Why is ATSC on VHF so damn finicky?

[Robert Grant]

When discussion of HDTV started getting serious in the early nineties, the plan was for all over-the-air television to migrate to UHF on channels 14-69, with VHF television to be discontinued.

Then. Congress got greedy. They added high-VHF (7-13) to the plan, in order to sell channels 63-69 to cellular interests. Quite quickly, they added low-VHF (2-6) as well, leaving channel 58 as the top channel. Later, channels 52-58 were cut out of the plan.

Well, when the VHF channels were added to the plan, the FCC worried that the DTV stations on VHF would have an "unfair advantage" of greater coverage compared to UHF stations (as lower frequencies do carry further over the cuvature of the Earth). Thus, lower power levels were prescribed for VHF high stations and much lower power levels were prescribed for low-VHF stations.

This had the drawback that high-VHF stations would have weaker signals in areas that were NOT on the fringe, and low-VHF stations would have MUCH weaker signals.

Go to rabbitears.info, and look at the longley-rice coverage maps of any lowband DTV, and you'll see a lot of red and orange shades!

To make matters worse, the early years of the DTV transition coincided with an electronics revolution that filled the spectrum with an unbelievable surge in the amount of RFI in the VHF bands.

How bad VHF is for DTV depends on where you live. While the power levels for UHF are the same throughout the US, VHF power levels are divided into zones. In areas where many cities are packed close together, low-VHF stations are limited to a paltry 10kW and high VHF are allowed a maximum 30kW (and many of these are limited further). In the rest of the country, lowband stations are allowed up to 45kW if their towers are not very tall, and highband stations with short towers may be as powerful as 160kW!

Finally, antenna manufacturers had heard that all TV was going to UHF and didn't get the memo when Congress shifted the TV bands (to this day, I hear from people who insist that all TV is UHF now), so a lot of people buy cute little antennas, and wonder why they're not getting all of the local stations.

[loopstick]

Originally Posted by Robert Grant

Finally, antenna manufacturers had heard that all TV was going to UHF and didn't get the memo when Congress shifted the TV bands (to this day, I hear from people who insist that all TV is UHF now), so a lot of people buy cute little antennas, and wonder why they're not getting all of the local stations.

I recently had my roof reshingled. So I moved my Winegard HD7084P into the attic. Boom length 131", max width 110". Getting it through the little trap door in the closet and reassembling it in the attic was like building a ship in a bottle. I didn't know about the repacking and was thinking about replacing it with something more UHF focused. But I wanted to keep the FM radio capability, so I made the effort to stay with the Winegard. Glad I did.