[mythtv-users] Antenna attenuator question

Alen Edwards allen.edwards at oldpaloalto.com
Tue Jul 1 08:34:28 UTC 2008



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.
>
> Thanks in advance.
>
> J.S.

JS,

There has been a lot of discussion about multipath that came from this 
thread.  That may or may not be your problem but as you say the problem 
went away when you disconnected the amplifier, my bet is still on 
overload, as you originally suspected.

If you have a 2 way coupler, you can make a cheap variable attenuator 
out of it by connecting one output to your Tv and the other output to 
your antenna lead.  This may be enough to cure the problem but if it is 
not, put a resistor near 75 ohms on the input.  If you put a perfect 
load on the input and the coupler were ideal, this setup would give you 
a high level of attenuation because the signal going in the output and 
coming out the input would be absorbed by the resistor.  If, on the 
other hand, the input is left open, then the signal will bounce off the 
open circuit and come out each output.  Half would go up your antenna 
wire and stop at your amplifier.  The other half would go to your TV, 
attenuated.  A resistor near 75 but not exactly 75 will give you a 
variable amount of attenuation.  I tried this and it works.



Allen



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