[mythtv-users] Antenna attenuator question

Brian Wood beww at beww.org
Mon Jun 30 14:14:04 UTC 2008

On Monday 30 June 2008 07:03:18 backuppc at sundquist.imapmail.org wrote:
> Thanks for all the previous help in setting up my antenna (e.g.
> http://www.gossamer-threads.com/lists/mythtv/users/329040)
> Things have been working great.  I ended up using a single CM 4221
> rather than my Radio Shack one because Solid Signal was having a special
> the same time I bought a CM 7775 preamp, which I needed after moving the
> antenna to the roof (and had a longer cable run).  I used just one
> antenna after pricing out what it would cost to do the grounding
> correctly.  After the pre-amp, I split the cable three-ways into a
> pcHDTV5500 and a HDHR.  I've got the antenna aimed at the further
> antennas about 30 miles away (FOX, PBS, etc.), with old big 3 networks
> closer by behind the back of the antenna.
> Here's the problem:  NBC, which is the closest antenna at about 5 miles,
> sometimes comes in too strong despite being "behind" the antenna.  The
> noise is apparently getting amplified above the signal because when the
> pixelation starts to occur, if I go downstairs to the preamp and unplug
> it, the station comes in fine (but then none of the other stations come
> in at all).  Other days (like last night's Olympic Swim trials) I need
> the preamp to get the signal (almost missed that program; family would
> have killed me!).
> I could fiddle with the aiming to try to get NBC in one of the
> poor-reception nodes out the back of the antenna
> (http://www.hdtvprimer.com/ANTENNAS/cm4221.html), but since the
> reception is so variable anyway, I'd have to be up there all day
> checking signals, and then weather would change.
> So I guess I need an antenuator.  I am looking at this one:
> http://www.solidsignal.com/prod_display.asp?prod=TA-8700   One question
> first.  The literature says this gets installed "between the amplifier
> and the TV" (replace "TV" with "mythbox" ;-).  If this is the case,
> wouldn't the noise already be amplified too high?  If then, how would it
> help?
> Possible answer (let me know if this is right): I suppose both signal
> and noise are amplified but the tuner clips the signal as it comes in,
> so the attenuator reduces both the signal and noise amplitude, so that
> the signal isn't so clipped.  Correct?
> I realize that antenna set up is an art more than a science (like this
> guy: http://www.hdtvoice.com/voice/archive/index.php/t-21117.html) and I
> don't want to end up with even more stuff I got that I ended up not
> using, so any help before I place my order is appreciated.
> If this attenuator is the ticket, hopefully I can find a magic setting
> that will balance the strong and weak signals.

The problem with that type of attenuator is that it lowers the level of all 
signals, the strong as well as the weak.

The best situation for an amplifier is to have all of the signals going 
through it at more or less the same level.

You might want to look into a tunable trap, which allows you to reduce the 
level of the strong local channel while leaving the rest of the channels more 
or less alone.

Also, things like strong local FM stations can cause problems with amplifiers. 
You can get either tunable FM traps to null out a strong station, or you can 
get traps that take out the entire FM band.

There are other tricky ways to handle a single strong local channel, there are 
systems that pick up the offending channel with a separate antenna and feed 
the signal out of phase into the main feed, thus reducing the level, but such 
systems are probably out of the price range for most Myth users.

Sorry I don't have specific model numbers for you, I've been out fo that 
business for a long time :-)


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