As described in W6PQL's article, I used VK5DJ's Yagi Calculator software to design the Yagi. The software provides a datasheet with the required element lengths and positions, and further specs for the coax connection (balun).

If you plan to follow this tutorial as-is, using the same parts described here, click the link below to download my datasheet for either 4G LTE or 3G (coax balun info included):

• Datasheet for Verizon 4G LTE antenna
  (787 MHz design frequency; 18 mm metal boom, 2.1 mm element diameter, elements through boom)

• Datasheet for Verizon 3G antenna
  (894 MHz design frequency; 18 mm metal boom, 2.1 mm element diameter, elements through boom)

For 4G LTE, Verizon currently uses 747-787 MHz (with a 20 Mhz gap in the middle of this range). For 3G, Verizon uses either 824-894 MHz or 1850-1990 MHz (again, with gaps in the middles). I did not provide a datasheet for the 1850-1990 MHz range because the dimensions of the resulting antenna are much smaller. It's certainly possible to create a Yagi for this range; just use the software to create your own datasheet and keep in mind that the boom length can decrease from 5 ft to approximately 2 ft.

The design frequency

Both of the above datasheets have been designed for the top ends of their respective frequency ranges--787 MHz for 4G LTE, 894 MHz for 3G. I chose the maximum frequency as the design frequency because a Yagi's sensitivity supposedly drops very rapidly for frequencies above the design frequency, but drops more slowly for frequencies below the design frequency. Thus, if your elements' lengths are a bit off during construction, you should still have a usable antenna. Although I have not tried it, you might want to experiment with a design frequency somewhere in the middle of each of the above ranges to achieve better performance (e.g., using 767 MHz for 4G LTE).

If you need to make a change

The above datasheets assume that the passive elements make metal-to-metal contact with the boom. Because such metal-to-metal contact tends to shift the antenna's sensitivity to a higher frequency range, the Yagi Calculator software has compensated for this effect by slightly lengthening the elements. The amount of compensation depends on the boom's diameter (and possibly also the elements' diameter). For this reason, the above datasheets are specific to the parts described here.

If you plan to use a different design frequency, a boom with a different diameter, or elements with a different diameter, then you'll need to use the software to create your own datasheet. Similarly, if you plan to use a non-metallic boom, or you plan to mount your elements insulated from the boom, then you'll also need to use the software to create your own datasheet. The software is free and very easy to use (click the image above to download it).

Four parts are needed for the antenna:

• 1/2" x 10 ft Electrical Metallic Tube (EMT) Conduit (approx. $2)
• Rigid Metal Rods (approx. $2-3)
• Plastic Standoff (approx. $0.50)
• Short Length of Flexible Coax Cable (approx. $4-5)

We'll use EMT conduit for the antenna's boom.

We need only 5 ft of EMT conduit for a single Yagi antenna, but you'll pay about the same price for a 5 ft length vs. a 10 ft length. Plus, you can save the other 5 ft for another Yagi.

Note that if you can't fit the 10-ft EMT into your car, just borrow a hacksaw from the Tools section of Lowes/Home Depot and cut it yourself in the store. Or, ask an employee to cut it for you.

EMT conduit is a fraction of the cost of aluminum or copper. Similar to aluminum, it's lightweight and more corrosion-resistant than copper. But, unlike aluminum, you can solder EMT as long as you sand-off the finish at the intended soldering point. (EMT does not accept solder as readily as copper, so it's still a bit of a challenge; be sure to use plenty of flux.)

We'll need some form of thick, rigid metal for the antenna's elements (reflector, driven element, and directors).

There are many options for this metal, spanning a wide range of prices. Two of the cheapest options that I've found are a galvanized steel tomato cage or metal coat hangers.

You can buy a galvanized steel tomato cage from your local hardware/garden store for $2-3. This metal is well-suited for outdoor use. Alternatively, 7-8 metal coat hangers will suffice. I purchased an 8-pack from Walgreens for about $2.50 (shown on the right). These particular hangers have a plastic coating, which can easily be stripped off.

