So many stations, so few antennas
Stations are still having problems finding room on their towers for DTV antennas. Many stations are realizing that the only solution to the problem is to eliminate the old antenna and replace it with a new one that will handle both the existing analog signals and the new digital signals. Combining signals into a joint antenna system doesn’t require more tower space nor does it significantly increase tower loading.
Richland Towers’ dual-purpose antennas on the Richland Atlanta broadcast tower stand 120 feet above the tower. The stack on the left shows these two broadband antenna systems, both carrying DTV and NTSC signals.
VHF + VHF
Perhaps the simplest of the combined operations can be found where both analog and digital signals are VHF and occupy the same portion of the band. That is, they are both either high-band or low-band VHF (either 2 to 4 or 5 to 6). For this application, the familiar batwing or superturnstile antennas are highly usable because of their excellent bandwidth.
In some cases, a batwing permits two stations to use the same antenna with a minimum of additional hardware. The stations, if analog, must be diplexed prior to the conventional hybrid combiner. Then, one station is connected to one of the inputs and the second station to the other input. The result is two output signals, separated by 90 degrees in phase, connected to the two inputs of the batwing.
If you want more flexibility in the antenna, the conventional solution would be to use a VHF panel array. In that configuration, it is easy to design a directional antenna to meet protection needs. Panel antennas also offer good power handling and bandwidth characteristics.
Some broadcasters have complained that panel antennas have too many components that can cause maintenance problems. Granted, such a configuration requires power dividers, matching networks, multiple feed lines, etc. But the good old batwing antenna has more parts than a ’36 Ford when you add in all the lines, straps, dividers, etc, yet broadcasters accept it as fully reliable. Panel antennas, both VHF and UHF, are just as reliable when properly designed and installed.
The real issue here is the installation. Someone with experience installing batwing antennas must install it or supervise the installation. Grounding and panel adjustments are critical to its performance and reliability.
VHF + UHF
The next combination is a little more difficult to deal with. When one station is VHF and the other is UHF, your choices dwindle. The obvious solution is to install two antennas. You can stack one antenna on top of the other, side-mount one antenna and top-mount the other, or use either a “T” top or candelabra on the tower. Most manufacturers can provide a VHF antenna on a sufficiently strong mast to support a UHF antenna on top. The transmission line is normally routed on the outside of the lower mast, which does have a minor effect on the antenna pattern.
Dielectric offers a more sophisticated approach to the VHF/UHF problem. The company has developed an antenna that will work with either low-band VHF and UHF or high-band VHF and UHF. In both cases, directional patterns are available and the two signals can be tailored somewhat independently of the other – a neat trick not normally available in multiple-station antennas.
By the way, you can find all of these antennas on the manufacturer’s Web pages. Try www.Dielectric.com, www.Andrew.com, www.Shively.com and www.Jampro.com for starters. Some of these sites offer a lot of really neat free software and catalog information.
UHF + UHF
Now for the final case: every station on the antenna is UHF. We’ve discussed this issue in these pages before, but some developments make it worth mentioning again. First, broacasters’ continuing requests for multiple station antennas that don’t have the windloading of a mainsail have prodded manufacturers to look for more bandwidth. The result is that several manufacturers have developed antennas that will cover a limited range of channels. Dielectric’s TFU-TC (three-channel) series offers omnidirectional or directional patterns. RFS’s RD series offers good VSWR over a range of up to 20 channels. Others have bandwidths of two to eight channels. Again, we advise the reader to go the appropriate Web pages for full details.
But before you make a mad dash for the catalogs, you need to evaluate carefully exactly what you’re using. If your station antenna is an old, extremely high-gain slot antenna, the bandwidth is 6MHz +/- about 1Hz. You can’t add another channel. But many newer antennas with reasonable gain have acceptable properties on a channel adjacent to the channel for which it was designed. For example, WTVP in Peoria, IL, operates on NTSC 47 with a DTV allocation of 46. The station bought its Andrew antenna within the past few years and has had fairly good response over the second channel. In this case, the station’s measurements of the antenna system indicated that the antenna would perform reasonably well if the station could solve a couple of problems.
WTVP uses circular waveguide for the vertical run on the tower. The input and output couplers on the waveguide didn’t have sufficient bandwidth. In addition, the fine matchers at the output coupler/antenna input didn’t offer sufficient range to handle the necessary 12MHz. Andrew designed and manufactured new couplers and fine matchers that offered acceptable VSWR performance. Calculations indicated that there would be increased beam tilt at the lower channel – about one degree more than the original design. But the antenna was designed for moderate gain, resulting in a fairly wide lobe in the vertical plane. The antenna also had 10 percent vertical polarization. The overall system worked remarkably well. The station didn’t have to buy a second antenna, and it didn’t have to modify its tower to support one. The station reports that its viewers’ DTV reception has been excellent.
Of course, the panel array is now widely used for multiple UHF stations. And many manufacturers offer them. Some are quite interesting. For example, RFS makes a panel antenna that has a ladder inside in case it requires maintenance. The test technician simply opens a port in the bottom of the antenna and climbs up to the point of the problem. This works best if the worker is slim, trim and physically fit.
RYMSA makes an even more unusual antenna. It is marketed in this country by Shively. The antenna uses Superturnstile (batwing) elements for UHF. The entire assembly is inside a large radome that has vortex-shedding ridges incorporated into the radome itself. It has a maintenance ladder inside and the diameter of the radome is over five feet. Of course, the windload is a bit on the high side. But its performance is very good, with minimal variation of the non-directional pattern.
One last thing you must consider is the antenna’s power-handling ability. If the antenna must accommodate five or six stations, each with an ERP of 5MW and non-directional patterns, well…lots of luck.
The best power-handling capability available is about 150kW average power. That’s ok for one or two high-power stations, depending on the pattern, and some lower-powered DTV stations. The ideal situation would be a bunch of approximately 200kW DTVs. One panel antenna can handle enough of those for a medium-sized city. The problem here is the power-handling capability of the individual panels along with their transmission lines. The size of the internal lines is restricted by limited amount of space available. The same applies to the power dividers.
When doing the original planning, work with the manufacturers carefully to determine the power budget on the antenna. Remember, as you modify beam tilt and null fill, the power distribution to the various panels will vary. The hottest panel will ultimately determine the power rating of the antenna. There is one additional advantage to panels: you can make some really weird designs. For example, beam tilt and null-fill amounts can vary with azimuth to comply with the terrain around a site. So far, patterns must stay essentially the same for all channels, but don’t be surprised if someone comes up with a cure for that problem as well.
Don Markley is president of D. L. Markley and Associates, Peoria, IL.
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