Drones Taking RF Testing To New Heights

Broadcasters typically think of unmanned drones in television as aerial newsgathering platforms, but as nearly 1,000 U.S. broadcasters prepare to relocate to new channel assignments some may soon begin rethinking the role drones play in television broadcasting.

On May 5-6, LS telcom of Bowie, Md., used its specially designed and equipped drone to conduct signal strength measurements of two Dallas-area stations from the antenna farm at Cedar Hill, Texas, about 17 miles southwest of the city center.

The testing, which validated the horizontal and vertical transmission patterns of a newly installed antenna, is believed to be among the first uses of a drone to conduct TV antenna validation testing in the United States.

“While this is the first measurement we have made in the United States, LS telcom through its subsidiary Colibrex has measured over 800 different frequencies at a variety of sites in 15 different countries,” says Casey Joseph, VP of LS telcom.

Using a drone for the tests was done in place of more conventional methods that are more time-consuming and expensive, he says.

“In a day or two, we can come in and give broadcasters a very good idea of how their new equipment is operating,” Joseph says.

“Typically, these signal strength tests consist of taking measurements at hundreds of points throughout the coverage area [with bucket trucks] and then working that data backwards through a propagation analysis to determine the transmission pattern of the antenna,” he says.

“With our UAS [unmanned aircraft system] broadcast measurement drone, we are able to come in and get a very accurate measurement that is close in to the tower.”

Joseph declined to identify the Dallas stations because of a nondisclosure agreement with the broadcaster.

For the Cedar Hill test, the LS telcom flight crew measured the horizontal and vertical components of the stations’ antenna signals from an altitude of about 1,500 feet.

The flights were done with a waiver from the Federal Aviation Administration of its Part 107 rules, which normally impose a ceiling of 400 feet for drone operation, he says.

Signal strength data was collected during a series of flights, which taken together created an orbit of the antenna. The distance from the tower varied from 3,900 to 4,900 feet during testing, Joseph says.

This type of drone-based signal strength testing can be especially important in helping to identify the source of antenna problems, he says. “This is a great diagnostic tool. There could be an incorrect down-tilt angle on the antenna or something faulty in the transmission line, such as a problem with a splitter,” he explains. “That has a particular fingerprint that shows up in the horizontal and vertical pattern files we create from the measurements.”

At the 2017 NAB Show, John Kean, a consulting engineer with Cavell Mertz & Associates, presented “Development of an Unmanned Aerial Vehicle for Measurement of Broadcast Antennas” during the Broadcast Engineering and Information Technology Conference.

For a little more than a year, Kean and Gary Cavell, president of the engineering consultancy, have been at work building an RF measurement drone to do near-field signal strength testing of TV and radio antennas. By the fall, the firm expects to be offering the service commercially, Cavell says.

“Our thesis is antenna manufacturers do a fine job of building the antenna properly, testing it on the range and shipping it,” he says. However, rough handling or some sort of accident in transit, a misstep during installation or even someone accidentally damaging a connector while undertaking ordinary maintenance can create problems.

“That’s where the onsite measurement program can be helpful. Finding out if something changed,” he says.

Kean, who was involved with signal strength testing of Sutro Tower in San Francisco using a manned aircraft in the early 1990s says using a drone offers the benefits of lower cost, faster turnaround and fewer ground reflections.

“Drones can fly at a potentially much smaller radius around the transmitting antenna,” says Kean, “so the angle of reflection from the ground is extremely steep — almost perpendicular [to the ground] so the efficiency of reflection is low.

“Transmitting arrays also tend to radiate very little energy straight down the tower. For both of those reasons ground reflections should have less impact on measurements done from drone platforms.”

Cavell, Mertz & Associates and LS telcom are both using highly customized drones for RF signal strength measurements.

Both have started with robust airframes designed and manufactured by commercial drone vendors for industrial use, and then take special steps to shield both the RF instrumentation used for measurement and the flight navigation system from the high RF environment in which the drones work.

Signal measurements are taken with extremely small spectrum analyzers, data is stored using an onboard computer and removable memory.

Cavell, Mertz & Associates is at work on its second generation of drone.

Cavell predicts that validation testing of antenna operation for new antenna systems installed for the repack will be rather small.

(Cavell, Mertz & Associates will not do any signal strength testing paid for by the Broadcast Relocation Fund as part of the TV spectrum repack because of the role it’s playing in assisting Ernst & Young, which the FCC has selected to be the fund administer, Cavell says.)

Dennis Wallace, a partner and technical consultant with RF consultancy Meintel, Sgrignoli & Wallace, concurs.

With the original analog-to-DTV transition, many broadcasters requested this sort of testing to make sure their digital transmission pattern replicated their analog coverage, he says.

Transitioning from DTV transmission on one channel to another channel won’t likely motivate broadcasters in the same way, he says, adding that this sort of signal strength testing may be done by fewer than 10% of the nearly 1,000 U.S. stations being repacked.

However, LS telcom’s Joseph disagrees. “When there is new equipment installed, testing will be required as station operators will want to assure consistent coverage,” he says.

“New channel assignments also present new interference potential with neighboring stations. Station operators (who may also be operating at a new transmit power) will be best served to test their coverage area,” he says.

“We anticipate that the broadcast industry will leverage a combination of UAS broadcast measurement and conventional test point field measurements to verify their equipment operation.”   

However, both Joseph and Cavell agree that drones will be used for testing as the industry transitions to ATSC 3.0 — especially with single frequency networks.

“I think SFNs might be one of the places where this will be quite handy,” says Cavell. “It is a very, very touchy situation to get these things [individual SFN sites] to work and play well together. The data collected from drone signal strength measurements will help a lot.”

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