[mythtv-users] Antenna attenuator question
allen.edwards at oldpaloalto.com
Mon Jun 30 15:54:32 UTC 2008
backuppc at sundquist.imapmail.org wrote:
> Thanks for all the previous help in setting up my antenna (e.g.
> 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
> 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.
Your amplifier has 26dB of gain.
Say your loss is about 13dB
100 feet of cable 6dB
Splitter loss 7dB (4 way, I think 3 way is the same)
You probably need about 10dB of attenuation just before the splitter.
Now some more detail.
You want to put the attenuator after the amplifier unless, like you say,
it is the amplifier that is overloaded. If the amplifier were
overloaded, you would see that on other channels than just the one in
question so it is probably OK. If it is not, you need a trap in front
of the amplifier (see below). You don't want to put an attenuator in
front of the amplifier as that cuts the signal to noise ratio. For
example, say you put an amplifier on the antenna. It may add 2 dB of
noise. If you put a 10dB attenuator in front of the amplifier, you are
now adding 12dB of noise.
You also want your amplifier before the cable. I hope you were talking
about unplugging the amplifier power supply downstairs and not the
actual amplifier. I have a very similar amplifier. These are made to
go on the antenna pole itself. I didn't want to do that so mine is in
the attic right at the antenna lead-in, which goes through a pipe in the
roof so that the distance from the antenna and the amplifier is only
about 10 feet. If you can't do that, put the amplifier on the pole.
The other place you will add noise is the TV tuners. I have no idea
what they are but if they add another 2dB of noise, you can make that
insignificant by adding more signal in front of them (amplifier). Then
their noise is compared to the larger signal so it doesn't matter so
much. This works until you overload the tuner, which you have done.
So, you want to add some attenuation after the amplifier but still give
the tuner more signal than it had. You have about 13dB of excess gain.
You want some excess gain to help mask the tuner noise but that is
apparently too much.
I don't have overload but I do have a place in my system where I am
combining two signals and need to cut the level of one to match the
others. I use these: https://www.tselectronic.com/pico/fam.html The
one you are looking at is adjustable between 0 and 18 dB so that would
be ideal. You could also get a 3, 6 and 10 fixed and experiment with
combinations. Whatever you get, put it just before the splitter.
Notice that these block DC so you need to put them after your amplifier
There is actually a product made just for what you are trying to do. It
would be for a case where the signal was even stronger that what is
probably the case in your situation but if you continue to have trouble,
it is worth knowing about. It can attenuate just the one signal. This
one could go in front of the amplifier and prevent it from being overloaded.
It will attenuate just the strong signal. Because it so strong, it is
far from the noise so you don't care that you are making its S/N worse,
it will still be more than good enough.
Hope this helps,
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