This month, I'll take another look at receive antennas for DTV. My article "Antennas for DTV Reception" in the March 6, 2002 column on antennas for DTV reception was published almost four years ago, yet hardly a week goes by that I don't get a request for Fig. 1, which shows the dimensions for a do-it-yourself UHF TV rhombic antenna. The article is available at www.tvtechnology.com in the Doug Lung on RF section. I've reprinted Fig. 1 here. Refer to the original article for information on how TV antennas work and suggestions for building your own.
(click thumbnail)Fig. 1: Dimensions for a do-it-yourself UHF TV rhombic antenna.Although I've received many requests for Fig. 1, I haven't received as many reports on how the antenna worked. I guess some readers found the size of the elements made construction more difficult than they thought or they built the antenna and weren't happy with the performance. As noted in the article, a rhombic that's five wavelengths on a side (about 100 inches at 600 MHz) will provide a predicted gain of approximately 12 dB. You will see later that smaller commercial TV antennas provide gains close to or greater than this.
RHOMBIC ANTENNAS
If you want to build a huge VHF TV antenna, an article from 1951 by Richard J. Buchan may be helpful, (See http://members.fortunecity.com/wtfdamem/Rhombic1.html). The antennas described are much bigger than the one I referred to. Buchan describes how a three-bay rhombic can be constructed with gain of more than 100 (20 dB). He notes that even though rhombic antennas are broadband, separate antennas will be needed for low VHF and high VHF, although the high-VHF rhombic can be strung inside the low-VHF rhombic using the same supports.
The article gives construction details, including a parts list, mast construction hints, dimensions and drawings for single bay and two bay antennas. If you want distant VHF-TV reception in a single direction and have the real estate, check it out!
A "Dual Rhomboid" antenna developed by Edmund Laport of RCA was modeled by L.B. Cebik and determined to have gains between 15 and 16 dB at 1296 MHz. This design should be able to be scaled for use at TV frequencies, (See http://www.cebik.com/vhf/rh.html ). In researching this article, I found what must be the ultimate UHF rhombic antenna, if it works as claimed. The Sveriges DX-FŸrbund Web site ( http://www.sdxf.org/alfa/dxinfo/Antenna%20Articles.pdf ), has a 60-page article on antennas. On p. 35, there is a description of DHR (dual hexamerous rhombic) antenna designed for the 430 MHz band. It should work well at UHF frequencies. The calculated gain of the 2x6 rhomboid antenna is 33 dB! Unfortunately, author Wayne Sarosi said details would be provided in a future posting, but I haven't been able to find it. If anyone can provide details on the DHR antenna, please e-mail me and I'll share the information.
Broadcast engineer Jeremy Lansman at KYES in Alaska has put together an excellent Web page titled "About TV and FM Antennas" at http://kyes.info/antenna/antennadex.html. The page contains links to articles on how TV antennas work, common TV and FM antenna types, and, of relevance to this article, a section devoted to building your own TV or FM antenna.
PRO PERFORMANCE
In my Dec. 14 RF Report, I reported that in the FCC's report to congress on the DTV field strength standards and test procedures for the Satellite Home Viewer Extension and Reauthorization Act of 2005, the commission found no specific changes are needed to the DTV field strength standards and/or planning factors.
The comments filed by the ABC, CBS and NBC affiliate associations in the proceeding included an exhibit by consulting engineer Jules Cohen with a survey of available TV receive antennas and amplifiers. In the report, the FCC cited this survey to show there are antennas currently available that meet or exceed the planning factor criteria. It should be useful if you decide to buy rather than build your own antenna. The affiliate associations' filing can be downloaded using the at http://gullfoss2.fcc.gov/prod/ecfs/comsrch_v2.cgi. Enter 05-182 in the "Proceeding" box and scroll through the listings.
FCC DTV planning factors are based on antenna gains of 4 dB, 6 dB and 10 dB for low-VHF (Channels 2-6), high-VHF (7-13) and UHF (14-69) respectively. Kerry Cozad of Dielectric measured the Channel Master Model 4228 eight-bay bowtie-with-screen UHF antenna and measured gains of approximately 3 dB, 9 dB and 15 dB for low-VHF, high-VHF and UHF. This UHF-only antenna exceeds the planning factor gain at both high-VHF and UHF!
For VHF-only reception, the Antennacraft Model CS1100 claims an average low-VHF gain of 6.9 dB and an average high-VHF gain of 9.6 dB. The Channel Master Model 3610 doesn't do as well at low-VHF, with an average gain of 5.8, but provides a claimed average gain of 11.4 dB at high-VHF channels. While combined UHF and VHF antennas usually do not work as well as separate antennas, the Winegard Model HD7084P specifications show gains from 6.2 to 7.6 dB at low VHF, 10.8 to 12.0 at high VHF and 14.6 dB in the UHF band.
In a table in his exhibit, Jules Cohen shows that if a low-noise amplifier is used at the antenna, reducing the system noise figure to 4 dB, the media noise limited field intensity for DTV reception drops to 19.8 dBµ at low-VHF, 25.8 dBµ at high-VHF and 34.2 dBµ at UHF, based on receiving antenna gains of 6 dB at low-VHF, 10 dB at high-VHF and 12 dB at UHF.
In addition, CEA has an excellent Web site, AntennaWeb.org that allows you to see what TV stations are available at a specific address and what size antenna is required to receive them.
I welcome your questions and comments. Send them to me at dlung@transmitter.com.
