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Hack 76 Cut Cable Omni Antenna

Make a high-gain omni out of bits of feed
cable.

Most of the
designs on the Web for a 2.4 GHz omni antenna seem to involve brass
tubing and LMR-400 cable, none of which are readily available to me.
I then found a coax-only design for 444 MHz that was based on the
same idea. The only reasonable cable I could get my hands on was
RG-213 from Maplin (http://www.maplin.co.uk/). I thought
I'd give it a try by scaling the 444 MHz design up
to 2.4 GHz and using RG-213. In order to get about a 6 db gain from
the antenna, it needs 8 sectors, with a 1/4-wave section at the top
and a fly lead with an N-connector at the bottom.

It should take about two to three hours to build an antenna using
this design, but don't worry if it takes longer. You
will get quicker, especially as you need to make the jig only once.
Figure 5-11 shows the completed antenna.

Each sector of the antenna needs to be 1/2 a wavelength long,
multiplied by the velocity factor of the cable. The
velocity factor of RG-213 is 0.66. If you decide to use a different
cable (such as LMR-400), then you need to get the velocity factor of
that cable (which will be different), and recalculate all the
dimensions.

                          V * C    0.66 * 299792458
1/2 wavelength = ------  = ----------------  = 0.0405m  = 40.5mm
2 * F     2 * 2441000000
V = Velocity  Factor of RG213 = 0.66             
C = Speed of light = 299792458
F = Frequency  of Signal = 2441000000 (mid  point of 2.4Ghz range)

The 1/4 wave element is not adjusted by the Velocity factor, as it is
in the open, so it works out at just 31 mm long, giving a total
antenna length of 355 mm + fly lead.

All of the parts needed to make this antenna are available cheaply
from either Maplin or any DIY shop. You need the
following:

1 meter RG-213U cable (available by the meter from Maplin). This is
enough for two antennas. Buy more for whatever fly lead length you
want.

1 N connector. Depending on what you want to connect the antenna to,
use either male or female connectors, and inline or bulkhead.
Remember that inline connectors need to fit 10 mm diameter RG-213
cable.

20 mm PVC conduit (available from any DIY store) It should have a 20
mm inside diameter, and 22 mm outside.

22 mm pipe clips (depending on how you want to mount the antenna).
Pipe clips make mounting the antenna easy.

You also need the following standard
tools.

Millimeter ruler for measuring

Junior hacksaw

Stanley blade knife

Pliers

Standard soldering iron (you don't need a heavy duty
one) and solder

Scraps of wood to make a jig to aid soldering

Bench or vice to hold the cable while you cut it

Cutting the Pieces

After much trial and error, I found that
the neatest way to cut the cable is actually with a junior hacksaw.
It gives a much cleaner finish than wire cutters. Each sector
consists of a short length of RG-213 cable, with the central core
sticking out each end.

When building the antenna, the exact length of each piece of RG-213
is not that important; it is the overall length of each sector that
counts. I found that cutting the cable to 37 mm with 6 mm of core
sticking out each end gets enough overlap to easily solder the
segments together. If you allow 1 mm for the width of the hacksaw
when cutting the sectors apart, it means you need 37 + 6 + 6 + 1 = 50
mm of cable for each sector. Making 8 sectors + 1/4 wave section
comes to 420 mm of cable for the antenna + cable for the fly lead.

The best way to cut each sector is to make the cuts where each end of
the sheathed section of the sector will be, before making the cut
between each sector. Figure 5-12 displays the top
three sections of the antenna, and the 1/4-wave section, showing the
order that the cuts should be made.

The best way to make the cuts is to mark them out on the cable first.
When sawing the cable, you'll find it has a tendency
to deform and bend, so lightly sawing round the outside sheath
firstbut not cutting throughhelps guide the actual cut.
I use the junior hacksaw to gently saw round the cable sheath to make
the mark for each section.

The first mark will be at 31 mm from the end of the cable, which is
for the 1/4-wave section at the top. Once you make the mark, it is
time to cut around the cable. You want to cut through the sheath and
shielding, and just into the central insulation, but not into the
central copper wires. You may need to practice a bit first, but you
should be able to feel it as you cut through the shielding into the
central insulation. By leaving plenty of sheathed section either side
of the cuts, the shielding stays in place when being cut.

Now with pliers, gently twist off 31 mm of sheath and shielding at
the end of the cable. This should leave the central insulator
exposed. Using the Stanley knife, score round through the central
insulator (not too hard, or you will cut the central cable). Now
twist off the insulation (Figure 5-13). You should
be able to see the twist in the central cable through the insulation,
which will show you which way to twist off the insulation, resulting
in the central core twisting more tightly.

