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Step 1: Preparing the Can Opening
The coffee can you purchased probably has a plastic cover on the top and a metal cover protect-
ing the coffee freshness. Remove the plastic lid and put it aside; you will use it later.
You will also want to make sure that the can itself is intact with no indentations. Most cans will
have ridges around the circumference of the can which are okay; you just want to make sure
that it has not been dropped or mishandled. These indentations or dents can affect the effi-
ciency of the can.
The coffee can will be sealed in one of two ways. With many of the older coffee cans, you
needed to open the can with a can opener and discard the removed lid. If this is the coffee can
you have, make sure that you grind down or file the inside edge of the can so that it is smooth.




FIGURE 3-5: The empty coffee can.
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Chapter 3 ” Building a Directional Tin Can Antenna


If it™s a newer can, it will have a thick tin foil covering with a ring to pull the cover off. Simply
remove the cover and discard it.


Step 2: Cleaning the Can
While having coffee grounds in the antenna will not affect its operation much, it sure can make
a mess of things, so make sure that you clean the can out well. Also make sure that you clean
the opening of any foreign objects, such as glue, pieces of the original tin or tin foil cover. The
coffee can should now look similar to that shown in Figure 3-5.



Where to Drill
We will be using a copper wedge as the driven element or radiator. The location and length of
this element is extremely important. Although we will not be going too deep into the math
here, it™s important to understand where this driven element is to be installed.
The rule of thumb is that the driven element should be at one quarter of the “closed-space
wavelength” from the inside edge of the can when the connector is installed. The difficulty here
is that the closed-space wavelength will vary based on can and radiator dimensions.
Table 3-1 shows some of these dimensions for channels 1, 6, and 11 using two different
types of radiators. A narrow-band pole, and wideband wedge. The wideband wedge needs
to be built only for channel 6 because it operates well across the entire Wi-Fi range of
frequencies.
On our can, with an inner diameter of 100 mm, this offset was slightly more than 1 inch or
27 mm. Once you have this measurement, it is time to prepare the hole in the can where the
N-Connector is to be installed.
Remember to measure this carefully, because a mistake means you need to buy and use more
coffee.



Table 3-1 Radiator Offset: Distances From Back of Can to Drill-Point
Radiator Channel Frequency Offset for Offset for Offset for
Type (Hz) Can Can Can
Diameter Diameter Diameter
90 mm 100 mm 110 mm

Round pole or wire 1 2.412 53 mm 45 mm 42 mm

Round pole or wire 6 2.437 51 mm 44 mm 41 mm
Round pole or wire 11 2.462 50 mm 44 mm 40 mm
Wedge 6 2.437 29 mm 27 mm 26 mm
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FIGURE 3-6: Marking the correct location for the N-Connector.


Step 1: Measuring the Distance to the Opening
As mentioned earlier, measuring the distance to the opening is critical to the operation and
efficiency of the antenna. The old saying “measure twice and cut once” is also true here. But
instead, remember to “measure twice and drill once.”
Measure the correct distance from the top of the ridge at the bottom of the can, as shown in
Figure 3-6.
When you measure this distance, disregard the bottom lip of the can. This crimped edge of the
can has no influence on the interior workings of the waveguide. You are only interested in the
bottom material of the can, which becomes the back of your can antenna.



Step 2: Starting Small
There are several ways to get the hole for the N-Connector the right size. The method you use
is up to you. The final hole should be the diameter of the N-Connector stem.
In one method you can drill a small hole and work your way up to the desired hole size.
Another method, which is more time-consuming, is to use a nail to make the initial hole and
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Chapter 3 ” Building a Directional Tin Can Antenna




FIGURE 3-7: Drilling a hole in the can.



use a file or cutter to enlarge the hole. If you use this method, make sure you don™t dent the side
of the can. The can should stay completely round.
We drilled the hole using a 3/4 inch drill bit, as shown in Figure 3-7.


Step 3: Preparing for the Connector
There are different types of N-Connectors that you can use for this project. The type does
not really matter. As you can see from Figure 3-8, our connector had four screw holes (one at
each corner). The best way to ensure that the connector is installed properly is to insert it
into the opening you created in Step 2, line it up, and mark out the holes for the four mount-
ing screws. Once this is done, you can drill them with a drill of the same diameter as the
mounting screws.


Step 4: Finishing the Hole
The final step in preparing the N-Connector hole is cleaning it. Using a small file, make sure
that there are no edges or burrs around the openings. This will ensure a tight fit when the con-
nector is inserted and connected to the coffee can.
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FIGURE 3-8: The N-Connector.


