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Guest Message by DevFuse

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dB (Signal Strength)


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8 replies to this topic

#1 OFFLINE   kevinturcotte

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Posted 01 July 2010 - 01:30 PM

What's considered too low dB (Signal Strength) for ATSC? Good? Too high?

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#2 OFFLINE   n3ntj

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Posted 01 July 2010 - 02:56 PM

What's considered too low dB (Signal Strength) for ATSC? Good? Too high?


It depends upon your actual OTA receiver and your setup. My two Panny receivers' ATSC tuners will normally lock on a steady OTA signal of at least 30%. My HR20-700's ATSC needs a bit more to lock on. If you have signals steady above 80% you should be fine in most cases.
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#3 OFFLINE   Jim5506

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Posted 01 July 2010 - 08:16 PM

WAG - at the antenna anything below -10 dB is very difficult to deal with and if you get much above +75 or 80dB you risk overload.
Tuners: Hopper 2000; Hopper w/Sling; 3 Joeys; Samsung SIR-T351; Accurian 6000;2 X HD TiVo; 2 X TiVo Series 2 Stand alone; Panasonic Showstopper 2000
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#4 OFFLINE   kevinturcotte

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Posted 02 July 2010 - 03:54 AM

WAG - at the antenna anything below -10 dB is very difficult to deal with and if you get much above +75 or 80dB you risk overload.


Is there anyway to "Tone down" certain channels without effecting the others? I've got one that's at 66.6, another at 70.5 and a 3rd at 75.4.

#5 OFFLINE   yogi

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Posted 02 July 2010 - 07:43 AM

What's considered too low dB (Signal Strength) for ATSC? Good? Too high?


It depends on what your talk about.
+66.66 dBm will light up your house at night.:)
+66.6 dBmv will overdrive you tv big time.
+66.6 dbuv is a little low.
A SQ of +66.6 is low. but, will give you a constant picture.

#6 OFFLINE   Cholly

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Posted 02 July 2010 - 09:27 AM

Getting down to the nitty gritty, there's no definitive answer, because it depends on the sensitivity of your receiver AND the signal level from your antenna. Without professional measuring equipment, you can't tell the antenna output. Receiver sensitivity depends upon make and model. Signal levels you see from a reading of the "signal meter" on your receiver aren't always reliable.

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#7 OFFLINE   kevinturcotte

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Posted 02 July 2010 - 10:53 AM

Maybe I'm not reading it right, but look at 44, 38, and 8. Too high to begin with?

#8 OFFLINE   Jim5506

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Posted 02 July 2010 - 12:45 PM

Those numbers are at the top of the range because you are less than 5 miles from the transmitters (2 of them any way).

These numbers are not too high, they are normnal for your proximity.

They simply preclude you from using amplification to try to get distant stations.

If, for instance, you want to receive the FOX station which is RF channel 23 at 41 miles and 13.7dB, you might be able to use a Winegard HDP-269 pre-amp but CBS channel 13.1 (RF 38) is in the same direction but 12 miles out may be a problem.

One way to help might be to use a jointenna as a narrow band attenuator (one leg of a channel 38 jointenna passes channels 36-40 and the other blocks it). If you can set up 2 jointennas for ch 8 and ch 11 to trap out 8 and 11 (your biggest problems) you might be able to rig a setup that will let you use a more powerful amplifier, otherwise forget it and move to a location more than 15 miles from a transmitter.
Tuners: Hopper 2000; Hopper w/Sling; 3 Joeys; Samsung SIR-T351; Accurian 6000;2 X HD TiVo; 2 X TiVo Series 2 Stand alone; Panasonic Showstopper 2000
Dish 1000.2 @ 110, 119, 129; dish 500 @ 61.5
Antennas - CM4228; RS U75-R; coathanger; Funke PSP.1922 (stillin the box); paperclip
Displays: Sony VPH D50Q with HD Fury HDMI input; Hitachi 57F59; Sony Bravia LCD;Sanyo 32" LCD; Panasonic 42" plasma
Sony 80GB PS3; Toshiba HD-DVD

Give me a Finco colinear array and I'll rule the world - HA-HA-HA-HA!

#9 OFFLINE   Scott in FL

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Posted 04 July 2010 - 07:06 AM

Any value in dB is not an absolute power measurement, it is a relative measurement used to compare two values. For example, on the tvfool chart, NM provides the noise margin. This is how high (or low) the signal is compared to the noise. After adding and subtracting the gain, losses, and noise of your receiving system you want to wind up with a positive number.

When talking about pre-amp overload the value to look at is the power in dBm. This is an absolute measure of power. 0 dBm is equal to 1 mW. 3 dBm equals 2 mW. 60 dBm = 1000 watts. 80 dBm = 100,000 watts.

To determine if your pre-amp will overload you must add all of the signals it will receive and amplify: VHF, FM, UHF, plus any nearby transmitter in the amplifier's pass band including non-broadcast. I tried to find the overload spec for a CM7777 pre-amp, but so far no luck. I did find on one dealer's web site where it said this pre-amp could handle 350,000 uV. That's 1.7 mW into 75 ohms, or about +2 dBm. I have no idea if that spec is accurate, but it gives you and idea. Again, this is for the total power of all signals amplified. The gain of the antenna before the pre-amp must also be considered.

Edited by Scott in FL, 04 July 2010 - 10:21 AM.





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