Antenna Construction

One of the biggest changes I wanted to make to the high-altitude balloon project after the first balloon was to increase the strength of the antennas. My ultimate goal in doing so was to remove the antennas from the car platform and instead make it possible to have a designated ground station which can track the balloon for the entirety (majority) of its trip. To achieve this goal I really had two options: increase the modem power or increase the antenna gain. Since RF amplifiers are expensive and require a HAM radio license I opted to create better antennas.

Antenna Terminology

I want to start this paragraph by referencing a great source for understanding antenna lingo and theory. This source is known as Antenna-Theory.com and provides a great introduction to many of the techniques used in creating and designing antennas. I would suggest reading through some of the articles on this website before considering building your own antenna.

I have also included below some brief definitions which will be useful for this article

Gain : Gain is used to describe the increase in power of a signal from a given reference signal. For example when discussing an amplifier the reference signal is typically the input voltage and the signal whose gain we want to measure is the output voltage. For antennas typically gain is expressed in reference to two ideal antennas.
dBm :Used to express the power consumed by a device on a logarithmic scale. It is calculated in the same way as decibels but instead of referencing to the watt we are referencing the mw. If P is the power in milliwatts(mw) the formula to calculate dBm is 10log(P(mw))
dBi : This is used to express the gain of an antenna. It gives the gain of the antenna in reference to an ideal isotropic antenna (IIA). The IIA is a theoretical antenna which has an equal gain in all directions. Meaning the field strength at a point away from the antenna is only dependent on the distance of that point from the antenna.
dBd : This is used to express the gain of an antenna in reference to an ideal dipole. This gain is typically lower than dBi since the gain of a dipole is more directional than that of an isotropic antenna.
Skin Effect : This is the tendency of currents to flow near the surface of a conductor at high frequencies. For amateur antenna builders this means that choosing between a hollow copper tube or solid copper rod will have almost no effect on the performance of the antenna

Getting Started

Before we can even begin thinking about constructing an antenna we need to start by designing the Antenna. Before we can begin thinking of this we need to decide what type of antenna we will design. For my case, I decided to design and construct a Yagi Antenna. The reason for this being that there already exists a lot of work by amateurs to design and build these antennas. Furthermore, these antennas can have very high gains, allowing me to maintain contact with the balloon over extended distances.

Now that we know what type of antenna we will be creating, we can start the process of designing the antenna. Before starting I have included the references below to show some additional examples of people who have built yagi antennas.

JTech Engineerig

Callsign W6PQL

The most important tool we will be using in this tutorial is the Yagi Calculator developed by an Amateur Radio Enthusiast named John Drew. Essentially this calculator allows us to plug in some of the traits we need our antenna to have and this calculator will produce the schematic of the antenna. Example traits are the thickness of copper used in our antenna, the frequency of the antenna, the number of reflectors, the type of coaxial we will connect to the antenna etc. The link to download this software is available through the link below. Note that the software is only made to work on Windows.

Yagi Calculator

Using Yagi Calculator

After downloading Yagi Calculator Open it. You will be greeted by the following screen. Once this screen is open, select the task menu found on the upper left corner of the screen. This pull-down menu will have several options, select "Design Yagi".

After selecting Design Yagi a new screen, shown below will be opened up. This screen is where we will input the parameters we are interested in for our Antenna.

This screen will be used to specify the requirements our antenna needs to meet.

Frequency: To begin we should specify the frequency of the Yagi. From some of the sources referenced above, I have read that it is better to choose the frequency such that it is slightly higher than the center frequency of the band you will be operating in. For example, my Antenna will need to operate between 915 MHz and 928 MHz. The center frequency would then be 921.5 MHz. I chose to specify the Frequency for my antenna as 925MHz, In the construction section of this tutorial, I will try to quantify the bandwidth of my antenna.

