This month, I'll provide some guidance on the optimum ratio of horizontally to vertically polarized power for TV broadcasting and examine some special situations where the use of elliptical polarization can fill in coverage. Also, readers sent several photos showing different construction methods and reports on performance in response to my description of the Gray-Hoverman antenna design earlier this year.
THE OPTIMUM POLARIZATION RATIO
Once the decision is made to transmit an elliptical polarized signal, the next decision is how much power to devote to the vertically polarized (v-pol) signal. In many cases, the amount of v-pol power will be based on the amount of extra transmitter power available after the desired horizontal polarized (h-pol) effective radiated power (ERP) is achieved. It could be argued that some vertical power, even if it is only 15 percent of the h-pol signal, is valuable, providing polarization diversity on obstructed paths and avoiding huge signal losses with cross-polarized mobile antennas.
For unobstructed paths, coverage with vertically-polarized receive antennas will be determined largely by the amount of vertical power. If mobile TV is the primary business model for the station, true circular polarization (1:1 H/V ratio) at the highest effective radiated power possible should be the goal.
|Chart 1: Experiment results—More than 4 dB of margin improvement with 20 percent < v-pol < 50 percent. ©2008 SPX Corp. |
For obstructed paths, where signal strength is more of a problem than with unobstructed paths, you may have noticed from last month's chart showing the results of Dielectric's tests that a pure horizontally polarized signal was 1 dB better than a pure vertically polarized signal in relative variability over margin ratio. This implies that if transmitter power is limited, there is a point where decreasing the h-pol ERP to add v-pol ERP will reduce the margin and probability of service when obstructions are present.
Dielectric's research found the optimum v-pol/h-pol ratio, for a fixed sum of h-pol plus v-pol power, was 33 percent of the total power, which equates to 50 percent of the h-pol power. (See Chart 1.) It is important to note that this ratio is based on a fixed combined ERP—transmitter power and antenna gain are fixed. This doesn't mean more v-pol power hurts coverage or provides little additional improvement, provided the h-pol power is not reduced. For stations operating at the maximum h-pol ERP, any additional v-pol ERP will improve reception to vertically polarized antennas. VHF stations are likely to find it improves reception on UHF antennas as well, but more on that later.
SPECIAL APPS FOR ELLIPTICAL POLARIZATION
Even if mobile/portable reception isn't your main objective, there are situations where adding some v-pol ERP will improve reception. One case is where two transmit antennas at slightly different locations on a building/mast need to be combined to provide full coverage. There are examples of this at the Empire State Building in New York City and Sears Tower in Chicago. Because the antennas are not co-located, there will be overlap locations where the two antennas will combine to produce nulls whenever the received signal level from the two is the same and the two signals are 180 degrees out of phase. Careful antenna design can minimize interference areas and move interference over less populated areas. By using elliptical or, ideally, circular polarization on one of the antennas and not the other, viewers in the interference area may still be able to get reception on a vertically polarized receive antenna.
Adding vertical polarization may also improve VHF reception on UHF-only antennas. EDN Senior Technical Editor Brian Dipert described the challenges he had receiving DTV at his rural location in Nevada in a series of blogs titled "Thin Air ATSC (And NTSC)." See the August archives of "Brian's Brain" for copies of the columns at www.edn.com/blog/400000040-August-2008.html.
He tried the Antennas Direct C-2 (two loops in front of a reflector) which is advertised as having "consistent gain through the entire DTV channel spectrum" even though the gain plot for the antenna starts at 400 MHz. In response to Brian's question about this, Antennas Direct President Richard Schneider said, "Since it was not possible to incorporate a VHF element into the design without severe compromises to the UHF performance (electrical coupling and insertion losses), the solution the engineers came up with was a redesign to the PCB balun to allow the feed line to act as a high VHF radiator. This will give modest VHF performance, between 174–216 MHz. We have found it to be a significant improvement over our traditional UHF/VHF combination designs."
Of course, in most cases the antenna feed line will be oriented vertically and more sensitive to vertical polarization. Dennis Wallace of Meintel, Sgrignoli & Wallace displayed test results in his NAB paper "Measurement Results of Consumer Indoor Antennas" showing the RCA ANT585 indoor antenna was more sensitive to vertically polarized signals than horizontally polarized signals.
|Fig. 1: G-H antenna built by George Morisette |
These examples show that some antennas in some cases will do a better job with a vertically polarized signal than a horizontally polarized signal.
