DON MARKLEY /
09.01.2002
Originally featured on BroadcastEngineering.com
Transmission & Distribution: Two or more aren't a crowd

Traditionally, each television station in the United States had its own antenna and transmission line. To a significant degree, each station also had its own tower. Years ago, stations started sharing towers. In fact, stations started going to multiple antennas on a major structure as long ago as the 1960s, when a candelabra was built in Maryland.



The Dielectric TUV-M dualband antenna combines both VHF and UHF signals into a common antenna design. The TUV-M is for mid-band VHF (channels 4-6) combined with a UHF channel.


The driving forces that caused stations to start sharing towers were the scarcity of good sites in congested areas, the opposition of zoning boards and citizen groups t towers (even then), as well as the significant cost of a tall tower. Still, on structures like the Mt. Sutro tower and many candelabra or “T” top towers, everyone had their own antenna and transmission line. There was no real technical reason for this — just that that was the way it had always been done.

At the same time, multiple-station antennas were the rule in Europe and Asia. Granted, these were mostly government-controlled stations and the allocation schemes were somewhat tailored to help get everyone at one location on the same antenna. Still, the technology was good, the antennas worked well and there was no real reason why that type of operation was not accepted in the United States. Multichannel operation was dismissed with statements ranging from “no one is going to tell me what kind of antenna to use” to “it may work in Europe but not over here.” One would almost think that the laws of physics are altered by the Atlantic Ocean.

The advent of DTV has caused a lot of stations to rethink this whole thing. Suddenly the stigma has been almost totally eliminated by the specter of either the cost of a new tower or the unavailability of space for a new tower. So, stations have started to share in a big way.

On the manufacturing side, the antenna makers have responded with a host of new products, as well as the updating of existing products to meet the new need. The first was probably the panel antenna from Allan Dick. They had been providing panel antennas for years in the European, Asian and African markets. A good part of their line came from EMI when that firm was into broadcasting. Some of the first multistation antennas in the United States came from that background. Then domestic manufacturers started handling European antennas to augment their product lines. In particular, Dielectric and Shively were offering panels under their own brand name or as agents for the originating company. Dielectric went on to develop its own antenna that is now in the product line. Shively continues to market the RYMSA product line of multichannel panel antennas. Andrew now handles the Allan Dick line of antennas in the United States, giving them a broader range of VHF and panel antennas.

Harris already had panel antennas in its product line. The cavity backed radiator (CBR) antenna has been used for many years for FM systems, as well as for both VHF and UHF applications where circular polarization was desired. Harris also had a line of panels that worked well in situations where they could be wrapped around the tower. Of course, that entire antenna product line has since been sold to Dielectric.

Panel antennas that would handle multiple channels also are available from RFS Americas, Jampro, MCI, ERI and SWR. RFS, Jampro and SWR have both VHF and UHF versions. Many broadband antenna products have evolved from the low-power world, where multistation operation has been widely used for years. RFS'' line now includes the old but much modified Bogner line. The old Bogner low-power antennas have a bandwidth of around 15 percent of center frequency, permitting their use over a fairly large group of frequencies.

RFS also has their RD series of slot antennas that cover up to eight channels in bandwidth in either omnidirectional or directional patterns. Through their experience in Australia and Asia, RFS has developed a wide range of panel configurations for small to high power levels. Their larger panel antennas, like the biggest RYMSA, have an interior ladder so engineers can go up inside the antenna for maintenance. However, no panel antenna is yet large enough for this author to climb inside, nor is there anyone with a big enough whip to force this author to climb the tower to the antenna.

TCI, a firm long experienced in HF antennas, developed a panel array consisting of stacked cavity radiators. The antenna has full UHF bandwidth with low VSWR and can be made directional. TCI has since joined the Dielectric/SPX corporate family. Dielectric is offering the slot panel antenna under new model numbers as part of their multichannel line. Dielectric also has two-channel slot antennas that have been widely used in situations where a station has first adjacent analog and digital channels. Andrew and Jampro also have two-channel slot antennas.

The primary application of multichannel antennas has been where the channels are in a single group, i.e. low-band VHF, high-band VHF or UHF. The problem is that many stations have VHF analog channels and UHF digital channels, with a few instances where that situation is reversed. To meet that situation, Dielectric has developed their TUV-M and TUV-L antennas. While a bit simplified, they can be described as a super-turnstile antenna mounted on a slot. That is, the cylinder for the slot becomes the mounting structure for the super turnstile. These can be massaged into a number of omni- and directional configurations. One neat feature is that the UHF and VHF portions can have different directional characteristics.

Some facts apply to essentially all of the multichannel single-band antennas. The gain is going to vary somewhat across the band, based on the simple laws of physics concerning the size of the aperture in wavelengths. In a similar fashion, the beam tilt, null fill and directional characteristics will vary slightly across the band. Due to some really good design work by the manufacturers, these changes are small and usually don''t cause any significant problems. However, these are the big drawback for multichannel users. The stations have to agree on a commonly acceptable pattern. One station cannot have an omnidirectional antenna with another having a single large lobe. The stations must also agree to accept the slight variations in beam tilt — again, not usually a problem. These are all problems that can be worked out by stations working with their consulting engineers. Any good firm has all of the information from at least the majority of the manufacturers to help their clients pick an antenna system that will properly serve their market while accommodating other users.

There is a limitation involved in how many stations can combine into a single panel — power. If there are several stations all seeking an ERP of 5MW, the power-handling capability of individual panels, as well as the interior feed lines and power dividers, can be exceeded. The power divider and feed line problem can be handled if sufficient space is available. For the panels, the solution is to add more panels. That means going to five or six panels around rather than three or four. It is also possible to add layers of panels vertically. With good designs, it is possible to do this without the vertical beam width becoming prohibitively narrow.

The main point here is that these multiple-channel antennas work very well. There are dozens of multiple-channel operations running today to support that statement. Those antennas must be a compromise between the preferred designs for any of the individual stations. However, it is usually possible to reach an acceptable compromise through careful selection of one of the many antennas available on the market today. Finally, look at it this way. While you may not end up with coverage that you can brag about as being superior to any other station, they won''t be able to make that statement concerning you. Now, the ratings fight goes back to programming, where it belongs, as you, the chief engineer, confidently tell the suits that the numbers aren''t your fault — it''s the lousy PD who must take the heat.




Don Markley is president of D.L. Markley and Associates, Peoria, IL.

Send questions and comments to: don_markley@primediabusiness.com



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