Antennas: The necessary evil

Old towers have a tendency to be fully loaded based on their original design criteria. No realistic engineer can argue that panel antennas don't work
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Old towers have a tendency to be fully loaded based on their original design criteria.

No realistic engineer can argue that panel antennas don't work well.

Antennas are somewhat like a heart beat, every station must have one. With the gleeful implementation of DTV, most stations enjoy the pleasure of having two. Again like a heartbeat, if an antenna isn't there or if it doesn't work right, everything else is moot. Without the antenna, the only viewers will be those who receive the station by a direct cable feed. That is similar to being on a heart-lung machine. It works for a while but doesn't last in the long haul.

At a recent seminar by Zenith, the problem of meeting the Commission-designated sign-on date for commercial stations was discussed in some detail. Based on the number of stations that have yet to sign on their digital facilities, approximately 100 stations per month must start broadcasting with some DTV facility from now until May 2002. In comparison, making pigs fly might be easier.

That returns us to the problem of the antenna, transmission line, tower and construction scheduling. Right now, antennas are residing in storage or on trailers waiting installation. Last winter was unusually difficult in many of the Northern states for tower construction or antenna installation. Numerous construction crews have experienced more down time than usual due to bad weather and wind. Therefore, the whole DTV scheme may have suffered regarding its time table. As discussed in this column previously, there are a limited number of crews capable of working on very tall towers. Yet, a lot of new towers were necessary to accommodate the many new antennas. Old towers have a tendency to be fully loaded based on their original design criteria. The addition of one or more new antennas caused modifications to be needed that, in many cases, required a new analysis in accordance with ANSI/EIA/TIA 222F. Guess what? Many old towers simply can't comply.

The point of this is very simple. If a station does not have its DTV antenna and the necessary tower work on order and, hopefully, scheduled with some margin for delay, the odds are high and growing that it won't be on the air by May 2002. The transmission line probably isn't as big a problem as the antenna and tower. There are several manufacturers out there for that product and the delays indicated by those manufacturers don't seem to be unworkable. The antenna is a problem. Some manufacturers are backed up for several months.

If an order is placed today, the antenna will probably not be delivered before late fall — just in time for winter. If normal delays are encountered in finishing any needed tower work due to the winter weather or if any difficulty is found in getting a tower crew scheduled, the May sign-on simply isn't going to happen.

The Commission is offering an out. Stations will be allowed to sign on with reduced facilities as long as they place a required level of signal over their community of license. To do that, a construction permit must have been granted and a Special Temporary Authority (STA) must be applied for. Your station's consulting engineer can help determine the minimum power and height necessary to meet that requirement. However, an antenna still will be needed along with transmission line and a transmitter. With less than a year to go, timing is going to be a problem.

The station is faced with two choices. One is to purchase an antenna that will be used only for the temporary facility at lower power. Some stations are looking at the smaller antennas that were originally low-power TV antennas and that have been beefed up to handle higher power. Indeed, there are a lot of stations using those antennas for the long term and they seem to be working out very well. Those antennas depart from the traditional main antenna systems in that they are side-mounted and use semi-flexible transmission lines from one or more power dividers to the elements. Power handling capabilities of 30kw or 40kw are available for that type of antenna. Its gain and VSWR performance is very good and the price is fair. At the lower power levels, the use of 3-inch or 4-inch transmission line is common, with some stations simply using semi-flexible transmission line instead of rigid.

Some stations are using this option with the intent to replace the lower power antennas with larger, high-power models at a later date before the big channel give-away. That will allow them more time to make the larger purchase and make any needed tower modifications. Indeed, when one of the channels goes away, the options are available of either using the old main antenna on the original channel for DTV or replacing it on the tower with a new main antenna on the DTV channel. Remember, in most cases, the station may select its original channel as the one to keep and give up their DTV assignment.

The problem with purchasing a small antenna and line for temporary DTV use is that it will have very limited value when taken out of service. Used transmission line is often avoided by stations unless they are totally familiar with its history. Used antennas have an extremely limited value unless a buyer can be found who needs that specific channel. It also should be remembered that there will be a glut of used antennas on the market when all stations go back to one channel, when and if that ever happens.

The other choice, and probably the better one if the DTV channel is to be chosen for retention by the station, is to buy the higher-powered antenna and line now even if it is to be operated at lower power for some time. That would result in the older NTSC channel being the one to be discarded with less long-term cost to the station. While that might be an attractive option, it returns the station to the big problem of the earlier paragraphs — if it isn't on order now or very soon, it may not be possible.

There are alternatives. The first and most obvious is to share an antenna with one or more other stations. The traditional hardware for sharing would be the panel antenna and that is still a very viable option. No realistic engineer can argue that panel antennas don't work well. There are simply too many of them in operation, even right here in this country, for that argument to be supportable. Of course, shared antennas of any type, including panels, require some compromise between users in selecting the design performance. But, they will do the job. In addition, almost all of the major manufacturers now have two-channel antennas in their product line for first adjacent channels. Those certainly apply to many stations that received either n+1 or n-1 assignments. The dreaded n+1 problem has dropped by the wayside as several manufacturers now have combiners available that have solved the filtering problem. That includes the major houses such as Dielectric, RFS and Andrew.

There are other antennas available that will permit operation with a slot type radiator over a broader range of channels. For example, the Dielectric/TCI slot panel radiator covers the entire UHF band. RFS has a slot antenna that covers up to a 10 channel range. Units such as these permit a great deal of flexibility in sharing antennas between stations. There is at least one application currently under consideration that would combine four adjacent channels with a fifth non-adjacent channel into one antenna. The antenna isn't the problem in this case but the combiner is going to be a bit of a problem.

It is time for the engineering departments to push a bit harder. While it is realized that the suits up front may be having a problem determining to authorize the necessary orders, it is time to leave the golf course alone for a little while and make some decisions. Two things are certain. One is if the orders aren't placed soon, there will be a lot of stations not on the air with DTV by May 2002. The second is that if the station doesn't make the magic date, the engineer will be blamed for missing the date without regard to memos, pleas, manufacturing delays, tower crews or weather. Someone is going to take the heat and you can bet it won't be the front office. Get those resumes ready.

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

Send questions and comments to: don_markley@intertec.com