The Omni vs. Directional FM Antenna

In the real world, most antennas are in fact directional

The use of directional arrays in FM broadcasting has increased greatly in 30 years. Especially in the NCE reserved band, a DA can increase the operating power of a station significantly while still respecting the interference ratios that are at the heart of an effective regulatory scheme.

Consultant Clarence Beverage provided this photo of a directional antenna system serving WFUV-FM3, a 2.5 kW booster for Fordham University from the roof of a Durst Organization building in midtown Manhattan. The yagis are Aldena AST.05.02.336 five-element models. The E and H plane patterns are very close, making them candidates for slant 45 degree elements to achieve equal horizontal and vertical ERP.

However, there are complexities and caveats to using directional arrays. Some stations with high values of licensed DA power don’t necessarily deliver desirable coverage. At the heart of every DA is a specially designed antenna that requires a different approach by the FCC, one that is often misunderstood.

We explored the topic with several industry consultants.

First, some caveats about the use of a DA. The FCC will not authorize the use of just any pattern that stays within the limits of interference. “The FCC has a number of rules regarding use of directional antennas,” said Gray Haertig of Gray Frierson Haertig & Associates. “When using a DA to protect domestic allocation, the licensed relative field pattern may not change more than 2 dB per 10 degrees over the protection azimuths, and the maximum to minimum ratio may not exceed 15 dB.”

FCC regulations for directional antennas are very different from those covering omnis. A station requesting to build a DA must provide certification from the antenna manufacturer that the pattern has been measured and tested on a range under identical mounting conditions to the proposed tower mount. Most important are differences in how the commission licenses omni vs. directional arrays.

“Nondirectional stations are licensed by the RMS power, irrespective of the actual power radiated in any given direction. Directional stations are licensed according to the measured power in the maximum,” said Haertig.

“When the antenna is manufactured and tested on the range, it cannot exceed the authorized pattern at any azimuth and its RMS must be at least 85 percent of the authorized pattern.”

Wait, what? Did he just say that the DA I’m building might not provide the full coverage to the pattern limits drawn by my engineering consultant, and the goal is to get it to a minimum of 85 percent? Why am I not getting the 100 percent I’m planning on and need for success?

Simple physics means it is difficult to meet the pattern limits exactly. The FCC allows the 85 percent minimum to give stations some leeway in their antenna designs.

WHAT IS A DA?

It turns out that in the world of real antennas and towers, most antennas are in fact directional.

“It’s well to remember that with the possible exception of panel, Lindenblad or spiral antennas, all antennas are directional,” said Haertig. “The problem is that you have no idea what the pattern of the actual antenna as installed is unless you have had the pattern measured. As I generally tell my clients, if you care what your coverage is, then have the antenna pattern measured.”

Clarence Beverage of Communications Technologies Inc. agrees.

Directional FM antenna systems raise some particular questions. Shown is a Dielectric DCR-H system with radomes.

“Omnidirectional antennas are never truly omni if they are single side mounts, as the pattern is distorted somewhat on a pole or 12-inch tower,” said Beverage. “Get up to 36- to 42-inch-face towers and the patterns start to deviate significantly. Mount on a 7-foot face or wider TV tower and the ‘h.pol’ and ‘v.pol’ patterns may start to look like spaghetti.

“Because of this, omni antennas have to be considered as providing an unpredictable signal if the pattern has not been measured on a test range.”

How does the FCC handle this real-world intrusion into RF engineering?

“That nominally omni antennas are actually directional is something the commission has largely steered clear of addressing,” said Haertig. “They assume that omni antennas are indeed non-directional. Indeed, some nominally non-directional antennas can have as much as 6 dB gain in certain directions.

“As I generally tell my clients, if you care what your coverage is, then have the antenna pattern measured.”

In simple terms, all antennas are directional. But if you require a directional antenna to increase power while maintaining protection to adjacent stations, the FCC requires significantly greater proof that your antenna will meet that protection. While an omni antenna might be allowed to exceed licensed field strength up to around 6 dB in a particular direction based on the way it is mounted, a station with a DA must show that its design prevents maximum field strength from exceeding the licensed maximum allowed in any direction. Six dB … isn’t that four times the power?

It seems like the FCC is giving a fairly strong advantage to the omni antenna relative to how it treats a directional array.

“While it is against the rules to purposefully build an omnidirectional antenna that produces more than a full field in a specific direction,” said Doug Vernier of V-Soft Communications, “an omni antenna, carefully positioned on a tower to prevent coverage holes in the direction of desirable population within its licensed coverage, is perfectly acceptable. Keeping the antenna omnidirectional, therefore, may result in better coverage than accepting a small null in a directional antenna.”

WHEN DOES A DA MAKE SENSE?

There are marketing advantages in being to claim a higher effective radiated power; but putting those aside, there are cases when a DA makes sense.

From the above, it would seem that a conservative rule of thumb for using a DA might be if you can achieve a 4 to 6 dB improvement in operating power toward a desirable market or population concentration in comparison with an omni pattern. For example, a station might have a nearby interference limit thanks to another station in the opposite direction from a major city. By going directional toward the city, this station could put substantial signal into the market while protecting the nearby station within the limits of the 15 dB front-to-back ratio.

“Another situation when a directional antenna should be used is when the station’s tower is close to a high ridge or mountain,” said Vernier. “Using a directional antenna with a null pointed at the mountain will help prevent the station’s signal from reflecting off the mountain and traveling back toward the desired coverage area as co-channel interference.”

A directional array also may offer operational savings. Vernier said that directional antennas are often used by FM stations in coastal areas to place a null over bodies of water where there is no population to be served. “Since directional antennas always have higher gain than omnidirectional antennas having the same number of bays, the transmitter can operate at a lower output, consequently saving a lot of electricity resulting in lower power bills,” he said.

This story is excerpted from the Radio World eBook “Propagation Analysis for Profit,” one of the growing Radio World library of free eBooks.

Haertig said the benefits of a directional approach may include lowering operating costs through reduced radiation towards unpopulated areas; reducing multipath by cutting down on radiation towards prominent reflectors; or increasing power towards a target audience while still staying within the FCC rules of allocation.

“In a situation where a directional antenna is used to reduce radiation towards unpopulated areas, its inherent higher gain means lower transmitter power and may permit using a smaller transmitter for a given antenna aperture.”

Dave Doherty of Skywaves Consulting LLC summed up when to use a DA:

“If you need a null of more than a few dB in the direction of your primary service area, it’s probably not a good idea. Deep nulls often are much deeper than planned, especially when panels are required to create the pattern.

“On the other hand, if there’s not a significant population in the null area, it may well make sense use a DA to reduce interference to stations in the null and increase your ERP in the desired direction. Power-saving is another reason to use a DA if the population in the null isn’t significant.”

And, perhaps most important point of all, Doherty added: “Be guided by a knowledgeable engineer.”



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