Antenna Ideas - Ramsey Electronics FM25A Assembly And Instruction Manual

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ANTENNA IDEAS

A simple, yet very effective, antenna for the FM25A consists of a "dipole", set
up either horizontally or vertically, and connected to the transmitter 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
107 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 for-
mula 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 calcula-
tions.
If you are equipped to make the field strength measurements required by Part
15 FCC rules, and if you think it would be best to aim or "focus" your signal in a
narrower direction, you can 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 modified for such a purpose.
FM25A • 20