■ FIGURE 2. The radiating pattern of a typical dipole
antenna. The signal is strongest in the direction of the
horizon and weakest vertically.
of a center conductor (usually stranded wire)
surrounded by a metal shield (usually a woven copper
braid). In most cases, between the two conductors is a
plastic dielectric insulator; surrounding the entire coax is
a plastic jacket for protection from abrasion. Coax
makes a good transmission line because the electric
currents on the center conductor and the shield have
opposite polarities. This limits current to only flowing on
the surfaces between the center conductor and shield,
and cancels any electromagnetic fields outside the coax.
The cancellation of the fields means coax doesn’t
transmit a radio wave.
Because the longest electron oscillations occur
when they take place perpendicular to an observer, the
strength of the radio waves emitted by a dipole antenna is
strongest parallel to the dipole elements and weakest at
the tips of the elements. The pattern of radio wave
strength is called the antenna’s radiating pattern. In
three-dimensional space, the radiating pattern of a dipole
antenna looks like a donut or torus. The top and bottom
where — theoretically — there is no signal are called nulls.
The thickness of the torus depends on the length of the
antenna’s elements (compared to the radio frequency).
By changing the length of the elements, the dipole
antenna can transmit more power towards the horizon at
the loss of power transmitted vertically. That is how the
effective power of the radio is changed (by redirecting
more of the antenna’s watts of power towards the horizon
and less to the sky or ground). The radiating pattern of a
dipole also indicates how sensitive the antenna is to
receiving radio transmissions. Two dipole antennas
communicate more strongly when they are aligned
vertically with each other and most weakly when they are
aligned vertically to each other.
I recommend mounting the antenna vertically to
the near spacecraft. If the antenna is horizontal instead,
chase vehicles would also need to mount their antennas
horizontally for the best reception from the near
spacecraft. However, the near spacecraft spins
constantly. As a result, chase vehicles would receive a
signal that varies constantly as the near spacecraft spins.
It’s better to have all the antennas mounted vertically
instead, so that near spacecraft spin and chase vehicle
orientation doesn’t modulate the transmissions from the
Readers should realize this leads to another problem.
The nulls of the vertical antennas
align when the chase vehicle drives
under the near spacecraft. In that
situation, a radio receiver in the
chase vehicle is incapable of
receiving transmissions from the
near spacecraft (this is less of a problem if the radio
receiver has lots of gain). However, since chase crews are
seldom directly beneath the near spacecraft, they
frequently avoid being within the null. However, in those
cases were they are, chase crews can rely on the Internet-gates (I-Gates), or amateur radio stations that put radio
transmissions on the Internet. I-Gates stations located
away from the near spacecraft will still receive strong
signals and will send position reports to servers like
■ FIGURE 3. After cutting
the perf board into a
square, the parts of the
antenna look like this.
■ FIGURE 4. The end of one of the
antenna elements. Leave the rest of
the insulation on the wire.
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