Keep in mind that you really just need metal rods or tubes that can be soldered. If you can't find coat hangers or a tomato cage, you can use an old grill, thick copper wire (6-12 AWG), welding rods, or some form of metal tubing, perhaps from an old TV antenna. In fact, using larger-diameter metal is supposed to increase the Yagi's bandwidth, which may lead to better results.

The total length of metal that you'll need depends on the number of elements you intend to put on the Yagi. For the 14-element Yagi shown in this tutorial, we need about 9 ft (2.7 m) of metal.

We'll need one plastic standoff to mount the Yagi's driven element to the EMT conduit (boom).

The driven element is the only element that's physically connected to the coax (and thus to the modem). The driven element needs to be electrically isolated from the boom; all other elements will be soldered to the boom.

Find something which you can repurpose for use as the standoff, preferably something durable. A plastic bottle cap, for example, will work; but, it might not last long in the sun. A 1/2" PVC T-fitting, an electric fence insulator, or even a 1/2" hose repair coupling are better choices if you plan to mount your antenna outdoors.

Here, I will demonstrate the use of a 1/2" PVC T-fitting as the plastic standoff.

We'll need about 1.5 ft of flexible coax cable to create the balun and pigtail connection for the antenna.

For this short length of coax, I suggest choosing the type of coax with the same impedance as your longer coax run (the long run of coax that connects the modem to the antenna). 

• If you're using 50-Ohm coax such as LMR400, then use RG58 for the antenna's balun/pigtail.

• If you're using 75-Ohm coax such as RG6, then use RG59 or RG6 for the antenna's balun/pigtail.

The previously described datasheets contain information regarding the baluns for RG58, RG59, and RG6.

RG58 is 50-Ohm coax; RG59 and RG6 are 75-Ohm coax. 50-Ohm coax is theoretically matched to the impedance of the antenna and the modem. But, in practice, the impedance of the antenna varies within the 4G LTE and 3G frequency ranges, so there's no way to be perfectly impedance-matched across the entire band of interest.

The antenna's balun/pigtail is very easy to construct. There's no harm in trying RG59 or RG6 if you already have some on hand. You can always replace it later with the RG58 version. (Click here for more notes on this topic.)

Here, I will demonstrate using RG59 because I had a remnant piece available (with a pre-attached F-type connector).

The following 10 steps will be used to build the antenna:

• Making the boom
  • Step 1: Cut the EMT conduit in half
  • Step 2: Draw guide lines along the length of the boom
  • Step 3: Mark the positions of the elements
  • Step 4: Drill the holes for the reflector and directors
• Making the elements
  • Step 5: Measure and cut the reflector and directors
  • Step 6: Measure and cut the driven element
• Mounting the elements to the boom
  • Step 7: Mount the directors to the boom
  • Step 8: Mount the folded dipole to the boom
  • Step 9: Mount the reflector to the boom
• Making and mounting the coax connection
  • Step 10: Create and attach the balun and coax pigtail

• Hammer
• Drill with 3/32" bit (or anything large enough to accommodate the elements)
• Hacksaw
• Metal file and coarse sandpaper
• Center punch (or nail punch, or awl, or strong nail/screw)
• Wire strippers
• 4-ft or longer straight edge (ruler or level or other)
• Ruler with cm scale and mm tick marks
• Soldering iron (40-80 W preferred); plus solder and flux
• Permanent markers (one fine-tip marker and one medium-tip marker)
• Epoxy and screws

 If you don't have a particular tool, it's certainly possible to use a substitute. A vise is also highly recommended.

It helps to use a vise to hold the conduit while cutting (see Figure 1). If you don't have a vise, you can use clamps or your foot.

After you've cut the conduit, briefly sand or file the cut ends to remove any of shards of metal.

Again, one of the 5-ft sections of conduit will serve as the antenna's boom.

To draw the top line, place your straight edge along the length of the boom, clamp the boom to the straight edge (or step on the boom to prevent movement), and then follow the straight edge with your marker, drawing a line on the boom.