Diameter of Dipole Bend (mm): Next, we need to specify the diameter of our Dipole bend in mm. This number can be anything but be sure it is large enough that the copper tubing you use will not kink in the bending process. For my antenna I chose 35 mm. This is because the PVC pipe I had on hand had an outer diameter of 35mm meaning I could easily bend the copper to the desired diameter. Be sure though to account for the thickness of the copper tubing in this process. For example, my copper tubing is 7 mm in outer diameter. Therefore I should make my diameter of dipole bend 38.5 mm to account for the fact that the center of the copper will actually have a diameter of 38.5.

Dipole Gap at Feed Point (mm): The next number to specify is the dipole gap at the feed point, I chose to just leave this value as the default. Depending on the coaxial you are connecting to the dipole it may need to be bigger or smaller, this will become more clear in the section below.

Number of Directors: The number of directors is proportional to the gain of the antenna, the more directors the higher the gain of the antenna and the higher the directionality of the antenna ther will be. The length of the antenna will also increase as the number of directors increases. For my antenna, I wanted to reach a gain of around 15 dBi. To achieve this I was required to use 14 directors.

Boom-Type: For my antenna, I was using PVC pipe as the balloon so I specified a circular boom.

Directors Metal Shape: Since I am using copper pipe for my antennas I selected a round metal shape.

Directors Mounting: For the mounting of my antenna I selected nonmetal insulated boom because my PVC boom is non-conductive.

Directors: Diameter of Element: The diameter of the elements I am using is about 7mm. This is the diameter of the copper pipe.

Dipole Metal Shape: Again since I am using copper pipe I specified round

Dipole Mounting: Since the dipole is being inserted into the same PVC pipe I select same as dir/reflector

Dipole: Diameter of Element: Finally, since I am using the same copper pipe for both the directors and the dipole I select 7mm.

Finally, note that in the bottom left of the window we need to select the type of coaxial we will be using. Note that this is used to calculate the length of the matching Balun we will be using and therefore must be specified correctly. Note that at the very bottom of the menu there is an option to "specify custom velocity factor". This number determines how fast EM waves move through the coaxial and will determine the length of the 4:1 balun. Typically for any of the coaxial materials, this number can be found on the datasheet.

Once yet are finished specifying the traits of the antenna press the "calculate" button in the lower right. This will open up a new menu that will specify the traits of the antenna you need to build

Tools

Before starting the process of building an antenna ensure that you have access to all of the following materials.

1/4 Copper Tubing: Copper tubing will be used to make the dipole and reflectors of the antenna. Any thickness of copper can be used but 1/4" is the cheapest. Note that solid copper can also be used but it is more expensive and will not increase antenna performance at all due to the skin effect. One last tip is to use copper refrigerator tubing. I am unsure why this is but I found that the refrigerator tubing was much easier to bend into place without the tubing kinking as opposed to regular copper tubing.
10ft of 1" PVC pipe: The PVC pipe will be use as the mast of the antenna.
Coaxial Cable: Used as a balun in this antenna. Also used to connect the antenna to a network analyzer.
Type N Connector: Used to make it simple to connect the antenna to the RFD900+
Drill or Drill Press: This is necessary to drill holes in the PVC pipe to hold the reflectors and dipole.
Hacksaw: Used to cut the PVC pipe in half
Copper Tube Cutter: This will be used to cut the copper tube into the correct lengths
Clamps: Used to hold the PVC pipe down while drilling and marking. Also useful for straightening the copper pipe
Pliers: Not necessary but useful for straightening the copper pipe
Sharpie: Used to mark the antenna
Soldering Iron: Used to solder the coaxial to the folder dipole
Solder: Used to solder the coaxial to the folded dipole
Safety Glasses: Used to shield your eyes while drilling

Building the Antenna

The first step of building the antenna is to cut the PVC down to the length of antenna that you need. The required length will be specified in the "comments" section of the antenna specification page, generated with Yagi Calculator. My antenna was specified to be 1481 mm long which is around 5ft. For this reason I cut my PVC pipe in half using the hacksaw. The cut does not need to be perfectly straight or especially accurate.

