Station Staff On Air Technology

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RFBM's Radio Tower


A tall tower is necessary for effective FM broadcast coverage. Line-of-site operation is only one aspect of trying to place an antenna as high as possible. Line-of-site states that what you can see from the tower without land obstruction well receive a transmitted VHF signal. For a 60-foot tower, the line-of-site rule translates into an expected range of about 12-miles over perfectly flat terrain which exhibits the true curvature of the earth.

Elevation of the antenna is only one part of engineering radio coverage. The transmitting antenna system employed can greatly enhance the receiver's perception of how the signal is being transmitted. Circular polarization has been used in the FM broadcast industry for decades. In real-world FM licensing, the transmitter's power is calculated to radiate a given amount of signal through a given antenna and a given transmission line connecting the transmitter to the antenna to achieve a specified signal strength away from the antenna. If the antenna is constructed in such a way as to radiate an expected horizontal signal and also an equal vertical signal, phased 90-degrees from the incident horizontal, then the antenna is said to be circular-polarized. The advantage to having the vertical component is that when the horizontal one reflects of an object or terrain, the vertical won't reflect the same way and will help to cancel the interference caused by the reflected signal at the receiver antenna. There are numerous designs for balancing the vertical and horizontal components and radiating them evenly around the antenna.

One antenna element is not enough, and several bays are stacked to improve the overall system gain. Adjacent bays are spaced a half-wavelength vertically from each other, which is not as good for system gain as full-wave spacing, but reduces the downward radiation near the site. The photo on the right shows a rare 1999 four-bay arrangement, although, in the end, only two bays were connected in this one. The standard RFBM antenna is a two-bay array connected with a 75-ohm, 3-quarterr-wavelength, coaxial phasing set; the simplest method available.

RFBM has 60-feet of radio tower available for several configurations. First installed as a 50-footer in 1995, this tower has been used in various configurations to hold the station's antenna array as well as serving several years as the support for the Black Rock City Communications antenna farm. In the fustercluck year 1997, the tower was set up in only a 30-foot configuration. From 1998 through 2000, the tower was installed in the Emergency Services Communications camp. In 2001 ESDR acquired its own crank-up tower, and RFBM has operated the tower on its own ever since. After severing with the BMORG, RFBM has had only modest success at setting up this tower.

1999, ESDR camp was 90-degrees from RFBM on the Center Camp Circle and with the tower. This was the first use of all six tower sections, and the guying was strong and tight.

2002, BM Org-supplied tower, when BMIR and RFBM shared space. When powered lifting equipment is available, a maximum number of antennas and hardware are applied while it is on the ground. This minimizes the climbing work needed when the tower is up.

2003, The tower is a pretty versatile structure, allowing for a height reduction from 60-feet to 40-feet on site. This one was lifted by the hands of station staff members, without the use of a crane.

This 40-foot configuration was cobbled together at the last minute to accommodate unexpected space restrictions for 2003. 2005 had a 30-foot height. Other years used small masts or only as much as 20-feet of tower sections secured to the RFBM building. The 2004-2008 era had significant logistical and cost challenges which eliminated the possibility of even thinking of running the station's antenna at an adequate height above the ground.

2005: 30-foot tower

2006: No tower, 15-foot mast on main building

2007: 20-foot mast for automated station in a box

2008: Antennas attached to main building above roof line

2009-2011: No RFBM

2012 saw the return of the tall tower, this time at 50-feet with a top pipe extending another seven-feet.
This installation provided a single pylon on the top for mounting a two-bay antenna and used two sets of guy wires. Setup was accomplished in two days by a crew of only two. Since no crane equipment was available, a 30-foot basic portion was tilted into place with the help of some passers by, and secured with the lower guy set. Two more tower sections were raised with a gin pole. With the top guy set secured, the antenna pylon was raised with the gin pole. This new setup method works well, but will need more than a two-person crew in the future.

Screw anchor installation and removal is now handled with an electric motor-driven tool. The anchors in use are 48-inches long with a 6-inch auger. This tool runs on 120V AC power and can install an anchor into the ground in about 15-minutes. Two people are needed to hold the tool and cheater bars are added to the handles for driving the anchor the final two-feet. Removal of an anchor is somewhat quicker as putting it in takes time to get the auger to grab enough soil. The anchors exhibit approximately 6000-pounds of holding force in the Playa soil.

RFBM camp from the top of the tower.
These vapor-proof industrial lamp fixtures serve as obstruction beacons on the tower. At the short heights of our tower, even within the distance to the BRC airport runway, they are not required to mark the structure, but they add a visual element to the tower, reinforcing the appearance of the station as a broadcast empire. The lamps are 75-watt clear envelope, E-24 base. BRC residents often thank us for having such universally useful nighttime markings to aid pedestrian navigation in the city. We've been doing this with our antenna structure in one way or another since 1994.
A reasonable effort at grounding the tower and coax feeds has become a recent obsession. Although not fully implemented, this ground method has shown a significant reduction in charge differential between the tower and the transmitter equipment. That's a good start.

2013: No RFBM  




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