YAGI 101 Low Cost Antenna Making

DW Butrick ITC Haywood County Schools, NC

Just about all of us at one time or another have broken the antenna off our car. What did we do? We went to the closet and got a coat hanger.

Now a 2.4 Ghz antenna is a lot more complex. But it still come down to the fact, IT'S JUST A PIECE OF WIRE!

Simple Antenna Theory

Waves

Imagine a boat making a wake in a river. As the wake laps up against the poles of a pier, it imparts a vibration of the same frequency to the flat boards in the pier itself. If you put your ear on the pier, you can hear them quite loudly because the pier amplifies the sound at the water level. The frequency is very low, because the length between waves is long. It can be up to a few seconds between waves. This length is called the period of the wave. With wireless, the waves are much shorter, the period is less than a second, much less. It is because of this that we name the wave by it's frequency. For 802.11b, it's called 2.4GHz, because the wave repeats 2,400,000,000 times every second.

Resonance

An antenna is an amplifier in a sense. It is the antenna that catches the signal from the air, much like a sail of a ship catches the wind. Any piece of metal will serve as an antenna, and the bigger the better if you use the sail/wind analogy. But regardless of size, some work much better than others. Why is this? Resonance. Some bits of metal are the same size in one dimension as the radio wave it's catching. This causes the wave to be felt stronger by the metal, resulting in the a stronger electrically induced current. It actually works well if you are some even multiples and fractions of the wave's size. Quarter wave length segments are very common and useful.

Frequency Bands

We need to know the electrical characteristics of our signal in order to understand an antenna. These are the wave's frequency, wavelength and amplitude. The frequency is inversely related to the wavelength. 802.11b uses several frequencies that are close together. The range as a whole is called a band. Because they are all close to 2.4GHz, it is generally referred to as the 2.4GHz band. It is the beginning of a band of frequencies on up to 2.4835GHz. A useful fact here is that one antenna can and does serve for more than one frequency, but generally the ones in the middle will work better than the ones on the ends. You should design your antenna around the middle of the band you are working with so that it will work more efficiently.

Light Speed Ahead

The frequency is the number of times your carrier signal cycles or repeats in one second. The wavelength is the distance your signal goes in one cycle. To calculate wavelength for 802.11b, we must know the speed of the signal. Electromagnetic waves, our radio waves, travel at the speed of light. That means the velocity is about 3.0 x 10^8 meters per second. It's a constant. This means that distance it goes per second will always be the same. One can imagine a sine wave flying through the air. This picture shows one cycle of a sine wave. No matter how long the cycle of the wave, it always goes the same speed, and therefore the same distance in one second as any other electro-magnetic wave. Now that we know the velocity , the frequency and the band or our signal, as well as a little bit about resonance, we are ready to do some calculations.

Here's a really nifty sine wave applet that I found on the web.

And a handy unit conversion table that I also found on the web.

This simple formula is from a high-school physics book.

Rate * Time = Distance

Or for our special case,

C (the speed of light) * Time = Wavelength

Notice how the time and the distance portion of our little equation are related. The time we are concerned about is the time it takes our radio wave to complete one cycle. This time is called the period of the wave. This will typically be measured as some small fraction of a second. Since the measurement of frequency is the number of cycles in 1 second, the frequency multiplied by the period will always be one. If we know either the frequency or the period, we know the other as well. We use frequency to name a wave because it's a number larger than one.

Period (length of time for 1 cycle) * Frequency (cycles per second) = 1

or

T * F = 1

The Frequency of our 802.11b wireless equipment is 2.4 x 10^9 Hz.

Wavelength is the distance traveled in one period, or the speed of

light times the time it takes the wave to complete one cycle. And

the period times the frequency is equal to one. So now we have two

equations with two unknowns. Sound familiar? Good, we all did this

in grade school right? It's basic algerbra to solve this problem.

Equation 1: W [meters] = 3.0 x 10^8 [meters/sec] * T [sec]

and

Equation 2: T [sec/cycle] * 2.4 x 10^9 [cycles/sec] = 1

Equation 2 in terms of T is: T = 1 / (2.4 x 10^9)

Using the first equation in terms of W, which it already is,

W = 3.0 x 10^8 [meters/sec] * (T)

and substituting the second equation in terms of T,

for T in the first equation we have,

W [meters] = 3.0x10^8 [meters/sec] * (1 / 2.4x10^9 [cycles/sec])

W = 3.0x10^8 * 2.4x10^-9 [meters/cycle]

= 3.0/2.4 * 10^-1 = .125 meter, or 1/8 meter, or 12.5cm

The same calculation for 2.4835GHz, the top of our band, yeilds 12.0797cm, which rounds nicely to 12.0 for now. So, we are dealing with waves that are between 12.0 and 12.5 cm in length. Cool, huh?

