Quote:
Originally Posted by jr_tech
The problem... The Calculation of the coverage area appears to overstate the coverage of the VHF channels when compared to the UHF channels. This calculation was based on assumptions/projections (mostly about receiver capability) that were made 20 years ago when the standards were set. Some projections were not realized. 
jr |
The calculations were based on a rooftop antenna with a decent gain at VHF, but most importantly neglected the high levels of manmade noise and interference and "atmospheric" noise in the VHF band. The receivers themselves have about the same sensitivity on all channels. When you go to an indoor antenna, which generally must be too small for VHF performance to equal UHF, plus taking attenuation due to lower height above ground, plus building attenuation, reception suffers. I can get reception at 38 miles from Chicago with an amplified indoor antenna on channel 12 and even channel 7, **on the second floor**, but not on the first floor. (This antenna has quite long dipole rabbit ears for VHF). The antenna I have in my attic picks up everything. The line of sight is through a neighbors' house kitty corners from me and some tall trees behind that.
When reception is truly impossible, it usually involves being behind terrain or over the horizon - the signals just don't go through dirt. On analog, some reception was possible by diffracted signals. The VHF coverage for digital (and hence the radius of interfering with other stations) was predicted to follow a similar result (except for the all-or-nothing nature), and therefore the VHF stations' power was reduced drastically compared to analog to prevent long-distance interference. (UHF is more predicatably limited to somewhat beyond the horizon.) Now that practical results are in, stations are asking for power increases, which have to be considered in the light of will they cause interference to other stations. Plus, increases along the northern tier of states have to be coordinated with Canada.
There were a few cases of digital low VHF signals (especially over water) interfering with analog VHF on the same channel before analog was shut off. With analog shut down, there is more possibility of adjustment.