FM100B
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7
ANTENNA IDEAS:
With the one watt output of the FM100BWT you must use a properly matched
external antenna. It is mounted on the rear F connector.
Ramsey Electronics now offers an antenna called the “Tru-Match FM Broadcast
Antenna”. This antenna has been specifically tested and designed for the FM-
series of micropower transmitters and will help you get the most out of these
products. When you call, ask for the TM100 antenna.
A simple, yet very effective, antenna for the FM100B consists of a "dipole" (set
up either horizontally or vertically) connected to the transmitter RF output jack
through a few feet of coaxial cable (either RG-58, RG-59 or miniature RG-174,
available at Radio Shack and other sources). Correct dipole lengths for major
sections of the 88-108 MHz band are:
88 MHz, each side: 2.7 feet; 5.4 feet total
98 MHz, each side: 2.4 feet; 4.8 feet total
108 MHz, each side: 2.2 feet; 4.4 feet total
You can see that there is not a great difference in antenna length from 88 to
108 MHz. Some antenna designers have the view that an "approximate" dipole
such as 2.5 ft. on a side will do fine, while others believe it is worth the effort to
calculate the length for your exact frequency, using the simple formula of
Length (of one side, in feet) = 234/Frequency in MHz.
If the dipole is installed vertically, the end connected to the center conductor of
the coax should be the upper (higher) end. If young children will be around the
set-up, a flexible wire antenna is preferable rather than rigid tubing.
A "ground plane" antenna can be quite effective. A ground plane consists of
one vertical element (the same length as one side of a dipole) connected to the
center conductor of the coax. Four "radials" are connected to the shielded side
of the coax at a 90 to 135 degree angle to the vertical element. The dipole
formula is also used to calculate the length of the radial; since radials should be
slightly longer than the main element, use 240 rather than 234 in your
calculations.
If you are equipped to make the field strength measurements required by Part
15 FCC rules, and you think it would be best to aim or "focus" your signal in a
narrower direction, consult an antenna handbook and design a suitable gain
antenna. See Appendix A concerning FCC field strength limitations. An FM-
VHF TV receiving antenna could be easily modified for such a purpose.
Ham radio books and magazines are filled with antenna principles and ideas
which can be adapted to your application. Also, you may wish to look at Radio
Shack book No. 62-1083 on antennas.
