THE DTV TRANSITION: JANUARY 2009
Home, Home On The Fringe
HDTVexpert.com revisits a remote Vermont location over New Year’s weekend to add a pair of VHF digital TV channels to the line-up, and see how the original UHF antenna installation is working.
Figure 1. The location was beautiful but c-o-l-d, with temperatures dropping to zero at night.
Figure 2. There’s that doggone hill again. Time to bend some TV signals!
Back in May of 2007, I wired up my brother’s house in a remote corner of southwestern Vermont to receive UHF terrestrial digital TV broadcasts from Albany, NY (DTV Reception Out On The Fringe).
The location was challenging because the house sits on the steep side of a narrow valley, with its view towards the southwest and the 54 mile-distant Helderberg Mountain TV transmitters completely blocked by a hill that is 100 to 200 feet taller and less than ½ mile away.
Nevertheless, I was able to successfully pull in four strong UHF DTV channels by exploiting a signal propagation phenomenon known as “knife edge refraction,” wherein radio and TV signals on VHF and UHF frequencies are actually bent downwards as they pass over the edge of hills and mountains, providing better-than-expected reception in their shadows.
Using nothing more than a Channel Master 4308 suburban UHF yagi and a Channel Master 7775 Titan 2 mast-mounted preamplifier, I picked up WTEN-26, WMHT-34, WRGB-39, and WCWN-43 quite reliably on a Samsung DTB-H260F 5th-gen ATSC set-top box, and my Acer notebook PC, equipped with AutumnWave’s OnAir Solution HDTV-GT DTV receiver.
So far, so good, except that there were a few problems with the setup. First, WRGB-DT was scheduled to vacate UHF channel 39 and return to its old analog channel (VHF 6) on February 17. Secondly, there are two Albany VHF digital channels that I couldn’t pull in with the CM 4308 — WXXA-7 (Fox), and WNYT-12 (NBC).
Finally, as well as Samsung’s DTB-H260F works, it’s really not intended to be used with basic, run-of-the-mill analog TV sets. The tuner’s operating menus and displays aren’t passed through the composite and S-video connections, making operation of the set-top box extremely difficult with older TVs.
IMPROVING THE FRINGE
For this trip (it really was for a holiday family visit, not an antenna installation!), I brought along a surplus Terk TV35 UHF/VHF antenna that I had removed from my roof here in November. Sure, it had a few years on it — don’t we all — but it still work nicely. I also tossed a Channel Master 7777 dual-band preamp in the kit bag, along with a Zenith DTT901 digital TV converter box, a bunch of coax, connections, and test equipment.
What I hoped to do was to replace the CM 4308 with the Terk TV35 and pull in every available channel. But it didn’t quite work out that way. Initial tests with the TV35 showed that I could indeed “see” channels 7 and 12 quite nicely — except that they peaked almost 45 degrees farther west than the Albany UHF channels!
Figure 3. Here’s the final placement of the TV35 (bottom) and CM 4308 (top) antennas.
Notice
how close both antennas are to ground level.
Figure 4. Another view of the split VHF/UHF antennas and the CM 7777 preamp
(mounted on
top of the mast).
As a result, I couldn’t find one heading for the antenna that would pull in all channels reliably. Indeed, at one point I was able to receive WXXA with the odd “hit” here and there while WNYT came in flawlessly, but I could only pick up WCWN-43 from my original group of UHF DTV stations.
Turning the TV35 more south and aiming it in the same direction as the original CM4308 brought all of the UHF channels back, but caused WXXA and WNYT to drop out frequently, if not disappear altogether. Grrr! (And “Brrr!” too, as it was in the low teens outside while I was a-fiddlin’ with the aluminum…)
After some deep contemplation and some warm liquids to get rid of the chill, I realized that the solution was to use the split VHF and UHF inputs on the CM 7777 preamp and orient each of the antennas in their optimum position. As shipped, the 7777 preamps are configured for combined VHF/UHF signal input. But remove four screws and flip a switch, and you now have separate coaxial input connections for each band.
Using a portable spectrum analyzer, I positioned the TV35 for strongest signal levels on channels 7 and 12. That meant sliding it even farther DOWN the mast (not up) and swinging it west to get clean 8VSB waveforms from WXXA and WNYT. In contrast, I had to slide the CM4308 UP the mast and peak it more to the south.
Figure 5. (Left to right) 8VSB signals from WXXA-7, WVER-9 (not receivable), and WNYT-12.
Figure 6. (Left to right) Reception of WTEN-26, WMHT-34, and WRGB-39 was still
as good as
ever, despite a bit of multipath and carrier notching.
The TV35 position worked so well that I actually saw a strong reflection from WVER-9, 32 miles north through the mountain in back of the house! The signal was not receivable, but with a bit more gain from the antenna system — who knows? (You can see WVER-9’s carrier between WXXA and WNYT.)
Reception was made that much easier by using the DTT901, which uses an even better adaptive equalizer than the 2+ year-old DTB-260F, and is much more tolerant of signal notches and multipath. You’ll see from the spectrum analyzer screen shots that while WXXA and WNYT are not nearly as strong as any of the UHF DTV stations, there’s still plenty of carrier to work with.
CONCLUSIONS
First of all, “knife edge refraction” works just as well for VHF TV signals as it does for UHF signals, although you may find the VHF antenna needs to be closer to the ground. (Now that’s a switch — no ladder required for TV antenna work!) The effect is more pronounced as you get closer to the hill (it also with tall buildings), although there is a point of diminishing returns.
Secondly, you will be surprised at how clean the signals are, despite the “obstructed” signal path. I was able to peak both antennas for minimal notching, making the DTT901’s job that much easier.
These results are similar to those I saw during tests nearly six years ago in Glendale, CA against the California foothills: Signal levels were predicted to be unusable, yet with the same CM4308 and 7777 preamp mounted on 10’ of mast atop a one-story house, I was able to pull in 12 UHF DTV stations from Mt. Wilson without a hitch — and that was using a 2nd-generation Samsung SIRT-165 DTV receiver.
Figure 7. WNYT-12 digital (left) and WNYT-13 analog (right). That’s about as clean an 8VSB signal as you’ll ever see from a digital TV station — and it’s bending over a mountain…
Third, although the TV35 is a nice suburban VHF/UHF antenna design, it’s a bit overmatched to pull in DTV signals from so far away. A follow-up trip this coming summer will see it replaced by a VHF yagi with a lot more gain, possibly one of the new Channel Master 2018 or 2020 models.
And finally, it is indeed possible to get reliable digital TV reception in remote,
out-of-the-way locations, provided there is enough signal strength for a yagi or log-periodic
TV antenna, combined with a mast-mounted preamp, to snag the signals and reel ‘em in. (In
fact, my brother and sister-in-law are now considering dropping their DirecTV service altogether
and just going with terrestrial DTV signals to save a few dollars.)