The next mark is 37 mm down (68 mm from end of the cable), and is the
cut for the other end of the sheathed section of the top sector. The
next mark is 13 mm down (this section consists of 6 mm core from each
sector and 1 mm for the cut between sectors; 81mm from the end) and
is the top of the sheathed section of the second sector. The next
mark is again 37 mm down, then 13 mm, then 37 mm, and so on, until
you have each of the sheathed sections marked out.

You can now start making the cuts, remembering to cut only through
the sheath and shielding, and just into the central insulation. First
make the cut at 37 mm down, then make the next cut another 13 mm
down. You may find that some of the shielding pulls out when you make
this cut, as the 13 mm length of sheath cannot hold the shielding
tight enough. Don't worry, it
doesn't matter.

Now you are ready to cut off the top sector from the cable. Cut
through the whole cable at the midpoint of the two cuts you just
made; that is, about 43.5 mm from the end of the sheath, or 74.5 mm
from the end of the cable. See position 4 in Figure 5-12. Just carefully saw the whole way through the
cable. Now you can pull off the sheath and shielding from the each
end.

Now score round the insulation as you did before, being careful not
to cut the central cable. Carry on, making cuts 37 mm down from the
end of the sheath, and then 13 mm further down (50 mm from the end of
the sheath). Then cut through the cable in the middle of the two
cuts. Another sector made. You need 8 sectors total. Make the same
cuts as usual for the eighth sector as it will make top of the fly
lead as well. Now that you have all 8 sectors, you need to check
round the end of each sector to make sure that none of the shielding
is touching the central cable, as odd strands can get left.

Next, you need to make a gentle V-shaped cut with the Stanley knife
at each end of the sectors to expose the shielding, which is where
the central core of the next sector will be soldered. See Figure 5-14 for an example V cut.

Make sure that the V cuts at each end of the sector line up;
otherwise, when you come to solder the antenna together, the whole
thing will be twisted all around. Once you have all eight sectors
finished, it is time to put them together.

Build a Jig

If you do not have a handy helper to
hold the sectors together, then you will find it easier to make a
small jig from scraps of wood to hold the sectors together as you
solder them. The clamps on the righthand side of Figure 5-15 need to be no more than 30 mm long. The
baseboard of the jig must extend to the right far enough to take the
whole length of the completed antenna, as the baseboard will need to
support the antenna during the soldering since the antenna is not
rigid enough to support itself. Don't make the
clamps too tight, as you need to be able to easily lift the cable out
after it has been soldered.

When you are ready to solder the sectors together, take care that
each sector is correctly spaced. The overall length of each sector
needs to be 40.5 mm. Measure from one end of the shielding of the
sector that you are adding to the same end on the next sector, and
slide the sectors together/apart until the distance between them is
40.5 mm. Try to get it as accurate as you can, as it affects the
direction the antenna transmits in if you get it wrong. There should
be a small 3 mm gap between the sheaths of each sector. Figure 5-16 shows the details of a soldered sector.

Once you have soldered each sector together, lift the sector up, turn
it over, and move it down the clamp to get ready for the next sector.
This results in a nice straight antenna. When soldering, remember to
heat both the shielding and core so that the solder runs smoothly and
fixes them together.

Once complete, test the cable with either a bulb and battery or a
multimeter. The center of the fly lead should form a circuit to the
1/4-wave section, and to the shield of the fly lead to the shield of
the top section. Now test that there are no crossed connections by
ensuring there is no circuit between the center of the fly lead and
the shielding of the top sector, and there is no circuit between the
1/4-wave section and the shielding of the fly lead.

Now fix the N connector of your choice onto the end of the fly lead.
The type of connector you use depends on what you want to connect to.
I use inline connectors, but you could use any connector you like.
Slide the antenna into a length of conduit. It should be a snug fit,
so you may need to gently ease it in. Now find an old soft drink
bottle top, and pop it on the top end of the antenna.

Voila, one complete antenna! Securing the antenna in the conduit is
best left until you are ready to mount it somewhere. You can cut 5 cm
slots in the bottom of the conduit and use a jubilee clamp to grip
the fly lead, drill a hole through the conduit and use a cable tie to
hold the fly lead, use a bulkhead mount connector on a bottle cap and
glue it to the bottom of the conduit, or glue the fly lead in place.
It's up to you.

Disclaimer

I should point out now that I don't claim that the
design just described is fit for any purpose, and
don't accept any liability for use of the design, or
any antenna based on this design. If you want to build an antenna
using this design, then you are responsible for ensuring that it
doesn't breach any laws where you are, and is
compatible with any hardware you connect it to. If in doubt, buy a
commercial antenna.

The original article on which this Hack is based is available online
at http://wireless.gumph.org/articles/homemadeomnil.