Peanut whistle DTV
Jan 1, 2004 12:00 PM, BY DON MARKLEY
It is common knowledge that many stations have delayed broadcasting at full authorized power and are operating under an STA at much lower power levels. The FCC's only requirement is that they provide a city-grade signal level over the city of license. These little stations are doing a pretty good job, and the service is much better than many expected. The required signal levels for low-power operation are 35dBu for low-band VHF, 43dBu for high-band VHF and 48dBu for UHF, all based on FCC F(50,90) values.
 Broadcasters should remember that lightweight, low-power antennas require more maintenance. Photo courtesy Dielectric. |
By the way, “peanut whistle” is a derogative term that used to be applied to any ham station operating with low power, primarily those hams that felt they needed a kilowatt to communicate across town.
Since the commission determined DTV standards, naysayers have predicted that the system wouldn't work and that the horizon was a definitive limit on any possible service. All had one belief in common — that DTV was going to be a terrible disappointment and wouldn't work in any acceptable manner. Basically, they were wrong.
Many stations are finding that they are covering their cities of license better than they ever anticipated, operating at low power levels. DTV is turning out to work, and work well.
Back to low-power operation — one needs to realize that it isn't necesary to have an antenna weighing tons to have good electrical performance.
We seem to forget that the reason for having big, massive antennas is because of their mechanical strength, not necessarily their electrical performance. If we desire a high-power nondirectional top-mounted antenna, we need a physical structure that will survive a hostile environment while maintaining its electrical characteristics. You can't do that cheaply. However, you can side-mount a lightweight antenna, feed it with flexible cables and still get excellent performance at limited power.
With panel and other flex-line antenna types, the power divider sets the initial power limit. Remember, the goals here are performance and low cost. A small power divider coupled with 7/8- or 1-5/8-inch lines will handle input powers in the 5kW neighborhood. This can easily result in ERP values of 100kW or so, which will serve a good-sized area when coupled with reasonable height.
On the negative side, effective radiated power values of 1MW are not going to be possible. While the suits may not be interested in that level of power now, they might be once NTSC goes away and they realize that some stations in their market have more power than they do.
Also on the negative side, more maintenance will be required on these antennas. The flex lines are more susceptible to lightning damage than larger rigid lines. They also need to be inspected periodically to ensure that they are being properly supported to avoid damage from rubbing on tower members. This isn't a big deal — batwing antennas and panels have been in use for years, with lines running all over the place. It's just that they must be maintained, even in an era when management seems to think one technician is enough to handle five or six transmitting plants.
As to coverage, the FCC describes a manner of using F(50,50) curves to generate F(50,90) curves. The problem with this analysis is that it is based on the average terrain between two and 10 miles from the site, along however many radials are to be studied. The commission says to use 36 radials to determine city service. However, any interference calculations still must be based on the Technote 101 method, commonly known as the Longley-Rice propagation model. Longley-Rice uses the actual terrain elevation along each radial all the way from the transmitter to the point where one wants to determine the field strength. This method is much more accurate in determining actual service than the older FCC curves. Unfortunately, it isn't a method that easily can be done with just the curves and a map. It requires massive number crunching.
To accurately determine signal strength using this method, you will need a reasonably fast computer, one of the many programs commercially available to run the study, and a database containing terrain elevations for the area of concern. The least expensive programs are identified as 30-second files, which contain the elevation of the area or the whole country for every 30 seconds of latitude and longitude. The next move up is to a three-second database, which contains the elevations of the logical points.
Perhaps the simplest way to handle this is to go to either the station's consulting engineer or to one of the many services that will perform that calculation for you. If all that is required is a simple coverage map or study, the price should be a small fraction of the cost of acquiring the software and databases.
Most engineers will get their biggest surprise when they compare the service area and population covered between operation with 100kW ERP and 1000kW ERP. For something really eye-opening, calculate the cost per person that is added to the service area by that power increase. It will give you a whole new respect for peanut whistles.
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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