Solving VHF DTV Reception Problems
Many TV stations with high-band VHF analog channels are moving their DTV transmissions to those channels after analog is shut down on June 12. Some have already made the transition and found viewers that were receiving DTV on UHF previously had problems with reception on VHF channels. Fortunately, rescanning solved many complaints, but not all.
This month, I'll look at reception options for VHF TV, both indoor and outdoor, new Gray-Hoverman antenna developments, and take a look back at some VHF antennas that worked well in the 1950s. My focus will be on high-band VHF channels, as few stations will be using low-band VHF channels post-transition. INDOOR VHF ANTENNAS
Indoor VHF DTV reception is a challenge. According to the analysis Bill Meintel presented at the IEEE Broadcast Technology Symposium last October, VHF stations need a field strength close to or stronger than that of UHF TV stations for indoor TV reception, (see "2008 IEEE Broadcast Symposium in Review," Dec. 4, 2008). They must do this with effective radiated power capped well below that of UHF TV stations—160 kW versus 1,000 kW. The two major issues affecting indoor VHF reception are low gain antennas (usually "rabbit ear" dipoles") and noise. Noise can come from many sources in the house—appliances, microprocessors (in computers and other devices), video displays, and the increasingly popular energy efficient compact fluorescent lamps.
Let's look at the antenna first. Given the space constraints on top of the TV set, it will be very difficult to obtain more signal from one of the fancier passive indoor VHF antennas than what you'd obtain from a set of rabbit ears, assuming they adjusted to the correct length for the VHF channel being viewed. The combined length of the two elements ranges from about 31.75 inches at channel 7 to 26.375 inches at channel 13. How many indoor antennas, other than rabbit ears, have you seen with a linear dimension greater than 30 inches? There are smaller indoor antennas without additional elements (directors or reflectors) that are able to receive VHF broadcasts, such as the RCA ANT-1500, which can't match the "gain" of a set of rabbit ears without amplifiers.
|Winegard's Indoor Antenna Model SS-3000 |
Winegard has taken steps to improve the VHF performance of its indoor antennas and offers some interesting options. Its top performing indoor antenna is the SS-3000. Unamplified gain is specified at –15 dB for channels 2 to 6, –5 dB for channels 7 to 13 and zero dB for UHF channels. It includes an amplifier that provides 7 to 10 dB gain over VHF and UHF channels with a noise figure of 1 dB or less. The SS-3000 is only 26.75 inches long and from the pictures, it appears the driven element is smaller than that, so the negative gain at high VHF is understandable. VSWR is 2.2–2.3 at high VHF and UHF. This will make a difference in reception.
Other indoor antenna options from Winegard include the GS-2200, which at 46.5 inches should more closely match rabbit ear performance; and the HD-1080, a two-bay bow-tie with extended elements for VHF. Specified VHF gain of the HD-1080 compared to a dipole ranges from –11 dB at channel 7 to zero dB at channel 13. VSWR is not specified.
If rabbit ears work so well, why consider a much more expensive antenna such as the SS-3000? First, the SS-3000 offers some directionality. If the desired TV signals come from the same direction, this directionality may help reduce noise pickup. Second, the rabbit ears work best only if resonant at the TV channel being received. Move from channel 7 to channel 13 without adjusting the antenna and the VSWR will increase.
While the coax from the antenna to the tuner is short enough that increased attenuation with higher VSWR is not likely to be a problem, as has been pointed (and debated) in previous articles, the mismatch could cause a reflection that stresses the equalizer in the DTV tuner. If the signal is strong enough, adding a 3 dB attenuator to reduce the mismatch at the tuner may improve reception. Antennas like the SS-3000, which have low VSWR across the band, shouldn't have this problem.
Adding an amplifier solves the tuner mismatch problem and also offsets the reduced gain of the VHF antenna. As a result, indoor antennas with amplifiers are recommended in all but the strongest signal locations. One potential problem is the amplifier required at VHF may overload with stronger signals at UHF due to the higher antenna gain.
Dennis Wallace from Meintel, Sgrignoli & Wallace reported on indoor antenna performance as measured in the anechoic chamber at ERI. The SS-3000 performance exceeded its specifications, showing –3.8 dB gain at channel 7. This is actually better than the measured performance of the simple ANT-115 rabbit ears, which showed –5.2 dB gain, 1.4 dB worse than the SS-3000. This is likely due to the ANT-115's measured –5 dB return loss compared to –15 dB return loss for the SS-3000.