READERS BUILD THE G-H ANTENNA
Earlier this year, I discussed the Gray-Hoverman (G-H) UHF antenna in RF Technology ("TV Receive Antennas," May 1, 2008). The article is available on the TV Technology Web site, but you can also find information on the antenna at www.digitalhome.ca/ota/superantenna/index.htm. I've posted a drawing with dimensions for the antenna at www.xmtr.com/articles/RF182-fig1.pdf.
Ray Tanner, chief engineer at WOHL-WLQP-WLMO-WFND-TV in Lima, Ohio, wrote to say he worked for Doyt Hoverman in 1964 repairing TV sets in his TV store, after he made the antennas. Ray said the original Hoverman antennas had yellow insulators, while the ones made and sold by AntennaCraft had black plastic insulators.
Ray said, "Also, Doyt said that his antenna used only one wire per side versus five per side for the four x antenna. This resulted in less corrosive damage to the connections."
Several readers built the antenna and were kind enough to e-mail me pictures of their antennas and the testimonials on its performance. If I were giving awards for G-H antennas, the award for best use of available materials would go to George Morisette for his use of scrap 8281 coax for the reflectors, 12 gage copper wire cutoffs from electrical installations for the driven elements, and scrap foam for the spacers. Fig. 1 shows a picture of his antenna. Comparing response on analog video carriers from his home northeast of Sycamore, Ill., he found signal strengths the same to 2 dB better than a Channel Master 4-bay bow-tie.
|Fig. 2: G-H antenna built by Tomas Colon |
Tomas Colon, an amateur radio operator in Puerto Rico, would win the award for best looking antenna. Fig. 2 shows his antenna in the air. He found that it works well on VHF as well as UHF channels, with less directionality (of course) at VHF. It even provided a satisfactory signal on low VHF channels! Tomas has built another version of the antenna using a screen reflector supported by PVC tubing.
Robert Lynch, assistant chief engineer at the Fox affiliate in Roanoke, Va., built a G-H antenna with a screen reflector (Fig. 3) and says, "I have built everything from Yagis to discones to single bay bow-ties, and stacked and phased bow-ties with and without reflectors. I have also built stacked and phased dipoles, also with and without reflectors. I thought I had tried most all practical UHF antenna types; that is until I saw your article on the Gray-Hoverman design. I built one based on the specs given with exception to the reflectors, and I must say that this antenna kicks butt!" He said, "The antenna in the pictures is the best performer of all of the UHF antennas I use, be they home-brew or store-bought. I can turn this antenna 180 degrees from the transmitters and do a channel scan and not one digital channel is detected."
Lynch offers these details on the design: "Mast is 1 inch PVC. The driven elements are mounted on a back plane of Lexan sheet, which is screwed to the mast with self drilling screws. The driven elements are wire tied through holes drilled through the Lexan sheet on both sides of the elements. The reflector array is made from a welded frame of steel rods salvaged from an old tomato plant cage with 1/4-inch nuts welded at 3 points inside the welded frame as mounting points for the reflector to the boom. The reflector is made from 1/4-inch hardware cloth tied to the frame with stainless steel tie wire, and it is mounted to the mast with long 1/4-inch bolts so I can adjust the depth of the reflector up to 4 inches in relation to the driven elements to see what effects it may have. The reflector edges are angled in at about 25 to 30 degrees making a kind of square frying pan shape in hopes of attenuating any RF from the sides, as this seems to give it a narrower beam width."
|Fig. 3: G-H antenna with a screen reflector built by Robert Lynch in Roanoke, Va. |
If I were giving awards, Robert Lynch would get the award for innovative design as well as most enthusiastic response. Lynch's improvements could make this the ideal receive antenna in on-channel booster systems. The high front-to-back ratio should help viewers located between two transmitters in a single frequency network eliminate interference from one of the transmitters.
It's obvious many RF Technology readers are interested in antenna design and experimentation. While some viewers will install outdoor antennas to get the best reception, that isn't an option for many viewers in urban and suburban areas. Even though the FCC prohibits restrictions on outdoor antennas, people I've talked to don't want to risk a confrontation with their homeowners' association. My challenge to readers is to come up with an easy to build indoor receive antenna design that ideally works on VHF as well as UHF. I've tested the RCA ANT1500, the Artec AN2 and AN2A flat antennas, and whip antennas. While these are more compact and easier to install, none has equaled the performance of the Terk HDTVa amplified indoor log periodic antenna. More on this in a future column.
Even though time constraints may make it difficult to respond to all e-mail, I read all comments. Your question could provide the seed for a future RF Technology column. E-mail me at Dlung@ transmitter.com.