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Old 30-Nov-2010, 6:28 PM   #1
Whyteboar
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Can I block Analog Channels?

Normally I wouldn't care, but as one is causing a co-channel interference with a channel we want to get (See here for specifics
http://www.tvfool.com/?option=com_wr...81a34ac4d0a6e3
We finally get channel 13 but now 17 goes out more than in. It's got to be analog 19, which wasn't a problem when our antenna was lower. (And 13 not so hot)
So rather than buying a filter (if one even exists) for channel 19, is there a way to simply block all analog channels?

The obvious answer is to use a rotor, but since my wife often wants to watch something besides football, another channel option is a must. Therefore multiple antennas.

I searched the forum for an answer first but came up empty. Thanks for any insights you guys can give us.

Craig
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Old 30-Nov-2010, 7:06 PM   #2
GroundUrMast
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An inline filter does exist, typically a commercial product. However, it will not differentiate between digital or analog, it will block both equally well.

Your best hope is to use the most directional antenna available. Aimed at the desired station and perhaps more importantly, aimed so that it's least sensitive direction faces the station that is causing the interference. Your situation is one that calls for "walking the roof", meaning, moving the antenna up/down and laterally to find a "sweet spot" were the desired signal is strong and the interfering signal is weak.

A possible candidate is the 91XG http://www.solidsignal.com/pview.asp...u=853748001910

Your TVF report indicates about 15 dB difference (in you favor) between the to two signals. That makes me think there is hope.

If you are very committed, consider: http://www.kyes.com/antenna/stackluge.html , http://www.hdtvprimer.com/ANTENNAS/ganging.html , http://www.anarc.org/wtfda/stagger.pdf the intended result is a more directional array. Done right and with a bit of luck, stacking identical antennas can help in marginal situations.
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Last edited by GroundUrMast; 30-Nov-2010 at 7:23 PM.
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Old 30-Nov-2010, 7:47 PM   #3
John Candle
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Tv Reception

You can also call the tv station , ask the enginering department if the analog channel is on the air.
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Old 30-Nov-2010, 7:52 PM   #4
Whyteboar
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Out of curiosity, how would that help? Something is coming through on that channel, and it isn't normally what we want there.
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Old 30-Nov-2010, 7:58 PM   #5
Whyteboar
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Talking

[QUOTE= Your situation is one that calls for "walking the roof", meaning, moving the antenna up/down and laterally to find a "sweet spot" were the desired signal is strong and the interfering signal is weak. [/QUOTE]

I tried that in the attic with limited success. Though trying to maneuver an 8' antenna through the trusses and not step through the ceiling accidentally really limited my options.

I've looked at the other sites you provided, even got a channel 13 only filter, but apparently the old VHF antenna I'm using for that is also helping with channels 3 and 8 from the south so I can't use the filter. Unless maybe a third antenna. Yeah, that's the ticket...
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Old 30-Nov-2010, 8:21 PM   #6
GroundUrMast
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I don't blame you for wanting to keep it simple. For many folks, a rotor is the best of several imperfect choices. A few people install an antenna / rotor for each set.

I am currently building out a multiple antenna install that uses a network attached tuner. http://forum.tvfool.com/showthread.php?t=820 This is certainly moving away from the 'KISS' principle though.

Back to your original question; If you find a passive band-stop filter tuned for channel 19, it will block both analog and digital.
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Old 30-Nov-2010, 8:48 PM   #7
mtownsend
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GroundUrMast is correct. The only defense against a co-channel interfering signal is to use a directional antenna to favor the desired broadcast. Using the spatial selectivity of the antenna, you can increase the signal strength of the desired broadcast, and decrease the signal strength of the undesired broadcast, as seen by your receiver.

If you are not able to do this, then your receiver is going to be getting a mixed version of both signals, at which point, it might be impossible to separate the two.

Digital TV signals require a theoretical minimum signal-to-noise ratio of about 15.2 dB in order to just barely function. To avoid occasional breakups or signal loss, you need to have some safety margin above this (more is better).

Right now, you have WXMI coming in at around -59.6 dBm and DW19BA coming in at around -62.2 dBm (these are just estimates based on propagation modeling). This means that without any selectivity from the antenna, you are starting with only about a 2.6 dB SNR advantage for WXMI. The way analog and digital signal power is measured is different, so in reality, you more likely dealing with a true SNR somewhere between 5-9 dB.

To get the minimum 15.2 dB SNR or more, you will definitely need a directional antenna to suppress the detection of DW19BA. Sorry to say this, but I think your best bet really is to use a highly directional antenna on a rotator.

Quote:
Sidebar (for the technically curious)

Analog TV signal power varies according to what picture is being shown, and the FCC regulates their broadcast power according to the maximum ERP allowed. An "all-black" picture will cause an analog transmitter to send out the most amount of power, and an "all-white" picture will send out the least. An "average" picture will be somewhere in between.

Digital TV signal power remains roughly constant no matter what picture is being shown. Due to the differences in signal structure, the FCC regulates broadcast power for digital transmissions according to their average ERP.

Since analog broadcasts are processed according to their maximum ERP values, the calculated dBm values should represent an upper bound on the amount of signal power received. Digital broadcasts, on the other hand, have a nearly constant power output, and the calculated dBm values should represent the average signal power received.
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Old 1-Dec-2010, 3:29 AM   #8
Dave Loudin
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The "D" means DELETED, so this station should not be on the air. The CDBS shows the license being cancelled in 1997.
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Old 1-Dec-2010, 12:04 PM   #9
Whyteboar
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Good information

Thanks all, I thought I had at least a clue as to what I was doing but obviously I didn't know how much I didn't know. It would have really helped had I started this testing before the snow flew, but that's the price of my foolishness. Back to the attic with me!
I think more reading is in order. It will take a few days for the roof to be safe again anyway.

Thanks again guys, for pointing out not only the obvious that I overlooked but 'edumacating' me as well.

Be well,

Craig
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Old 1-Dec-2010, 3:21 PM   #10
Tower Guy
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Quote:
Originally Posted by Dave Loudin View Post
The "D" means DELETED, so this station should not be on the air. The CDBS shows the license being cancelled in 1997.
Finally, the right answer.
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