 60 feet away kill the signal anyway. ...If your trees are 70 feet high, then you should upscale the geometric dimensions of the power diagram on the hdtvprimer.com website. You see that there is only negligible power being transmitted into the shadow of the tree for several tree diameters behind it [except through the center of the shadow]. Siting your antenna 60 feet behind a 70 foot tree is a waste of time. You need to get out of the shadow of the tree. If you can find a line between your favorite transmission towers and your antenna that passes to the side of the tree, then your should be OK. However, a 70 feet tree may extend from one side of your lot to the other. If you have a big lot, then this may not be an issue. If not, then your best bet may be a tower mast that rises above your trees.

[No static at all]

After seeing the chart, I'm now thinking the 7777 preamp may be overloading. Can you run the cable directly to one TV with both parts of the preamp removed? The 7777 preamp is designed for fringe areas & does not like strong signals.

Also, have you tried tweaking the aim? Sometimes pointing directly into trees or anything in the way is not always the best. I need a rotator for a few stations even though they are all in the same direction because of the trees & houses right in front of the antenna.

[John Candle]

If there are very few trees or smaller trees or no close by trees on the other side of the big tree that is blocking the signal , then put the antenna on the other side of the big tree. It might help to put a antenna lower to the ground and shoot under the big tree. Or move the antenna to the side of the big tree. To go higher , here is a fold over tower , can do all antenna work standing on the ground , http://www.rohnnet.com/towers-foldover. Here is tall tripod tower that is mounted on the roof , http://www.ronard.com/Tripods%200703/4712.html

[John Candle]

Tree thinning can also so help. Tree thinning is not done much now days. This makes the tree much less dense , sunlight can then go through the tree and help the grass grow , and as a added benefit the tv transmissions will go through the tree.

[hillpc]

This is getting interesting.

Thanks for all the good input. Small adjustement to rotator helped me get 42 this afternoon (several hours ago). Then later I did some experiments to see if it's the preamp overloading something. I don't think it's preamp overloading (at least not on most of the channels) - here are the results of preamp power supply completely out and in, with the preamp power supply and a converter box and a portable analog TV at the bottom of the 20 foot downlead at ground level. I left the preamp at the antenna up there, as I didn't want to climb up there. I was using the auto channel scan as a poor man's field strength meter:

1.) Mast downlead directly into converter box: real channels 6 and 12 locked in. Repeated, with same results.

2.) Add preamp power supply plus 10 feet of RG-6 between downlead and converter box: real channels 6, 17 (lost 12!), 22, 26, 27, 29, 31, 34, and 35 locked in. Note that 42 still didn’t come in, though I had it this afternoon with this exact setup. Gone now.

3.) Remove preamp power supply to repeat 1.): same results as 1.) above.

4.) Add preamp power supply to repeat 2.) above: same results as 2.) above, except 2 more weak channels added this time, 25 and 32. Still no 42, which I had this afternoon.

Time of day, weather, or something seems to make some kind of difference here. It’s not raining.

[GroundUrMast]

Both the power supply and the mast-head preamp unit need to be removed when testing the reception without amplification. The amplifier unit does not pass any significant amount of signal when power is disconnected. Most amplifiers will attenuate 20 to 40 dB when power is removed. That a few strong signals can be seen is not uncommon. Signals with NM figures of +50 and higher may leak through the 'dead' amplifier.

[John Candle]

I agree with GUM , disconnect/remove All parts of the amplifier , the outside preamp unit , the inside power supply , any power injectors , any in line amplifiers , any and all.

[hillpc]

Thanks guys. I'll get up there on a ladder and test with and without the preamp. It'll likely be at least a week or so.