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Discussion Starter · #1 ·
Back in July, I got an RCA DTC-100 (real cheap), but the nearest DTV station then was 70 miles away. Recently, two stations within 50 miles have begun onair DTV testing and I can receive them sometimes late at night with a rooftop CM 4248. During the day and at other times when I can't receive the stations, the DTC-100 shows a fluctuating signal strength within the 22 to 25 or 30 to 35 range. What do these strength values mean on the DTC-100? S/N rations, percentage of maximum possible value, or what? When the strength reaches 45 to 50 the picture begin to display.
 

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I'm no expert, but I believe they are like the meters on DBS receivers: they give an indication of signal quality, not strength. The tuners have automatic gain controls that can compensate for a weak signal, as long as the S/N ratio is high enough. That's why multipath is a bigger problem for digital signals than analog. Reflected signals introduce incorrect data, and you reach a point where the error correction built in to the signal can no longer put together the correct data.
 

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My understanding is that it reflects error rate. Something like the percentage of data packets that do not require error correction. Unfortunately, at signal strengths below 30 on the DTC-100, there are enough packets with uncorrectable errors that decoding a usable picture is difficult. The antenna you have should be able to easily cover that distance. Does it have a rotor? An antenna that directional would be hard to peak without a rotor. Are you in a valley? In that case, raising the antenna another 10 or 15 ft could make a big difference. I guess the other thing to check for is a bad run of coax. In the SF bay area, I get 9 stations off the SF tower at a distance of 42 miles with signal strengths on a DTC-100 between 65 and 90. So, I think you are in range, but something is not quite right with your antenna.


Ernie
 

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Discussion Starter · #4 ·
Ernie: thanks for your response. Admittedly, I have yet to optimize my antenna for the DTV stations I want to receive. Currently, I have the CM4248 joined with a lessor UHF yagi pointed in different directions and mounted on the roof about 31 and 34 feet above the ground. This works quite well, but with some ghosting, for NTSC UHF stations. I don't think that either of these points to my intended DTV stations.


Presently, I am gathering info to determine how to optimize reception. I am thinking of vertically stacking two CM4248s with a rotator and preamp. Part of this info is to get an idea of what the strength numbers mean.


Yes, I am in a slight valley where there are rolling hills 200 feet higher than my location, beginning about 3 miles out between me and one of the stations not yet onair. Being this far out, do these hills affect the signal, though I can get a medium quality signal from the existing NTSC Channel 28 station.


Another question: why do the values improve at night and why do they fluctuate?
 

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Tech, i have a dtc100 and a sony hd200, i can get steady picture with no dropouts on the sony, while the dtc100 doesnt even register a signal.The dtc100 is not good at pulling in marginal signal.

Also, i have found in my situation the cm4228 out perfoms a stacked 4248
 

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Quote:
Originally posted by Teeh
Ernie: thanks for your response. Admittedly, I have yet to optimize my antenna for the DTV stations I want to receive. Currently, I have the CM4248 joined with a lessor UHF yagi pointed in different directions and mounted on the roof about 31 and 34 feet above the ground. This works quite well, but with some ghosting, for NTSC UHF stations. I don't think that either of these points to my intended DTV stations.
Joining antenna systems, even if they are pointed in different directions, is hard to do correctly. I have found pointing a high gain antenna to be quite touchy and being off by 15 degrees either side of the station will give a signal that is not usable. A rotor really helps getting it exactly pointed right.

Quote:
Presently, I am gathering info to determine how to optimize reception. I am thinking of vertically stacking two CM4248s with a rotator and preamp. Part of this info is to get an idea of what the strength numbers mean.


Yes, I am in a slight valley where there are rolling hills 200 feet higher than my location, beginning about 3 miles out between me and one of the stations not yet onair. Being this far out, do these hills affect the signal, though I can get a medium quality signal from the existing NTSC Channel 28 station.


Another question: why do the values improve at night and why do they fluctuate?
At night an atmospheric temperature inversion occurs in many places, which allows to signal to follow the curvature of the earth better. Its called "ducting". Since you can get an ok NTSC picture, I would suggest forgetting about stacking and combining antennas, and just get a rotor. You are very close to getting good numbers as it is and most likely you just need to get it pointed exactly in the right direction. If that doesn't do it, then you can try stacking or a different antenna.


Ernie
 

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Discussion Starter · #7 ·
Ernie, thanks for the info and advice. Following your advice, last week I discarded the two joined UHF yagi idea and just used the single CM4248 yagi raised to the top of the mast and the mast extended by a few feet. Result is the CM4248 was raised from 31 to 39 feet above ground. I mounted the Radio Shack crossed bi-dipole FM antenna 3' below the yagi for FM and VHF and joined both antennas with a CM7777 preamp on the mast. I also installed a CM9521A rotor.


Now, I can receive all 17 analog station in the two ciites that I'm in between very clearly. Their towers are located from 34 to 79 miles away. Seven of these stations now transmit digital signals off and on. When they're on, I can get them very well, with strength numbers mostly in the 70-90 range. Yes, I'm very pleased. I'd had quite a lot of advice from 'experts' but yours made the most sense and gave me the results I was looking for. Again, thanks.
 
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