Originally Posted by rockandchelle
....how accurate and reliable is the antennaweb website. Does it take into account possible elevation changes and other factors I am unaware of that could cause potential problems. Because if that is the case I can understand, but if not we are only talking about an additional 5 miles of distance from my location to the towers and his location to the towers. ..I am just trying to figure out why 5 miles of distance you go from a yellow -2.5 dbd to a red -25 dbd. Especially since ch 4 is my strongest signal. To me that just doesn't make since how that little bit of distance, can have that large of an impact.
Where did these -2.5 and -25dBd figures come from?
An antenna has to develop a suitable signal to noise ratio from the ambient signal, and then your transmission medium has to sustain an adequate signal to noise ratio from there to our tuner input. We think that digital, off-air tuners need to see an input signal to noise ratio of around 16dB in order to reliably lock onto a signal, that cable boxes need to see about a 32dB S/N ratio (which is easier for them to meet, because it is a different kind of signal and transmitted under more controlled conditions), analog tuners need about a 45dB S/N ratio to develop pictures generally considered to be excellent by most viewers, and DBS satellite receives need an S/N of 8dB to reliably lock.
The amount of gain needed by an antenna to develop such a level depends on the ambient signal level at the reception point, which is a functionally related to signal transmitter's output, the distance to the reception pint, and is irregularly adversely affected by signal path obstructions.
If a person understands what any of that means, he surely wouldn't be coming to this thread looking for antenna advice, and unfortunately, unless he even has some idea, conceptually, what that might mean, he probably will not be able to utilize any advice given here by those who do understand it to develop remedies to their own antenna problems. They may find it intellectually stimulating, but they will not be able to make any definitive decisions regarding product sufficiency based upon it, other than, perhaps, to be able to rule out the absolutely unsuitable antenna selections.
When I was young, I used to primitively calculate signal levels and S/N ratios (which we more commonly called C/N ratios when working with analog signals), based on the assumption that I had line-of-site to the tower, but I haven't done so for broadcast TV reception analysis in thirty years because I can always do a good enough job estimating it. (I still do it for satellite signal sufficiency, however)
I am a commercial installer and I only use "Antenna Web" for analyzing azimuth relationships and estimating likely relative signal levels over generally line-of-sight transmission paths, usually under 40 miles. I have never bothered to learn what their arbitrary cut-off points are for calling an antenna low, medium or high gain, or directional versus non-directional, as I have much more detailed and specific information available to me when I develop multi-antenna headends for tall buildings, and I have a spectrum analyzer to furnish me with the actual signals levels, I am working with, rather than having to rely on the calculated or estimated ones.
That all having been said, I can't imagine any product claiming to have an output at a given frequency of -25dBd, as I think a paper clip antenna would outgain that, at least for channels 7-69. If consumer antenna dealers are now furnishing VHF lowband "gain" figures for antennas designed for UHF only use, they are furnishing numbers that are as useless for that application as itheir product is.
There is a lot of confusion in discussions in which lay people exchange published information regarding antenna signal levels, as sometimes they believe that numbers they are seeing describing different products are expressed in like terms and are describing the same signals or needs when they are not.
Those seeking advice on hardware additions to their home antenna system are always better going to their local sub-forum and opening a thread there, as there will be other participants who know the local considerations, like relative signal strengths, azimuth angles and sources of interference or signal degradation, and can offer solutions more specific to the unique circumstances of installations in their own back yard.