Instruction manual

TM100 Page 4
INTRODUCTION TO THE TM100
At Ramsey Electronics we are constantly bombarded with questions on how to
get the micro power transmitters to go as far as possible without worry about
FCC rules being broken. Here is the answer all you callers and others
interested in getting the most out of the FM micro-transmitters. We have come
up with a sturdy, easy to build folded dipole antenna with wide band
characteristics perfect for transmitting or receiving.
HOW DOES A FOLDED DIPOLE ANTENNA WORK?
A folded dipole antenna is a pair of wires that are suspended between two
points. The wire is cut to a specific length, and is fed in the middle with a radio
frequency (RF) signal. In our case the folded dipole has an impedance of
around 300 ohms, while we are using 75 ohm cable to feed it. We use what is
called a matching transformer to convert the impedance from 75 ohms to 300
ohms, which also gives us a better bandwidth where the antenna is efficient at.
We also add a ferrite choke core on the coaxial cable to prevent radiation along
the coax feed line. This little core really does the trick in providing a proper
match to your receiver or transmitter.
As an RF signal is applied to a piece of wire, both an electric and a magnetic
field is set up around the wire. These waves propagate through the air (or
ether!) without limit. As these magnetic and electric fields reach another
station’s antenna (or about any other metal object in its path) the exact reverse
effect takes place on our antenna that our RF signal did. The rapidly changing
fields produce an RF current that your receiver can detect.
Although it is impossible to actually see the waves coming off of a dipole, we
can try to visualize it to understand it better. Take a piece of paper and push a
pen or pencil through the center. Hold it up in front of you. Let’s pretend that the
pencil represents our antenna, and the paper represents where our signal is
concentrated. As you can see, most of the radiation is given off at a 90 degree
angle to the antenna, with very little radiation occurring parallel to the pencil. It
is this property that makes the dipole directional. Another advantage is that the
radiation is at all angles around the wire, causing the antenna to cover all
directions.
If you have any questions, there are many excellent books on antennas
available at any good electronics store. The classic ARRL Radio Amateur
Handbook is also an excellent resource on antennas.