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Guidelines for Designing or Transitioning Systems to Digital Operation

Joshua King on all-digital AM

In a recent ebook, Radio World explored the implications of allowing all-digital operation on the AM band in the United States. Here’s an excerpt in which Kintronic Labs Project Engineer Joshua King shared his thoughts on the idea. Read the ebook here. Also, readers are invited to send a letter to the editor on this or any subject.

Joshua King, project engineer for Kintronic Labs, cites bandwidth guidelines for designing or transitioning systems to digital operation.

“The guidelines are important, because if the load impedance characteristics are not symmetrical, this can complicate the error correction process and erode the reliability of the digital transmission channel,” King said.

“It’s more important for antenna systems designed for digital operation to maintain really good bandwidth, so that all the energy in the sidebands, which contains the data needed by receivers to reconstruct the digital signal, makes it through. This can help with fewer blackouts, regions where you can’t get the signal.”

[We Look Forward to the Day of All-Digital AM]

Consider the whole system, he recommended. This is key to get the best digital performance out of your antenna system.

“So working back from the antenna, we need to consider the tower and mast characteristics. Is it a guyed tower? Is it a self-supported tower? Is it series-fed? Or is it skirt-fed? If it’s skirt-fed we need to know complete design details,” King said.

“Take into consideration the matching networks, the phasors, your intermediate matching units (IMUs), your antenna tuning units (ATUs) and the phase shift associated with each of those networks; also the feeder system including your coax from the transmitter to the ATUs.

The exciter and transmitter also need to be considered because the output filter network has a certain phase rotation associated with it.”

King offers ideas to improve the bandwidth of the system; these are summarized below. However, “If there’s one point I’d like to get across, it’s that it’s not that difficult to transition your system to digital operation. It’s probably easier than people think. There are a lot of options.”

[Finally a Solution to the “AM Problem”]

Guy Wire Top Loading — Towers with narrow faces, and guyed towers that are electrically short, typically have narrow bandwidth, King said. Top loading can improve the bandwidth. “It’s like stretching the tower, and it makes it electrically taller without making it physically taller. It’s a cost-effective way to do this.”

Rhombic Skirt Feed on a Guyed Tower — King said a rhombic skirt makes a tower seem fatter and can yield optimum normalized impedance characteristics for digital broadcast. This approach is particularly useful when multiplexing several frequencies on a single tower.

Broadband Folded Unipole Design — Quality of the design is important with this technique. “A good unipole can really improve your bandwidth, and a poorly designed unipole can hurt your bandwidth.” These are also useful for putting other equipment on an AM tower such as cellular infrastructure.

Phase Rotation Methods — “This can be as simple as adding a single element like a capacitor or inductor,” King said. “Sometimes you need more control, and you can add a full T-network or an L-network to get that extra control of the match and phase rotation that you need.”

Sideband Impedance Improvement Methods — Frequently used methods for new systems include broadbanding techniques such as slope correction, pre-matching and cascading networks. For existing systems, King said, be sure to check the design of current filters and remove unused equipment bonded to the tower; at WWFD, he said, incorrectly tuned filters affected the impedance of the overall antenna system.

King also shared links to products made by Array Solutions and Rig Expert that he said are affordable and lightweight, for use in troubleshooting or tuning an antenna system.