To determine the location of the bottom line, use your ruler to measure 18 mm down from the top line (the diameter of 1/2" EMT conduit is 18 mm). Mark this location. Then, flip the boom over and use the straight edge and marker to draw the bottom line--starting at your mark--in the same fashion used for the top line.

Using your ruler and marker, measure along one of the guide lines and mark the positions of the elements according to the Yagi Calculator datasheet. Repeat this process for the other guide line. Thus, you should have marks for the elements on both sides of the boom (i.e., along both guide lines).

Note: The idea is to have marks...

As suggested in W6PQL's article, the idea is to have marks on both sides of the boom so that you can drill halfway through from each side of the boom rather than drilling all the way through in one shot. Trying to drill all the way through in one shot is a recipe for misalignment. By drilling from each side, you have a better chance of having aligned holes (though, slight misalignment is inevitable and can be corrected later by bending the elements slightly).

Figure 3 shows a screenshot of the datasheet for 4G LTE with the element positions highlighted in yellow.

• The reflector is to be located 30 mm from the beginning of the boom.
• The radiator (driven element) is to be spaced 76 mm from the reflector, which is 106 mm from the beginning of the boom.
• For the directors, the "Spaced" column indicates the spacing from the previous element. The "Boom position" column indicates the position relative to the beginning of the boom.

Figure 4 shows an illustration of the relative position ("Spaced") and absolute positions ("Boom position") of the first four elements.

Note: Unless you happen to have a metric ruler...

Unless you happen to have a metric ruler that's 5 ft long, you'll inevitably need to use the numbers in the "Spaced" column to measure the spacing from the previous element rather than the position from the beginning of the boom. The disadvantage of using the "Spaced" column is that it can lead to error-propagation. But, if you take your time, the errors can be negligible (sub-millimeter).

You can round the datasheet's numbers to the nearest millimeter without affecting performance. But, when rounding, you must be consistent for both guide lines; otherwise, you'll introduce misalignment.

Marking the element positions along each guide line is a tedious and time-consuming process. But, it's important that you take your time and measure correctly. The element positions are not as crucial as the element lengths (see Step 5). The most important thing is to be consistent for both guide lines in order to maximize the alignment of the holes on both sides.

Figure 5 shows the boom with the marked positions. The "FD" stands for "folded dipole;" this is the driven element (radiator).

Figure 4: Diagram of the relative and absolute positions of the first four elements based on the 4G LTE Yagi datasheet.

Figure 5: Mark the position of each element along the guide lines on both sides of the boom.

To keep the drill-bit from drifting during drilling, use a center punch to make an indentation at each marked position (except for the driven element); see Figure 6. If you don't have a center punch, a nail punch, awl, or even a strong nail or screw will work. (As you can see in the background of the left image of Figure 7, I had an actual nail punch, but I couldn't find it when I needed it; so I resorted to using a plain nail.)

Next, drill the holes halfway through from each side of the boom (see Figure 7, left). If you don't have a drill bit that matches the size of your elements, it's better to go slightly smaller, since you want a tight fit during soldering.

As you drill the opposite-side hole for each element, run the drill bit briefly though both holes to remove metal shards (Figure 7, right). If your drill bit is too small, you can rock the bit in a circular motion to slightly enlarge the hole (but, don't overdo it--remember, you want a tight fit so that the elements don't move during soldering).

Figure 7: Drill the holes halfway through from each side of the boom.

Figure 8 shows a screenshot of the datasheet for 4G LTE with the reflector and director lengths highlighted in yellow.

• The reflector is 197.6 mm long (round to 198 mm).
• For the directors, the "Length" column indicates the length of each director (again, you can round these to the nearest millimeter).

The datasheet states that the elements lengths must be within one millimeter of the stated lengths.

Use a pair of metal-cutting dikes or a hacksaw to cut your elements. You can remove bends by gently pounding them out with a hammer.

Be sure to cut each element a bit long, and then file the element down to the correct length. This filing also helps remove jagged edges from the cut ends.

