Ramsey Electronics FM25B Assembly And Instruction Manual page 23

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:
You can see that there's not a great difference in antenna length from 88 to
107 MHz. Some antenna designers have the view that an "approximate" di-
pole 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 di-
pole 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 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 suit-
able gain antenna. See Appendix A concerning FCC field strength limitations.
An FM- VHF TV receiving antenna could be 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. Our new TM-100 Tru-match FM an-
tenna is an ideal mate to your transmitter. It features proper impedance
matching for optimum power transfer and range. See our catalog for details.
ANTENNA ALTERNATIVES
If your situation involves a single large building or multi-level home where re-
ception from the FM25B antenna may tend to be uneven because of walls and
other VHF path obstacles, you might set up the FM25B's output in a "carrier-
current" configuration. If you know how, then do so - safely. If not, you can
show your FM25B and this book to a licensed radio engineer and negotiate
with that person for a safe installation which will feed your signal through inte-
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
FM25B • 23