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Whip and Mast

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Chapter 7

Whip and Mast

"Whip and mast" refers to a short vertical antenna, 2 to 3 meters tall, mounted (along with the transmitter) on top of a much taller conductive support structure. The support might be a section of aluminum electrical conduit or some grounded object that is already in the environment such as the metal frame of a roadside billboard. The antenna, obviously, serves as the antenna, and the transmitter's ground wire is connected to the support structure.
The whip and mast approach is an attempt to exploit the vague wording of 47 CFR 15.219. According to the rule, the combined length of the antenna, the cable that connects the transmitter to the antenna, and the ground lead cannot exceed 3 meters. The exact nature of the ground itself is not specified, and many experimenters have assumed that an elevated object such as a roadside billboard or a metal pole would be permissible. However, a reasonable person could argue that a 20-foot metal pole between the bottom of the transmitter and the soil is in fact a 20-foot ground lead, or the bottom half of a dipole antenna either way, a flaming violation of the Part 15 rules.
Many of the factory-made Part 15 transmitters that are on the market perform best when mounted this way, and in some cases the instructions that come with the units advocate the whip and mast approach. For example, the famous advertising transmitter on 1570 kHz in Tustin, California used this type of set-up. An engineer from a licensed station claimed he heard this unit in a neighboring city and asked the FCC to investigate. The station survived two inspections by FCC field agents.
In a Radio World article (June 24, 2000), Alan Peterson described his experiments with an AM2000 unit made by LPB. When he clamped the transmitter-and-whip assembly to the railing of his front porch, he got poor range. When he clamped it to the chain-link fence at the edge of his property, the results were even worse. Then he clamped it to a 20-foot section of one-and-a-half inch diameter metal electrical conduit. (The bottom 18-inch section of the conduit was "planted" in the soil.) With his whip antenna raised above the rooftops in his neighborhood, he got a dramatic increase in range.
Finally it's worth mentioning that a 100 milliwatt transmitter mounted near the top of an 80-foot bell tower was heard (by a well-equipped listener) 10 miles away.
It's not clear why whip and mast set-ups work so well. Are the ostensible ground structures actually becoming de facto antennas and radiating some signal, or is it just a case of plain old line-of-sight radiation coming from the whip antenna and working best when the antenna is high and in the clear, as is always the case with line-of-sight? (If the latter were applicable, wouldn't the manufacturers put a set of ground radials under the transmitter and suggest mounting the assembly on a tall non-conducting support?)
Some whip and mast installations have been allowed to continue operating by some FCC field agents. However, Mr. John A. Reed of the FCC Office of Engineering and Technology wrote the following comment in response to a query: "If the installer/user places the transmitter atop a metal pole or other conducting structure and grounds the transmitter to that structure, the height of that structure also is counted toward the three meters maximum length." (Radio World, August 2, 2000) In view of this published opinion from the OET it's a little unclear why the manufacturers of FCC-approved transmitters are advising their customers to put the rigs in whip and mast installations.
Because they often achieve a very good coverage radius, whip and mast stations are often inspected by FCC field agents. Anyone using the whip and mast technique should do it with an FCC-certified transmitter. The field agents have been more liberal when it comes to interpreting the rules if the transmitter is factory-made and has an FCC ID number stamped on the back. Even so, don't be surprised if they tell you to switch to a more "down to earth" antenna system.
Whip and mast installations are lightning targets. Anyone using this type of set-up should take proper precautions.

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