Once the antenna has been cut to length we need to mark an straight line onto the antenna. This straight line will help us when we are drilling the pipe. It will ensure that all of the elements are for the most part lined up with one another and not at angles in relation to one another. To do this clamp the PVC pipe to either a straight piece of wood or another piece of PVC pipe. Run a sharpie along the intersection of the straight edge and the PVC. This will create the straight line we need to drill the PVC. See the pictures below for some details

Next we need to mark the placement of the holes for the reflectors. To find the position of the holes view the reflector and directors section of the document generated by Yagi Calculator. To do this I used the clamps to hold the metric yard stick to a single spot on the PVC pipe and then marked the position of each hole.

Once this is done we are ready to use the drill press to drill the PVC pipe. I found that it was helpful to use a drill press rather than a drill because it in part ensured each hole was in line with the next hole. Select a drill bit that is the same or smaller then the outer diameter of the copper tubing you are using. Ideally, the hole will be small enough that the tubing is snug but not so small that it does not fit. It is really crucial for proper antenna operation that all the directors be in line with each other. To aid in this I did my best to ensure that each of the holes drilled was perfectly in line with line that was drawn in the first part of this tutorial. To do this I would first line the drill bit up with the line on the pipe. Once this was done I would start the drill and drill the tiniest divot into the pipe. If this divot was not perfectly lined up with the straight line and the mark for the position of the hole I would stop drilling and realign the drill bit. Be sure to where safety glasses for the entire drilling process. I also found it helpful to clamp the pipe done when I began drilling. Otherwise the drill bit had a tendency to pull the PVC pipe towards it.

In the photo below an image is shown of one of the divots used to ensure the drill was properly lined up with the center of the line. Also included is a video of me using the center of the drill bit to initially line the straight line up with the center of the drill bit.

Once all of the holes have been drilled into the pipe it is time to cut the copper tubing to length for each of the reflectors/directors/dipoles. Before we start this process we should begin by straightening the copper tubing. To do this clamp one side of the copper tubing to a sturdy table or door. Next grab the other end with a pair of pliers and bend it to a forty five degree angle. Finally, hit the pliers with a hammer. The pulling motion on the copper will stretch it out slightly but will also help to straighten it. See the video below for details.

Once the copper is straightened use the tubing cutter to cut off the crimped ends of the pipe. Next lay the straight piece of tubing on a flat surface and cut out each section of tubing according to its length as specified in the document created by the Yagi Calculator. To do this use the metric yard stick to measure each section of copper tube and then use the tube cutter to cut it. As you are cutting be sure to mark which director/reflector each piece of tubing is. At this point do not cut the dipole out of the copper tubing.

Now we are ready to insert the reflectors/directors into the PVC pipe. This is greatly simplified by using the drill press to push the pieces of copper into the PVC. Before we can start this process measure or google the outer diameter of the PVC pipe you will be using for the mast. Now divide the outer diameter by two to get the radius of the PVC. Now take each of the reflectors/directors and find the midpoint. From the midpoint measure out a distance of the outer radius and place a mark on the reflector/director. This mark will be used to determine when we should stop inserting the reflector/director into the PVC. When the mark is just entering the hole in the PVC pipe the reflector/director should be approximately centered.

To insert the copper piece into the chuck of the drill and tighten it. Do not over tighten the chuck as this could damage the copper. Next line the hole up with the tubing so that when pressing the drill press down the tube will pass through the hole. Push the copper tube into the hole until the line we marked above reaches the hole. At this point the tubing should be nearly centered. To ensure that the tubing is perfectly centered measure each side of the director and ensure the two sides are equal. If they are not use a block of wood or rubber malet to gently tap the copper tubing one way or the other within the PVC pipe.

Also note that for some of the larger pieces of tubing the drill press will not be able to move far enough to fully insert the copper tube into the hole. To overcome this issue raise the table of the drill press to push the copper tube in further, see the image below for clarification.