Noise will be an issue for VHF reception in many homes. If analog VHF pictures have any sparkles or lines in the picture from in-house devices, digital reception may be difficult. Re-orienting the antenna or identifying the device causing the interference and moving it or turning it off may help. When talking to viewers complaining about losing VHF reception at night, ask if they are using compact fluorescent lamps. Adding an amplifier will overcome the low gain of the indoor VHF antenna, but if noise is present it will amplify that as well with no net gain in the ability to receive a DTV signal.
Pete Putnam, an HDTV expert, tested a variety of indoor and outdoor TV antenna, (see www.hdtvexpert.com, for details and spectrum analyzer plots). His pick for an indoor antenna was one I've used and recommended, the Terk HDTVa. His tests didn't focus on high-band VHF reception, although he noted that adding an amplifier to the Terk as well as an RCA ANT-1450B raised the noise on VHF channels by 20 dB, likely due to noise generated in the house. He suspects this will cause reception problems when WABC-TV, WPIX and WNET return to their analog VHF channels. OUTDOOR ANTENNA OPTIONS FOR VHF
The disparity between VHF and UHF stations should drop when an outdoor antenna is used, provided the outdoor antenna is designed for VHF reception. Unfortunately, viewers that installed UHF-only antennas for DTV may have problems. VHF stations transmitting a circularly polarized signal, or even an elliptically polarized signal may have better results as the vertically polarized component may be received on antenna coax or the phasing harness on a bow-tie array. Fortunately manufacturers are recognizing there will be VHF DTV signals after the transition and are introducing new antennas that work better at high-band VHF. Small antennas—while claiming VHF compatibility—offer little gain at VHF.
Putnam said, "While reception of WFMZ-46 and WLVT-62 isn't particularly difficult at my location, WBPH's signal on channel 9 requires a decent VHF antenna to pull in reliably. Only the CM2016 was up to the task, providing solid reception with or without the Titan 2 preamp. Winegard's SS-1000, equipped with the Titan 2, delivered an erratic signal with frequent dropout, while the SS-2000 couldn't hack it at all."
The CM2016 is not a high gain antenna, but it works well in suburban locations like Putnam's. For reception in more rural areas, you'll need a bigger antenna. Some multiband antennas are for high VHF and UHF bands only, allowing more gain at high VHF in the same or less space. For example, AntennaCraft's HBU44 has a 116-inch boom and claims 8.2 dB gain with a 50 degree beamwidth on channels 7–13.
The smaller HB33, with an 85-inch boom, has a specified gain of 7.6 dB on channels 7–13. While their slightly longer HD-1200 (120-inch boom) channel 2–69 antenna has 8 dB gain on channels 7–13, the low-band VHF elements make it significantly wider, with greater wind-load. For a high-VHF only antenna, the Winegard YA-1713 has a 100-inch boom and specified gain ranging from 9.1 dB to 10.3 dB. A high-VHF only antenna like the YA-1713 combined with a separate high performance UHF antenna like an 8-bay bow-tie or the Antennas Direct C4 would be my choice for distant signal reception. DO-IT-YOURSELF
From the response to my past columns, many readers want to build their own TV antennas. I previously reported on the success readers had building the Gray-Hoverman UHF antenna. You will be pleased to know that the experimenters on the Over-The-Air Digital Television/Antenna Research and Development sub-forum at www.digitalhome.ca are working on a VHF version of the Gray Hoverman antenna. They have even been able to reduce its size by using fractals. There are many variations on the design, so I won't provide a drawing here, but you can find all the details, including NEC models, on the Antenna Research and Development sub-forum.
One of the designs resulted in an antenna that has an overall width of 70 inches and an overall height of 86.91 inches. There are also some pictures of the antennas. If you don't want to page through the forum searching for the information, do a Google search on the words "Gray-Hoverman VHF fractal antenna" without the quotes.VINTAGE VHF ANTENNAS
While researching VHF antennas, I came upon a Web site old TV aficionados will enjoy: www.earlytelevision.org
. Check out the page with antennas from 1950–1960. I remember seeing the Finco "Bedspring" antennas as a child in western Maryland and being impressed by their size. We had one of the stacked conical antennas shown on the same page and I remember the pictures from WRC and the other Washington, D.C. stations looked fine on our black and white set. I'm not sure of the model but I think it was a Zenith.
Finally, while considering what you will do with your analog transmitter, take a look at the size of the 1 kW "high power" transmitter sold to CBS for broadcasting color using the CBS color field system on the Web page
. The transmitter operated on 490 megacycles and is larger than any UHF transmitter I've seen in use today. We've come a long way! I welcome your comments on VHF reception with indoor and outdoor antennas, even ones from the 1950s! E-mail me at firstname.lastname@example.org