The coil in the antenna does three things:
1) It gives the antenna a little springiness so it
snaps back if bent slightly.
❑ R1 12K
❑ R2 100K Pot (*)
❑ R3 12K
❑ R4 470Ω
❑ R5 30K
❑ R6 47K
❑ R7 47Ω
❑ R8 4.7K
❑ R9 3K
❑ R10 4.7K
❑ R11 240Ω
0.1 μF (*)
2) Series inductance makes the electrical length
of the antenna longer than its physical length.
The wavelength (λ) at 100 MHz is three meters, or
about118 inches. Power radiated would be higher
if the electrical length was equal to λ. To make that
happen you would need to calculate the required
inductance and the inductance of the coil.
❑ Q1 MPSH 10 (*)
❑ Q2 MPSH 10 (*)
❑ V1 MV104
❑ IC1 78L05 (*)
3) It looks cool.
If you don't want a whip antenna, then a
length of stranded insulated hook-up wire will
work. Use a length that gives you the best range;
maybe 20 to 30 inches or so.
❑ C1 470 pF
❑ C2 0.1 μF
❑ C3 470 pF
❑ C4 0.1 μF
❑ C5 470 pF
❑ C6 0.1 μF (*)
❑ Printed Circuit Board
❑ Electret Microphone
❑ Battery 9V
❑ Battery Clip
❑ Two-Pin Male Header
0.1 inch centers
❑ Header Jumper
❑ Hook-and-loop Strip
Mount the Battery
First, solder the battery snap-connector to the PCB
near the antenna end as shown in Figures 9 and 10.
Next, get a length of hook-and-loop strip (RadioShack
#64-2345 or equivalent) and cut off a one inch piece.
Separate the two halves. Peel the backing off one half and
attach it to a 9V battery. Peel the backing off the other
half and attach it to the back of the PCB. The half on the
PCB should be positioned so that when the battery is
attached to the board, the assembly will stand up on a flat
surface (see Figure 10).
The on-off switch is a shorting jumper seen in the
upper left corner of Figure 9. When placed across the
two-pin header strip, it connects
power to the circuit. When the
jumper is removed, power is
disconnected. (Note: The small loop
of bare wire soldered to the board
in Figure 10 is a place to clip the
ground lead of a scope or meter.)
transmitter is tuned. You might have to adjust the radio's
tuner slightly for best reception. When done, remove the
jumper to conserve the battery.
Have fun with it! But remember that using the transmitter as a bugging device may not be legal. To use the
transmitter as a wireless microphone, increase the value
of R3 to reduce the sensitivity of the electret mic. I've
used it over a distance of 100 feet inside a building, and
it worked fine. NV
The PCBs and/or a complete kit with SMT or TH
components for this project can be purchased through
the Nuts & Volts Webstore. See ad on page 90.
You will need a portable FM
radio. Also, it helps to have an
assistant. First, find an empty spot on
the FM dial and set your radio to that
frequency. Have your assistant take
the radio about 30 feet away. The
radio's volume control should not be
set too high to prevent feedback; if
the radio has an ear-piece, use it.
Next, power up the transmitter.
Talk to yourself as you adjust the
frequency with the trim-pot (talk
softly, the mic is very sensitive).
When your assistant hears you, your
September 2008 45