At this point we should have everything constructed on the antenna except for the folded dipole and the matching Balun. If any of the reflectors/directors are not in line with the rest gently bend them until they are in line with other reflectors/directors.

To simplify the construction of my folded dipole remember that I chose the dipole bends such that they could be made with the PVC pipe. To begin the creation of the folded dipole I started by creating the first bend of the dipole, far away from the edge of the copper. This way I could focus on bending the copper, without having to worry about ensuring the copper was a certain length.

After finishing this first bend I looked at the file output by Yagi Calculator to find the other dimensions of the dipole. Below is an image of the folded dipole from the pdf output by the Yagi Calculator.

The distance between each of the points will be specified within Yagi Calculator. To begin fabricating the rest of the antenna I first measured the distance between I and H and used this to trim the edge of the dipole down. Then I used the distance between B and D to find where the next bend should begin. I also marked the distance DE and DF on the pipe to help me as I bent the second curve of the tubing. Next I carefully bent the second curve of the pipe again using the PVC pipe. Finally, I measured out a distance of FG on the tubing and cut the completed dipole off of the rest of the antenna.

All that is left now is to connect the Dipole to the rest of the antenna and fabricate the balun. Of all the other tutorials above I believe that the tutorial by Callsign W6PQL gives the best method for this purpose.

Unfortunately by the time I found W6PQL's tutorial I had already ordered my N-Type connectors and did not want to spend money ordering more. Instead I decided to create a 3d printed part to mount the dipole onto. The benefit of this was that since the Dipole was very close to one of the directors, I was able to create the print such that it would use the director to accurately position the dipole. The print would also need to hold the connector and dipole so that they could be soldered together with the balun. The part I printed is shown below.

Screen Shot 2019-09-24 at 6.22.38 PM.png

Now that the dipole can be mounted to the antenna, all the ise left to do is connect the balun and N-type connector to the antenna. For now just use the image below to correctly make connections to the antenna.

Before worrying about connecting the antenna, it is necessary to cut the antenna to size. In my case the size of the balun is 107 mm. To cut coaxial down to size there are a couple options. First you can go online and find a stripper tool for the coaxial you will be using. Second, you can carefully cut each layer of the coaxial using a razor blade or scissors. I opted for the second option. Be sure to note that there are four layers to a coaxial wire. There is the outer insulation, which is exposed. Beneath the outer insulation there is a conductive layer made of braided wire strands. Following this, there is another layer of insulation, which is then followed by the center conductor. Below is an image of the layers of a coaxial.

Now it is time to cut the connector to length. Before cutting I find it helpful to first strip one side of the coax. To do this start by removing 1.5-2 cm of the outer layer of insulation, without removing the outer conductor. Once this is done pull the outer conductor back carefully, try to break as few of the wires in the mesh as possible. Next cut of 0.5 to 1cm of the outer insulation off. Now it is important to note that the 107 mm length is specified from the last point the inner conductor touches the antenna. This means you will want your coaxial to be 5-10 mm longer than necessary to allow for a strong solder to the antenna. Accounting for this, cut the coaxial to size and strip the other side. Once this is done it is necessary to cut and strip a 4-5 cm length of coaxial.

With this complete it is time to connect the balun and feed to the N-connector. To do this I will be using solder, but it might also be possible to use electrically conductive paste. If soldering it is helpful to buy a soldering tip which is large so that more heat can be transferred to the copper dipole. Start by connecting the two ends inner conductor of the coaxial to opposing ends of the copper tube making up the dipole. Once this is done take the 4-5 cm piece of coaxial and solder it to either end of the copper tube of the dipole. On the other end of this 4-5 cm piece of coaxial connect the N-type connector. Be sure to connect the inner conductor of the coaxial to the center pin of the N-type connector and connect the outer coaxial connector to the outer jacket of the N-type connector. Finally, finish the process by connecting the outer conductors of both ends of the balun to the outer jacket of the N-type connector. An image of my first complete balun/feed line is shown below.