Fitting the Radiating Element
One of the most important parts of the antenna is the radiating element. While different
shapes will change the way the antenna works, its signal strength, and its efficiency, it is not
super critical if you do not get it bang on.
The radiating element can be of three shapes (see Figure 3-9):

Round
Wedge
Cone




FIGURE 3-9: Three shapes for the radiating element.
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Chapter 3 ” Building a Directional Tin Can Antenna


Table 3-2 Length of Radiator Elements
Radiator Channel Frequency Wavelength Length of Radiator
Type (Hz) of Frequency (mm)
(mm)

Round pole or wire 1 2.412 124 31.0
Round pole or wire 6 2.437 123 30.7
Round pole or wire 11 2.462 122 30.5
Wedge or cone 6 2.437 123 24


While the cone shape is the most efficient, it is also the hardest one to make. So, we will only go
into detail for the round element and wedge element. The round element is very simple to make
since it is simply the core copper conductor from an LMR-400 coaxial cable (used extensively in
Chapter 1). The round element also has the most narrow frequency band. If you create an antenna
using the round element, effective power will drop considerably across all of the channels.
The wedge element is a bit more complex to make, but it has great coverage of the Wi-Fi
channels. It™s a little less efficient than the cone, but not significantly. The wedge is worth the
extra time and effort, and it™s what we use when playing with the can antenna toy.

The cone element follows closely with the wedge shape in dimensions. If you want to try to
make one, use the same dimensions and spacing as the wedge (6 mm diameter at the top, 1 mm
diameter at the bottom).


The length of the radiating element is important. Table 3-2 shows specific lengths for your
radiating element. Note that can dimensions do not factor into the element size.
With our wedge antenna, we needed the radiating element to be 24 mm in length. Figure 3-10
shows how to measure the radiating element.
Length




Length




Wedge
Round
FIGURE 3-10: Measurements of a radiating element.
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FIGURE 3-11: Starting with a piece of coaxial cable.


Radiator element length is measured from the end of the connector jack, not the length of the
piece of copper alone. Wait to make the final cut until after soldering the connector in place and
measuring the connector and element together.




A Round Radiating Element
Making the round radiating element is extremely simple because it is the core copper conduc-
tor from a coaxial cable. To make the round radiating element, follow these steps:

Step 1: Cut Too Much
Once you know the length of the radiating element, you will need to make sure you have the
correct length. Having said that, make sure that you don™t trim the coaxial cable to the exact
length of the element. Instead, cut extra. Starting with a piece of coaxial cable that is twice the
final length is a good starting point, as shown in Figure 3-11.

Step 2: Strip the Insulation
Using a single-sided razor blade, strip the outer jacket, inner shield, and dielectric core as
described in Chapter 1. As Figure 3-12 illustrates, you will be left with just the core copper
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Chapter 3 ” Building a Directional Tin Can Antenna




FIGURE 3-12: The core copper conductor.


conductor. Make sure that you add a little bit to the length of the connector with respect to the
length determined in Table 3-2, to make sure that the radiating element can be inserted and
soldered into the N-Connector.

Step 3: Cut to Length
Now that you have the core conductor to work with, start by trimming one side as square with
the edge as possible. Then measure the desired distance from that trimmed edge and cut the
final length. Ensure that you take into account the portion of the conductor that will fit into
the N-Connector body.
If you are not sure about the length, wait until after you solder the conductor into the body
before making any final length adjustments.


A Wedge Radiating Element
Making a wedge radiating element is not a complex process but it does take some talent to sol-
der it to the connector.
To cover the Wi-Fi spectrum, the wedge radiating element needs to be 1 mm wide at the
base (where it connects to the N-Connector) and 6 mm at the tip. Figure 3-13 shows the
wedge.
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6 mm
24 mm




1 mm
FIGURE 3-13: The dimensions of
a perfect wedge radiator.

One way to make this wedge is to start with a copper embossing sheet (this is available in most
arts and crafts stores). Simply trace out the desired wedge shape of the radiating element and
cut to size with either a knife or strong scissors. We created the wedge in Figure 3-14 using the
one foot-square copper embossing sheet.
A second way to make this wedge shape is to use a hammer to hammer out the shape. As you
can imagine, this process is more work-intensive, but requires less precision on your part. The
object in Figure 3-15 is a cross between the wedge and round element. The forgiving nature of
this antenna design allows for a good bit of fudging.




FIGURE 3-14: Copper wedge and embossing sheet.
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Chapter 3 ” Building a Directional Tin Can Antenna




FIGURE 3-15: A bashed-out round connector with a wedge end.


