When Sample Loops Just Won’t Do
ATLANTA — When
the FCC approved Method of Moments as a valid tool to verify AM directional
performance, I was immediately interested in employing the approach at CBS
Radio’s WAOK(AM) 1380 in Atlanta. However, because WAOK had a mix of tower
types, and by virtue of their height, we had to wait just a bit longer to
VSU-1 is at work in the field at the WAOK(AM) transmission site.
I had heard that Kintronic Labs was
working on a voltage sampling device specifically for stations that could not
employ sample loops.
WAOK is a 5 kW NDA-Day/4.2 kW DA at
night. The night antenna system consists of two self-supporting towers and two
uniform cross-section towers, each 360 feet tall.
In December of 2010, I was contacted by
Tom King at Kintronic Labs. They had just finished developing the VSU-1 voltage
sampling device. I was interested in testing these devices on WAOK.
A few weeks later the VSU-1s arrived at
our facility. The initial inspection of the units revealed that installation
would be fairly easy. Each VSU has an N connector, to attach to the existing
antenna monitor. There is also a bowl insulator and threaded stud to attach to
the tower base. This connection is routed through a J Plug, so the VSU-1 can be
installed and then connected to the antenna when ready. The J Plug is also
handy during the testing phase. A grounding point is also provided.
Each VSU was attached to the tower using copper tubing. The
existing sample lines were disconnected from the old loops and attached to the
Physically, the VSU can be installed in
a number of ways. Each VSU has four “ears” that enable the user to select the
best method for installation. In our case, two of the VSUs are installed using
galvanized fence posts. At the other two towers we elected to install the units
on the sides of existing equipment at the tower base.
Each time I connected a VSU-1 to a
tower, I checked the parameters of our antenna system. In three of the
installations, the VSU-1 had little or no effect on the antenna system. On the
fourth tower, we did detect a slight change in the tower parameters. This was
attributed to numerous items across that tower base. We were able to “wash”
this change out in the day and night LTU.
The WAOK array.
Consulting engineer Don Crain came in
and did extensive measurements of the WAOK antenna system. This included
verifying sample line length, transmission line length and the self-impedance
of each tower.
Armed with this information, Don
established the theoretical MoM parameters we would need to achieve the
pattern. The night phasor was adjusted to these theoretical numbers.
The next day, we went to the “old”
monitor points toverify that the night pattern was correct. To my amazement, all of
the points were in tolerance. We then ran the required set of reference
measurements for the array. We obtained a license for the MoM operation in late
spring of 2011.
These VSU-1 units survived a stormy
spring and summer here in Atlanta. In late April, our Tower 3 was apparently
struck by lightning. The top beacon was heavily damaged, including vaporized
wiring, burned sockets etc. The VSU-1 connected to the base survived and to
this day is providing stable samples of the tower. A peek inside the VSU-1
revealed no damage from this strike.
The VSU-1 Voltage Sampler is designed
for easy installation and appears to handle “real-world” conditions with ease. For
stations using tall towers or other nonstandard antenna configurations, the
VSU-1 offers a way to move forward with MoM, and take advantage of the
significantly easier monitoring of directional performance.
author is market chief engineer with CBS
For information, contact Tom King at
Kintronic Laboratories in Tennessee at (423) 878-3141 or visit www.kintronic.com.