Now that we know our wavelength, what do we do with it? Well, remember that we should design our antenna for the middle of the band. This would be something with a wavelength of 12.25 cm. I've made them with all kinds of lenghts from 11.8 to 12.7, due to the tools and parts I have access to, and the ammount of time I'm willing to spend filing down spacers, and they all seem to work well. But for our calculations, we should try to be exact. The more effort you put into your antenna, the better it will work.

With putting all this knowledge to use and a quick trip to the local hardware store (Not a computer store, but the kind that sells toilet seals and duct tape) I have built a $299.00 Yagi antenna for about $12.72

Making the Antenna
PARTS:

Access Point w/ removable antenna
Kitchen Slip Joint Extention 11/2"-12"
Carriage Bolt 6" 1/4" 1/4-20 thread
Nut for above bolt (1/4-20 thread)
Copper Tubing 3/8" id
Fender Washers 1 1/4"od 1/4"id
Crimp type Connector 1/4" id
Spray Can Lid with inner support (don't ask yet)
Heavy ABS Plastic Square (like from an old HP)
Wall Mount Speaker Support (swivel type)
Foam rubber (2")

TOOLS:

Pliers (2 pair)
Multi Purpose Knife
Tubing Cutter
Hand File
Small Vise
Dremel Roto Tool
Measuring Tape

After carefully doing all the calculations to get the wavelength that is between 12.0 and 12.5 cm.

It worked out that the spacers (tubing) needed to be 1.2" in length and 4 would be needed for each antenna

File down the square at the head of the bolt so that the first washer is flush with the rounded bolt head.

Alternate Washers (5) and Spacers (4) then place the Crimp Connector on and snug together with 1/4 -20 Nut

You Just Made a Yagi Antenna!!!! Now for the housing.

Reusing the RP-SMA Connector from Your Radio
Carefully score the removed antenna from the access point (I used a Dlink AP-2000). You can then remove the tip cover and see what you are getting yourself into.
With the Dremel you can now Groove the upper part of the hinge housing on both sides. (Be very careful not to cut too deep)
With the knife pry appart the 2 scored grooves this will expose the 2 hinge pins that you can now remove with pliers.
CAREFULLY strip about 1/2 " of the insulation from the end of the antenna whip. This will be what is crimped to the connector on the antenna you just made.
Building a weatherproof housing
Awl or drill a hole just slightly smaller than the lower part of the SMA hinge in the wide part of the Slip Joint.
the hole should be about 1 1/4 " from the back of your housing. (Thread side of the Slip Joint)
Cut the foam in 1 1/2 circles and divide to the center so it wraps around the first and fourth spacer. (this will help keep the antenna from sliding and ratteling in the housing.
Insert the connector to the apex of the hinge so the end of the antenna whip sticks out of the back of the housing.
Insert the stripped end of the antenna connector whip into the crimp connector and ..........crimp.
Do not try to solder this connection , the antenna is to big of a heat sink and by the time you get it hot enough you have melted something you shouldn't have
Ease the antenna back past the connector hole and slowly push the R-SMA into the opening so that the lower hinge housing is about 1/2 way in. Then ease the antennt forward untill the whip is paralelle with the outer housing.
It just so happends that the center guard of a spray can lid is the perfect size to press fit into the end of the housing.
Cut out the center guard of the lid leaving about 1/2" to 1/4 "of overlapping plastic. (you can trim it up later. If the cap is loose you can use some double stick take to secure it. If the lid is too tight to get started, try heating the end of the housing to expand it. (just be careful not to melt it.)
Next I took a piece of an old HP deskjet cut out a square and used the Slip Joint seal as a templet.
With a sharp knife, score the edges around your mark and break off with pliers.
Place the backplate in the vise and smooth the edges with a file untill it just in the threaded collar of the Slip Joint.
Now Awl or drill a 1/4" hole in the center of the back plate. This will be for the swivel speaker mount.
Remove the plastic piece that mounts it the speaker and place the Threaded Slip Joint Collar face up. Now put the back plate you just made on the 1/4"-20 threads and tightly secure the nuts around the back plate. Then place the Slip Joint seal on the top of the back plate and thread on the antenna housing.
You've Done It...Now add on a R-SMA extention cable and you are good to go!