In the February 2006 Technical Tidbit titled "Construction of a Coaxial Antenna for Troubleshooting
the theory of operation, construction details, and use of a coaxial
antenna built on the end of a coaxial cable are discussed. Since then I
have had a need to build a few of these antennas and worked out an
improved construction technique.
Figure 1 shows a close-up picture of one of these antennas cut to
resonate at 1850 MHz. Instead of just folding the shield back
over the cable jacket to form one half of a dipole antenna, I slipped a
small brass tube over the end of the coaxial cable and soldered the
shield around the end of the tube. The tube should be sized so as to be
a close fit to the jacket of the coaxial cable. The use of the brass
tubing makes the antenna more rugged and helps the antenna maintain its
shape. The center conductor is extended just as in the February 2006
article and covered with a few layers of heat shrink tubing (the red
tubing in Figure 1) so that a final piece of heat shrink tubing will
fit snuggly over the entire antenna (the clear tubing in Figure 1).
resulting antenna is easy to use and will stand up to rugged lab use.
The brass tube and extended center conductor are both 3 cm in length in
Figure 1, resulting in a dipole tuned to about 1850 MHz. When building
one of these antennas, only a little extra work is needed to build a
second one since two pieces of brass tubing can be cut at the same
time. Figure 2 shows the pair I built one evening. The heat shrink
tubing on the lower antenna is a little longer, but the two antennas
are electrically identical.
Figure 2. Pair of 1.85 GHz Coaxial Dipole Antenns
improved construction technique is described for a small coaxial
antenna that is useful in near field immunity testing for possible
mobile phone caused problems in equipment. The resulting antenna is
rugged and easy to use.