UPDATES FROM THE FIELD: MAY 31, 2007
DTV Reception Out On The Fringe
Live in a rural area, without cable TV service? Wish you could receive terrestrial digital TV programming? You just might be able to.
I recently spent Memorial Day weekend visiting relatives in Vermont at a beautiful, four-story lodge, perched on the side of a hill in a narrow valley — the quintessential Green Mountain location, complete with a small pond, a windy road, and mostly trees for neighbors.
My brother and sister-in-law bought the place a few years back as their “get away from it all” home, once he retires from the military. It’s definitely away from everything, including traffic, cable TV, DSL, and many of those conveniences we often take for granted.
This location in southwestern Vermont is so tucked away that the only TV service available is through satellite (they opted for DirecTV), as is high-speed Internet (DirectWay).
Figure 1. Ahhh, the lush green hills of Vermont. All that’s missing is Bob Newhart.
I’d been to the house a few times before and wondered if the location was truly a black hole, bereft of any analog or digital TV service. This time, I decided to find out, so I packed up a couple of antennas, a preamp, tools, a spectrum analyzer, and lots of coax, along with a couple of kayaks (you know what they say about all work and no play!).
A bit of research beforehand revealed several ATSC stations broadcasting from Albany, Mt. Greylock in Massachusetts, and one or two Vermont stations. It looked like my best shot would indeed be Albany, even though I calculated the straight-line distance to the Helderberg Mountain antenna farm (Albany/Schenectady) to be about 53 miles.
What antenna to use, and where to place it? I started by hooking up a Channel Master 3022 UHF suburban yagi and a Titan 2 UHF preamp to an old camera tripod, resting on the house’s front deck. That exposure looks directly southwest, with the base of the house at 990’ above sea level.
Figure 2a-b. The hills across the valley make for a beautiful vista (top),
but not if you’re a TV antenna (bottom).
Sounds great, except that the range of hills just ½ mile to the southwest vary between 1350 and 1650 feet — meaning they were between 400 and 700 feet taller than my location!
However, there was a possibility that I could still pull in some DTV stations via a VHF/UHF signal propagation phenomenon known as knife-edge refraction.
Knife-edge refraction of VHF and UHF radio waves is a familiar concept to amateur radio operators, who observed many decades ago that mobile VHF transmissions seemed to get stronger as the operator’s car got closer to hills and mountains. The waves are literally bent and scattered at a downward angle as they pass over the top of the hill.
With that in mind, I was pleasantly surprised to see five distinct 8VSB carriers from Albany stations WTEN-DT (ABC, UHF 26), WMHT-DT (PBS, UHF 34), WRGB-DT (CBS, UHF 39), WCWN-DT (CW, UHF 43), and WYPX-DT (PAX, UHF 50). Not only that; the first four stations were quite strong, and none of them really exhibited serious waveform distortion from multipath except for WYPX.
Figure 3a-b. The 8VSB carriers from WTEN-26 and WMHT-34 (top) and
WRGB-39 and WCWN-43 (bottom) were surprisingly strong and clean.
The next step was to verify reception, which I did using the OnAir Solution HDTV-GT receiver (5th-gen) and my notebook computer. Sure enough, I was able to lock up and read PSIP from WTEN, WMHT, WRGB, and WCWN. Each DTV station was multicasting a pair of minor channels, two of which were local weather forecasts and Doppler radar. Local news and weather aren’t yet available on my brother’s DirecTV system, which only carries New York City channels.
Tests with an older (3rd- gen) ATSC set-top receiver weren’t as encouraging. I couldn’t find one antenna position where all four stations came in reliably. After mulling over the matter, I decided to move the antenna down — not up — to see if I couldn’t peak up the refracted signals.
Figure 4a-b. The final antenna position (top),
nestled in amidst the other RF catchers from DirecTV (bottom).
Sure enough, I found a spot on the deck support about 4 feet below where I ran my initial tests where four of the five waveforms looked solid. Tests with the older set-top box showed I could now find positions where three of the four stations were reliable, and all four were rock-steady with the HDTV-GT.
A quick trip (well, about an hour, actually) to the nearest Radio Shack in Bennington provided the needed 5’ mast section and mounting brackets. After a few hours of drilling and routing the coaxial cable, the house was now receiving local UHF DTV stations just dandy.
Discussions with Fred Lass, the chief engineer at WRGB-DT, revealed that the pesky hill just ½ a mile away was the only significant obstruction between the house and Helderberg Mountain, which is southwest of Albany in the foothills of the Catskill Mountains.
Before you ask – no, I didn’t think to bring a VHF antenna to see if I could also pick up WNYT-12 (NBC) and WXXA-7 (FOX). That will have to wait for a future trip, but my guess is that with some gain in the antenna and a VHF preamp, they should be receivable as well. The index of refraction will certainly be different for high-band VHF channels than UHF, but hopefully not significantly different.
The point of all this? Just because a given reception location appears to be very unfavorable to terrestrial DTV doesn’t mean it will turn out that way, particularly now that 5th-generation technology is widely available in both set-top boxes and integrated HDTV sets.
Figures 5a-d. Top to bottom: OnAir Solution HDTV-GT screen grabs of
WTEN 10-1, WMHT 17-3, WRGB 6-1, and WCNW 45-1.
I’ve seen similar results in other deep valleys, specifically Laurel Canyon in Beverly Hills, along the foothills of the San Gabriel Mountains near Los Angeles, and right below the Ramapo Mountains in Ringwood Lake, NJ. In each case, I picked up several if not all DTV signals using either the short yagi with a preamp, or a 4-bay collinear UHF antenna.
The trick is to have enough of a signal to start with. The original ATSC standard called for a minimum carrier-to-noise ratio of 15.3 dB for 8-level VSB, and it looks like the 5th-gen chipsets are finally hitting that benchmark. In my case, the receivable signals measured from 32 dB to 40 dB C/N, with multipath echoes creating notches as deep as 10 dB in one instance.
WYPX-DT didn’t make the grade because its signal topped out at about 24 dB C/N and had many notches across its carrier, carrying from 2 to 6 dB, which is just not enough for the receiver to work with. (It doesn’t help that the WYPX transmitter site is also quite a few miles farther west in Amsterdam, NY!)
If you live in a rural area but have some digital TV stations located within 50 miles, it’s worth a try to see if you can pull in some of those elusive signals. One advantage of rural locations is that there generally aren’t any restrictions on outside antennas, and if your path is blocked by a mountain range, the knife-edge trick might pull you through.
One excellent, brand-new Web resource to get you started is www.tvfool.com, which is maintained by a former television engineer. You can find out which stations are on the air near your location. The site will also provide a Longley-Rice signal propagation chart for you after you enter your street address or latitude and longitude, which you can get from Google Earth.
(Oddly enough, the UHF stations I was predicted to receive at the Vermont location were not receivable, and the UHF stations I did pull in weren’t listed on the chart I was provided. Fortunately, I had a spectrum analyzer to “sniff around” for RF.)
