[mythtv-users] OT - Antenna tuning/aiming

Brian Wood beww at beww.org
Fri Nov 3 15:50:37 UTC 2006


On Nov 2, 2006, at 6:33 PM, Brad Fuller wrote:

>>
> As you say, as the freq goes up, the beamwidth gets narrower. I  
> suppose
> that is for all antennas.

That's pretty much true for conventional antennas using reflectors/ 
directors, like yagis (and their derivatives like log-periodics),  
parabolic reflectors etc. It's not necessarily true for things like  
phased arrays, but those are not generally used for TV reception. The  
reason is simple, as the frequency goes up the fixed size of the  
elements becomes a larger portion of the wavelength.
>
> From the spec sheet:
> chan: 7 (175.25MHz
> Beamwidth at Half Power Points: 95 deg
> Front-to-back ration: 6dB
>
> chan: 10 (193.25MHz)
> Beamwidth at Half Power Points: 93 deg
> Front-to-back ration: 2.6dB
>
> chan: 14 (471.25MHz)
> Beamwidth at Half Power Points: 68 deg
> Front-to-back ration: 20dB
>
> chan: 32 (579.25MHz)
> Beamwidth at Half Power Points: 67 deg
> Front-to-back ration: 16dB
>
> chan: 56 (723.25MHz)
> Beamwidth at Half Power Points: 58 deg
> Front-to-back ration: 12/5dB
>
> chan: 69 (805.75MHz)
> Beamwidth at Half Power Points: 54 deg
> Front-to-back ration: 12dB
>
> Here's the general spec:
>
> Avg. beamwidth ................................................ 61
> Avg. VSWR across band ............................... 1.3:1
> Avg. Front to back .........................................13 db
> Avg. gain across band 470-806 ................... 4.5 db
> Maximum Width Housing ................. 16" x 16" x 4"
> Preamp gain (SS-2000)
> 300,000 μV Total Input
> S/N ratio ......................................................  
> 2.8 db
> VHF ................................................... 12 dB avg.
> UHF ................................................... 12 dB avg.
>> But in the real world "field factors" usually outweigh the
>> theoretical performance issues, and I suspect a pragmatic approach
>> will work as well as a "scientific" one. I'd just try aiming in the
>> middle of the azimuth range, if you're anything close to line-of-
>> sight you should be OK on most if not all channels. You might also
>> want to try just aiming for the best signal on the most distant
>> channel (KTLN?) and see what you have on the others.
>>
> That's way too far, I believe. I doubt that I could get that. Never
> could before.
>

I guess I was assuming you could receive all the stations you  
mentioned. That kind of range can be achieved under ideal conditions,  
but that's pretty rare except for mountaintop-to-mountaintop situations.

I suspect you are trying to get too "scientific". Simply playing  
around to get what looks best to you will likely take less time and  
give the same end result. Your beamwidth is wide enough that you  
should be able to get a median setting that will work OK for all the  
stations that you can receive.


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