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Hack 71 "Spider" Omni

Possibly the smallest and simplest
omnidirectional antenna there is.

This is one of the simplest and
smallest antenna designs I've seen for 2.4 GHz. It
isn't much larger than a standard N
connectorbecause that is exactly what it is made of. It has
been dubbed the spider
omni, since it looks a bit like a crazed
spider crawling up your antenna feed (Figure 5-3).
Technically, it is a ground plane antenna, but practically
speaking, it acts like a vertically polarized 3 dB omni.

The spider is very simple to construct if you have a good soldering
iron and some basic tools. You need a standard N connector and about
a foot or so of solid copper 12-2 romex (common 12-gauge electrical
wiring). You also want a good vice to hold onto the pieces as you
solder them, as well as a pair of needle nose pliers, some good
solder, and a bottle of flux.

First, cut five pieces of bare copper romex, each about 3 cm long.
Straighten out each piece as best as you can. Using needle nose
pliers, make a small 180-degree bend on one end of four of the
pieces. Now, tin the bent tip of each piece, as well as one end of
the remaining straight piece. This will make your soldering job much
easier later.

Now solder the straight piece to the gold cup on
your N
connector [Hack #63].
Don't use too much solder; there should
be just enough to fill the cup without overflowing. Prepare to solder
the four legs directly onto the N connector's
chassis. You need to use a lot of heat, and liquid flux will help the
solder to flow better and bond to the body of the connector. I found
it easiest to clamp the straight piece of wire, rather than the
threaded bottom of the N connector. This helps to keep the heat from
dissipating into your vice while you solder to the chassis.

Take your time, and don't use too much solder on the
legs. When you are finished, let the whole thing cool for several
minutes, as the chassis will be quite hot.

Now, trim all of the leads to about 20 mm past the edge of the
housing. Trim the center lead to about 20 mm past the end of the gold
cup. Bend the four radials connected to the housing down at a slight
angle. Physically mounting the omni is straightforward if you use
heavy feed line, such as LMR 400. Mount the antenna with the center
lead pointing up.

The spider omni doesn't provide a tremendous amount
of gain (about 3 dB or so, as far as I
can tell from informal tests) but it does work quite well for what it
is. Higher gain antennas are certainly possible (as we see will see
later in [Hack #76]), but tend to be
more complicated and much larger. For many applications, you just
can't beat the size and cost of this tiny little
antenna.