AM DAs Benefit From Advances
The destructive forces of Mother Nature often
create opportunities for manmade technological improvement.
Such was the case for Jim Dalke, contract
engineer for KKDZ(AM), a Radio Disney station in Seattle. Fire destroyed most
of the Rose Hill main transmitter building in 2005, including the six-tower
phasing and antenna monitoring systems. KKDZ 1250 shares the site with KARR
Insurance covered the hardware replacement; but the need to rebuild the
DA monitoring infrastructure presented a unique chance for Dalke and Steve
Lockwood of Hatfield & Dawson Consulting Engineers to develop something groundbreaking.
will discuss their innovation for the legacy AM service in detail at the NAB’s Broadcast
Engineering Conference, on Tuesday April 9.
“When two engineers spend a lot of time on a
project, they can get esoterically creative,” Dalke said.
“Discussions included ways to replace the miles of coaxial sample lines
and better methods of tuning the array. We considered the possibility of using
fiber optics to reduce the costs and improving reliability, plus different ways
to graphically display the directional parameters during the laborious and
time-consuming phasor adjustments to bring each tower in each mode into licensed
studies and experiments were conducted at the rebuilt Rose Hill facility in
Kirkland, Wash., using one of the KKDZ nighttime towers. A subsequent
proof-of-concept project was completed on one tower of the daytime array of KTBK
1210 kHz Auburn, Wash. A complete system is now being installed at the three-tower
directional array of KXPA, Multicultural Broadcasting, 1540 kHz in Bellevue.
chose multi-fiber heavy-duty cable suitable for direct burial to feed the digitally
converted tower loop samples back to the phasor and antenna monitor. The cost
of the multi-fiber cable is generally less than the traditional heliax type
cable used for DA sampling systems, Dalke says.
The heart of this completely digital antenna monitoring system is based
on FPGA (field programmable gate array) technology.
“The custom antenna monitor uses the latest high-speed Linux-based
processor and employs very fast IOs and bidirectional data buses,” he said. “High-speed
A-D converters are installed right next to the tower sampling device outputs
and operate at 50 megasamples per second with 14-bit resolution, driving the
fiber at a data rate over 700 MHz.”
The FCC rules regulating directional antenna systems currently are based
on the use of coaxial cable in the sampling system and analog antenna monitors.
With the fiber optic sample system and digital antenna monitor, many of the
technical concerns related to cable length and temperature coefficients are
advantage of the fiber optic sample cable is the ability easily to traverse the
insulated base of a series-fed tower with high RF voltages present without
disturbing the impedance.
Dalke advises, “These advancements in antenna monitoring technology will
require revisions to the FCC rules that govern sampling systems and antenna
monitors. The new antenna monitor will require FCC type-approval per
47CFR73.53. The design specifications for the new AM should easily exceed FCC
The fiber optic system is well suited for arrays designed with the new
MoM (Method of Moments). Dalke points out, “The FCC rules (73.155) require
recertification of the MoM licensed directional antenna every two years to
verify the integrity of the antenna sample system. The fiber optic sample
system has a function in the digital antenna monitor to calibrate each of the
fiber optic sample cables to determine the precise electrical cable delay in
real time. This will eliminate the need to manually test the sample cable
periodically as required in 73.155.”
1 shows a display on the digital antenna monitor representing the relative
phase and amplitude of a three-tower array. The boxes represent the FCC limits.
Tower 1 is the reference tower. The measured parameters from Tower 2 are out of
tolerance and are represented by a red dot. The parameters from Tower 3 are in
tolerance and represented by the green dot. The antenna monitor has an internal
Web server with Wi-Fi capabilities. This makes it possible to use a laptop or
tablet for adjusting the phasor or even making adjustments at the ATU while
remotely viewing the directional parameters.