Homemade 440Mhz Yagi Antenna

Antenna built July 17, 2010
Page written on October 6, 2010

 

For an upcoming weather station project, I want a serial link between my house and the backyard treehouse that will house the electronics. To maximize the range of the radios, I decided to use directional yagi antennas. At first, I looked into 900Mhz RF links, but I couldn't find any affordable, yet reliable ones. When I came across the RFM22 at Sparkfun, decided to use it. The RFM22 is a low power transceiver (60mW TPO) that has built in encoding, FIFO, and other features such as RSSI. However, the model Sparkfun carries only works on the 440Mhz band and I couldn't find a good 440Mhz yagi anywhere online. I guess it isn't practical to make them because they are so big. However, 440Mhz happens to be in a HAM radio band (70cm) and there was no shortage of tutorials of homemade yagi antennas for that band.

So, I decided to make my own antenna. The project was cheap (less than $10) and everything I used came from the hardware store. Besides a little online research, it took less than one hour to build. This page details my experience building this antenna. Just note that I am completely new to antenna design and radio theory. I simply never had to deal with that kind of stuff, so this was a neat learning experience. So, if there is some amateur radio expert out there reading this an you would like to point out a mistake I made, please do so!! -I would appreciate any help.

Research

These websites helped me a lot, so hopefully they might help you. If any of these links go dead, I made PDF copies for anyone that may be interested.

Yagi Basics

A yagi antenna is a directional antenna with a driven element, a reflector behind the driven elements, and any number of directors in the front of the antenna to direct the RF field forward. The driven element is usually some kind of dipole, in this case I'm using a half folded dipole to offer wideband support in case I need to tweak the frequencies due to interference. When an AC current such as the output of the transceiver is applied to the driven element, it resonates and emits an electric field around it. The other elements direct this field in the direction the antenna is pointed. This uni-directional system is great for point to point links because you no longer waste energy transmitting in unwanted directions.

Size & Dimensions

In order for the driven element to resonate properly and for the other elements to work, the spacing and height of each element must be precisely calculated based on the intended frequency. See http://www.qsl.net/w4sat/antlegn.htm and http://radio.meteor.free.fr/antenne/schema.jpg for info about how to do this. My antenna has the following dimensions: Diagram showing the spacing of yagi elements including the connection of the coax feedline.

A note on radiation resistance and impedance

Before finishing this antenna, I knew nothing about impedance or radiation resistance. I did know that transmitters and receivers have a certain output impedance (the RFM22B is 50 ohm), but though that that impedance came from the electrical resistance of the coax feed line because "A 3/16 inch steel rod antenna certainly has a better resistance than 50 ohms". Wrong! When the transmitter's output RF signal hits the antenna, the electrons get stripped off the metal and produce a surrounding electric field creating a radio beam. When the electrons radiate, this is appears as a load (impedance) to the transmitter. The radiation resistance is not because of the antenna's material, but rather it's shape.

Origionally, I built a full folded dipole. Normally, these antennae have an impedence of 200 to 300 Ohms, and this was too high. When an antenna is not properly matched to the transmitter's expected output resistance, you will experience SWR (Standing Wave Ratio). This will cause the transmitter to run inefficiently because not all the power will be resonated off the antenna. It may even damage the radio. You could use a balun to convert between different impedences, however, I decided to modify the folded dipole into a half folded dipole. A quick cut with a dremel cut-off disc removed half of the bottom section.

This new antenna shape has a high impedence in free space, but when near the antenna's other sections (such as the reflector or directors) the impedence falls to around 50 Ohms. Perfect! Because I changed the antenna after I took the photographs, some of the pictures may look a little different than described.

Construction

Two rods and PVC tubing.
I used the following materials:
UPC Label of Rods.

Drilling and Cutting the PVC tube

The first stage of construction after obtaining the materials is to cut the PVC pipe to length. You need at least 24" of pipe to properly mount all the elements, but you may want to add a few inches onto one end to help mount the antenna later. This is up to you.

After you have cut down the pipe to the right length, you can drill the holes. The metal elements of the antenna will be stuffed through these holes to support them. Because drilling by hand through a curved surface is a bad idea (especially when all the holes must be straight), I put together this jig to hold the pipe in place under a drill press. It is basically a wooden base with two square dowells spaced one pipe-width apart. The PVC pipe is squeezed between the dowells and adjusted so the next hole will align with the last one.

Wooden drill jig, angled view. Wooden drill jig, straight-on view.
Top Left and Right: The empty jig fastened to the drill press base. Bottom: Finished pipe with holes.
Pipe with all four holes drilled.