This process produces a semi-wedge-shaped element that is not as efficient as the “true” wedge
radiating element, but is better than the round radiating element.



Final Construction and Weatherizing
This is the final stage of the coffee can antenna project. Time to put it all together!


Step 1: Building the N-Connector
Putting this together is fairly straightforward. The radiating element should fit inside the
opening of the N-Connector. This might be a snug fit, but that™s okay. If you find that the radi-
ating element will not fit properly, sand one end of the connector using some sandpaper until
the element fits.
When the fit is snug, solder the radiating element to the connector to make a permanent and
electrically strong connection. Soldering even a small amount will make the difference between
a great antenna and a lousy one.
Once this is done, you should have a completed Connector/Element assembly similar to the
one shown in Figure 3-16.
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FIGURE 3-16: The completed Connector/Element assembly.




Step 2: Mounting the N-Connector
You can now insert the completed assembly into the hole you drilled in the side of the coffee
can. As we mentioned, your N-Connector might be different than the one we used. Ours had
four mounting holes. Mount the connector assembly to the coffee can using the screws pro-
vided. Make sure that the screws are just the right length or only slightly longer. If they are too
long and protrude into the can more than a few millimeters, the signal will be adversely
affected. Figure 3-17 shows the connector mounted to the can.
Also ensure that the wedge™s flat edge is parallel to the bottom base of the can (see Figure 3-18).
This isn™t critical, but it looks better!



Step 3: Weatherizing the Antenna
Weatherizing the antenna is easy. If you plan on using the antenna outdoors, you may want to
spray paint the exterior of the antenna with a rust proof paint. To protect the interior of the
antenna, simply cover the opening with the original plastic cover that came with the can
(you may want to glue this cover on).
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Chapter 3 ” Building a Directional Tin Can Antenna




FIGURE 3-17: The connector mounted in the can.




FIGURE 3-18: The mounted N-Connector assembly.
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FIGURE 3-19: The completed can antenna hooked up to a laptop.


Some plastics block microwave energy. To test if the lid to your can is microwave safe and will
keep your antenna working great, try the following: Using a standard microwave oven, place the
lid on the floor of the microwave at least 2 inches from a cup of water. Start the microwave and
run it until the water starts to boil. Carefully check the lid to see if it™s hot or not. If it™s cool to the
touch, microwave energy does not affect the plastic and the lid is a good covering.


The cover of an antenna that keeps weather out but does not interfere with the antenna opera-
tion or signal is called a radome. In a sense, the plastic lid on your can antenna is the radome.
Neat!


Your can antenna is now done! Now put the antenna to use by attaching it to the cable and
pigtail you built in Chapter 1 (see Figure 3-19).


Extra: Antenna Simulation and Patterns
There is some free antenna simulation software available on the Web called 4NEC2. The basis
of the simulation is the “Numerical Electromagnetic Code,” hence NEC. This software can be
used to simulate thousands of different antennas. One such antenna is our sweet little waveguide
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Chapter 3 ” Building a Directional Tin Can Antenna




FIGURE 3-20: Can antenna simulation using 4NEC2.


antenna. The screenshot in Figure 3-20 shows the simulation for the can antenna we built in
this chapter. You can obtain a recent copy of NEC2 from the “links” section at www.nec2.org
and download the coffee can antenna simulation model from www.nec2.org/coffee.txt.
Antenna radiation patterns are created to show the strength and direction for antennas. There
are two types of antenna radiation patterns: vertical plane (or E-plane) and horizontal plane
(or H-plane). Antenna manufacturers use both of these pattern diagrams to show how an
antenna works.
The antenna radiation patterns in Figure 3-21 are for a common dipole omnidirectional
antenna. This type of antenna simulation makes distinction between the vertical versus hori-
zontal. An omni antenna radiation pattern looks a lot like a flattened donut.
The diagram in Figure 3-22 shows the relationship between the two pattern diagrams. Notice
the vertical plane is perpendicular to the Earth™s surface, while the horizontal plane is parallel
to the Earth™s surface. Each plane is like a two-dimensional slice or circle with the center of the
slice at the center of the antenna.
Moreover, 4NEC2 has a neat 3-D engine to show you what the antenna would look like if you
could see RF radiation. See the screenshot in Figure 3-23 for a 3-D simulation of the can
antenna from this chapter.
78 Part I ” Building Antennas




FIGURE 3-21: Antenna radiation patterns for an omni antenna.


Vertical




Horizontal

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