RFBM's Radio Tower

A tall tower is necessary for effective Part-15 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. On the Black Rock Desert playa we get such a place like few others on the planet, but the reality of Part-15 transmitter signal limits mean that the effective border for the signal should be no more than a 2450-foot sphere around the antenna. Part-15 limits keep the station legal, but they do not limit the quality of the signal within that allowable border. Tower height affects the performance of a radio signal as it approaches the receiver and reflects off of objects and structures around the receiver. The greater the angle of attack the signal has from being elevated above objects and structures, the less interference the reflections will cause when they get to the receive antenna.

Elevation of the antenna is only one part of engineering radio coverage within the boundary. The antenna system used 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 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 (much greater than Part-15). The legal standard is to specify horizontal polarization of the signal. Any vertical component is allowed to equal, but not exceed the strength of the horizontal signal. 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 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. RFBM uses a home-made, delta-match, crossed dipole design pioneered in the 1970's by the Jampro Corp. This system means that RFBM can use twice as much transmitter signal into an antenna and still be Part-15 legal because the measurement standards call for vertical or horizontal readings, but not both.

One antenna is not enough. When two adjacent antennas are transmitting the same signal and are phased to present a combined wave front, the receiver will perceive a higher signal strength. This is called antenna gain and is specified in a license to control the exact signal strength to cover a given area. While it is common practice in professional broadcasting to design an antenna array with multiple bays to increase the effective radiated energy, thereby decreasing the amount of transmitter power (electric bill) required to fulfil a given license, another benefit is realized, that of space-diversity. The two adjacent antennas are spaced only a half-wavelength on the tower, but the receiver sees a kind of shadow canceling effect. This is like looking at shadows from a light bulb in a dark room. Turn on one light and it produces hard shadows in one direction. Place another light bulb nearby and the shadows get more light than before, increasing the coverage. RFBM has used a two-bay array using 75-ohm, 3-quartrer-wavelength, coaxial phasing set. This is simply two lengths of 75-ohm RG-11a coax cable, cut to 3/4 of the wavelength of 99.5MHz corrected for velocity factor, connected to a tee connector on the main transmission line and connected equally to each bay. This is the simplest form of antenna phasing.

1999, ESDR camp is 90-deg from RFBM on the Center Camp Circle and so is the tower. 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 its height to be changed from 60-feet to 40-feet on site. This one was lifted by hand by station staff members.

This 40-foot configuration was cobbled together at the last minute to accommodate space restrictions for 2003.