The drill bit should match the diameter of the rods. If possible, use a slightly smaller drill. This will create a press fit. If you drill the holes and find that the rods fit too losely and slide around easily, apply hot glue to bond the rods to the plastic. Hot glue will adhere only to plastic and metal if you ruffen up the surface with a grinder or sheet of sandpaper. This exposes more surface area to the glue and allows it to stick better.

Cutting and Installing the Yagi Elements

The next step of construction is to prepare the rods. Each rod is 36" long. This is how I cut mine:

Rod 1: 13" Reflector Segment + 19" Driven Element + 4" Leftover = 36"
Rod 2: 11 7/8" Director + 11 3/4" Director + 12 3/8" Leftover = 36"

Measure Twice, Cut Once!
Yes, there will be a lot of rod leftover, but this is the only way to fit everything.

Using a tape measure and the chart above, use a marker to mark where you will need to cut. Then, use a hacksaw to carefully cut apart the segments. There will be about 17" of unused rod which you can save for another project!

The elements are now ready to be installed. The only extra work that has to be done is to shape the driven element into a "J"-shape. Look at the diagram in the 'Size & Dimensions' section to get a better idea of the shape. The longest section of the element must be 12". The "half" bottom section will be 6". The extra inch will be used up in the 180° bend.

To bend the rod easily and neatly, I put it into a vise and used a pair of needle nose as a fulcrum or pivot point to bend the steel around. The rod can be easily bent by hand.

Clamp the rod into a vise so that twelve inches of the rod is being held in the vise, with the other seven inches sticking out the other end. Place the pliers or any other tool at the edge of the vise by the 12 inch mark and fold the short section of rod over the pliers. Look at these pictures to see how I did it:


Because this will be an outdoor installation, rust-proofing will be key. This is why I chose zinc-plated steel. The rod has a steel core that is surrounded by a thin layer of zinc. Because zinc will not corrode as easily as steel, it adds rust protection. This works well along the sides of the rod, but not at the end of the rod where we cut it because we have exposed the inner steel. This is why we add rubber caps to the end of each rod. These caps will...


Left: Ruff edges and exposed metal from cutting. Center: Rod protected by rubber cap. Right: Comparasion photo shows rod with and without caps.

The final step is to slide in the elements through the correct holes. Antenna with all elements mounted:

Now we can move onto the electrical work...

Mounting the junction box and coax connections

As mentioned before, the coax box is a european electric box. They work really well because they have plenty of holes to mount plugs and antenna connections in. I honestly have no idea where to get one if you give in the States. In Germany they cost €1 and can be bought at any Baumarkt (home improvement store).

In the above picture is a 50 ohm BNC connector. This will attach to a feedline coming from the weather station's telemetry radio. The other coax cable is connected to the antenna. There is a hole in the back of the box which through which a wire passes to the mid point of the driven element.

The entire box is screwed onto the PVC pipe with two screws.

Mounting the antenna

That's all for construction. Now we can move on to mounting the antenna.

To do this, I made a custom mounting plate using off-the-shelf hardware store components and a piece of wood from the scrap pile. At first I planned on using U-bolts to attach the antenna to the plate. How do you properly fasten a PVC tube? The answer took an embarassingly long time to figure out: electrical conduit clamps! They're made for this. I screwed two of them onto the wooden plate. Becuase I used 3/4" clamps and I have a 1/2" pipe, I had to overtighten the clamps, but they still hold well.

The mounting plate is then attached to a metal pole that holds most of the weather station instrumentation and sensors. Two U-bolts hold the mounting plate tightly to the pole.

Notes

  1. As of the writing of this page (Nov. 21, 2010), I have not tested the antenna yet. The weather station electronics are still under development.
  2. The antenna has done excellent so far in the weather. The design is very sturdy and weather-proof. It has been deployed for about five months (July-Near December) without any mechanical problems.
  3. PVC tubing if often not recommended for antennas because it can sag under intense sun, resulting in misalignment, or loose some of its strength due to excessive UV radiation. Fortunately, this pipe is sunlight resistant:
  4. Bird Magnet? The treehouse has attracted a lot of attention over the years from the local birds. I wonder why they never land on the antenna, though? Are the vertical elements spaced too tighly for a bird's wingspan to fit through when landing or taking off? On a side note, why don't birds ever nest on cell phone towers or broadcast masts? Can they somehow sense the intense electromagnetic field?

    Left: A Great Blue Heron. Oct 17, 2004. Right: A Red Shouldered Hawk stalks the new antenna. Aug 20, 2010.
  5. Please contact website@tristantech.net for any questions, comments, or corrections. If you need any help with your antenna, I'd be glad to help!