[mythtv-users] Antenna amplifier

[Allen Edwards]

Yan Seiner wrote:
> Brian Wood wrote:
>> Yan Seiner wrote:
>>   
>>> More on the Olympics.  NBC is broadcasting it.  I never watch NBC.  It 
>>> comes in with the weakest signal - my pcHDTV won't lock in on it.  (Now 
>>> I find out.)
>>>
>>> So two alternatives: 
>>>
>>> 1.  Get up on the roof and fsck with the antennas to the great amusement 
>>> of my family and neighbors.
>>>     
>> Certainly the cheaper approach.
>>
>>   
>>> 2.  Get a 2 way amplified splitter.  About 9 to 12 DB should be plenty.  
>>> Any suggestions on model/brand/suitability?
>>>     
>> Depends, you may just be boosting the noise floor by 9 - 12 db.
>>
>> To do it properly you need to know where the xmtr is, what it's ERP and
>> countour is, your antenna specs, your height, losses in your signal path
>> etc. etc.
>>
>> An antenna in good condition, properly aimed and mounted is a good
>> start. More basically - can your neighbors get a usable signal on NBC
>> from an antenna? If not, you are not likely to do much better.
>>
>> I've read that capture cards require more signal than TV sets, but this
>> doesn't make sense to me as the card makers are almost certainly using
>> off-the-shelf tuners. If you can get +10dbmv. great, even zero should be
>> watchable.
>>   
> 
> Well, here's what's really bizarre.
> 
> I have two highly directional antennas at 90 degrees.  They're far 
> enough apart where interference is not an issue.  They go to an 
> unamplified splitter, there to an old Blonder-Tongue 6DB amp and 4 way 
> splitter, and then to the TV and the myth backend.
> 
> The TV has a passive splitter that splits the signals to the analog and 
> digital antenna inputs.  The TV shows a consistent signal strength 
> across all channels of about 60% (meaningless except that the signal 
> strength is approximately the same).  Myth gets a straight run from the 
> B-T amp to a pcHDTV card.  That card *also* shows a consistent signal 
> strength across all channels of about 60% - except NBC on channel 16 
> which comes in at a dismal 7 to 13%.  Can anyone take a shot at 
> explaining this?
> 
> Channel 16 is the closest station and has the strongest signal, so the 
> issue could be overfeeding, but I get dismal results with a straight run 
> to the pcHDTV with no splitters or amps....
> 
> --Yan
> 

Let me see if I understand you correctly.  You are combining the output 
of two antennas with a hybrid combiner (splitter).  Then going to an amp 
that has 4 outputs, each of which has 6dB gain???.  One of these outputs 
goes to a 2-way splitter that goes to the two TV inputs.  Another of the 
outputs goes to the myth pcHDTV card.

You have some unexplained behavior with your NBC station.

Try disconnecting one of the antennas (the one without NBC) and see what 
happens to the signal strength of NBC relative to the other channels.

You asked for an explanation.  My guess is that you have a nasty notch 
in the passband of that station due to the two antennas and the TV set 
deals with it and the pcHDTV doesn't.  This would be more likely if NBC 
is not directly down the throat of its main antenna but is a little off 
on the side lobe and the other antenna is picking up enough signal to 
mess up the equalizer in the pcHDTV.  Just a guess which you should be 
able to verify by removing the second antenna.

I tried combining two antennas and the results were terrible, similar to 
what you are describing.

Allen