[mythtv-users] OT: 'problem channel'

Alen Edwards allen.edwards at oldpaloalto.com
Wed Jul 2 15:54:10 UTC 2008



Kevin Bailey wrote:
> On Tue, Jul 01, 2008 at 08:32:53PM -0400, Jay R. Ashworth wrote:
>   
>> Big ChannelMaster corner-reflector widebands aimed at Tampa, and one
>> little dinky tuned 14 element yagi pointed 90 degrees from it.
>>     
>
> And you join them with a common splitter ? Or separate tuners ?
I just wanted to point out that if you combine two antennas with a 
common splitter there are a few things you might want to consider. 

First is that if there are hills or buildings around, you might get 
multipath problems.  It has been pointed out on this list that most 
multipath is taken care of by the tuners and not generally a problem, 
but it can be. 

Second is that each antenna is a noise source as well as a signal 
source.  If you just add them together, you might increase your S/N for 
the distant stations.  If you have an amplifier on the distant stations 
and a combiner after that with no amplifier on the local station, you 
would be drastically increasing the noise on the local station, but that 
may not matter. 

Finally, even with the antenna pointed off at 90 degrees, it is going to 
be receiving some of the signal from the distant station.  This will 
either increase or decrease the signal from those stations depending on 
the phasing.  Again, if you have an amplifier in one path, this won't 
matter.  By the way, one way of increasing the power of an antenna is to 
add a second identical one on the same pole phased just right so they 
add.  This increases the directionally up and down so it increases the 
signal.  I think the math is the signal goes up by 6db (double the 
voltage) and the noise by 3db (double the power) so you are 3db ahead. 
Of course, that is what you see in those UHF bow-tie arrays.

Just some things to consider if you have problems or are thinking of 
doing this.  I have not thought to try combining with a passive splitter 
because of the theoretical problems but it may very well work fine in 
practice.

Allen
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