[mythtv-users] Antenna amplifier

Yan Seiner yan at seiner.com
Tue Aug 5 12:45:49 UTC 2008


Allen Edwards wrote:
> 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.
>
>   
This actually makes sense....

I think I'll get on the roof and realign the antennas as a first step.  
The next step is to buy an HD HomeRun for Myth.

--Yan


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