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

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. (See Ionospheric
Messenger #6.)

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.
(To search Google for information on lightning arrestors,
click here.)