■ FIGURE 5. The
■ FIGURE 6. The inner conductor and its
insulating jacket were pushed through an
opening in the braid. I carefully spread
several strands of the woven braid apart
until there was an opening large enough
for the inner conductor and insulator.
APRS.fi and FindU.com. So, a smart phone or notebook
PC equipped with a wireless Internet card can provide
tracking reports to a chase vehicle while it remains inside
The length of the dipole’s elements influences the
antenna’s AC resistance (impedance) and radiating
pattern. The dipoles I recommend are 5/8 wave, or cut to
a length that is 5/8 of the wavelength of the radio’s
frequency. The impedance of a 5/8 wave dipole is around
73 ohms if there are no structures nearby to disturb the
antenna. It’s not enough to simply cut a wire to 5/8ths the
length of a radio wave. For several reasons — including
that the speed of light is slower in a metal conductor like a
copper wire than in a vacuum — we must use a different
equation to calculate the best length for the dipole
antenna elements. That equation is:
Length (in feet) of dipole = 468/Frequency (in MHz)
Be sure to divide the calculated length by two to get
the length of each element in the dipole. For a frequency
of 144.390 MHz, you should calculate that each element
is 19. 5 inches long. The dipole will have an impedance of
about 73 ohms and is center fed by a RG-174 coax
transmission line with an impedance of 50 ohms.
Although there is a mismatch between the impedances of
the coax and dipole antenna, the length of the coax is so
short and the power level of the radio is so small that the
mismatch is of little concern (only about 4% of the power
is lost due to reflection). Okay, so now we know the
■ FIGURE 8. The completed antenna will look like this.
There is no electrical short between the dipole elements
of the antenna, and both conductors in the coax are
soldered to the same trace used by each element.
■ FIGURE 7. The now "properly"
terminated RG-174 coax. Don't
tin the coax at this time.