Figure 9 shows one of the elements, stripped, straightened, and then cut/filed to length. The element shown is Director 1, which has a length of 178 mm. Figure 10 shows the reflector and all 12 directors cut to length.

After all of the elements are cut to the proper lengths, mark the midpoint of each element. Also mark half the boom diameter (9 mm for 1/2" EMT) away from both sides of the midpoint; see Figure 11. These marks will later serve as guides when mounting the elements to the boom. The particular element shown in Figure 11 is the reflector, which has a length of 198 mm, a mark at the midpoint of 99 mm, and flanking marks at 90 mm and 108 mm.

Note: It's wise to label the elements...

It's wise to label the elements as you cut them. The lengths are so close to each other that it's very easy to get them confused. As shown in Figure 10, I simply poked the elements through a labeled piece of paper to keep them in order.

As a final sanity-check, you can line-up the elements against a straight edge to ensure that they gradually decrease in length from the reflector to the last director. As you can see in Figure 10, this is indeed the case. Also observe that the lengths are extremely close to each other, so only pull them out from the paper one at a time, on an as-needed basis.

Figure 9: Cut each element a bit long, and then file it down to the correct length.

Figure 10: All 12 elements cut to length and organized.

Figure 11: Mark the midpoint and two 9-mm-away-from-midpoint points.

The driven element is arguably the most crucial piece of the entire antenna. It's the only element that's directly connected to the coax feed; all other elements serve to focus the signal onto the driven element. The driven element's length will affect the antenna's frequency tuning, and its shape and placement will affect the antenna's impedance.

The Yagi Calculator datasheet lists the specs of the folded dipole, a screenshot of which is shown in Figure 12 (for 4G LTE) with the most important parts highlighted. As stated in the datasheet, the distances should be measured from the insides of the bends (inner edge on one side to inner edge on the other side).

For the 3G version of the antenna, the lengths will be shorter because the design frequency is higher--refer to the actual datasheet PDF.

To create the folded dipole, cut a length of your metal slightly longer than the datasheet's "Total rod length" (403 mm). Then, mark the distances HI and GF (70 mm). Next, bend the coat hanger around some round object to create the bends (e.g., a can, bottle, jar, etc.). Figure 13 illustrates the basic structure for which you should aim.

Note: Unless you have specialized bending tools...

Unless you have specialized bending tools, you'll probably find it nearly impossible to meet all of the distances between each of the labeled points in the datasheet. However, the most important distance is the folded dipole's length (183 mm in the datasheet; the distance AE). As long as you achieve that length, and come reasonably close on the bend diameter length (BI = DF = 37 mm) and the gap (HG = 5 mm or smaller), the tuning and impedance will be fine.

Figure 14 shows the resulting folded dipole, which I created by bending the coat hanger around the spindle of my vise. It's not perfect shape-wise, but it's extremely close to 183 mm length-wise. Also, note that I used a brass-looking coat hanger for this folded dipole only because I already had a spare cut to a total length of 410 mm. You can use the same metal as used for the other elements.

Note: The bend diameter and gap...

The bend diameter and gap are free parameters that can be adjusted in the Yagi Calculator. I chose 37 mm for the bend diameter because that happens to be diameter of the spindle on my vise. If you have something else like a can, bottle, or jar that you plan to use to create the bend, that's fine. Just be sure to maintain the folded dipole's length (AE = 183 mm). In fact, I've created folded dipoles in the shape of a rectangle (and even in the shape of sunglasses), and they have worked fine.

Figure 13: Illustration of folded dipole shape and important dimensions.

Figure 14: Actual folded dipole created from a coat hanger.

Because we'll be soldering the directors to the boom, we need to prep the surfaces to ensure that they accept the solder. To do this, sand around each hole on the boom (see Figure 15).

Now, for each director:

1. Place the element through its hole in the boom.
2. Ensure that the element is centered by using the 9-mm-away-from-midpoint marks drawn previously in Step 5 (see Figure 16). You can also measure the amount by which the element protrudes from each side of the boom. It's centered when these protrusion amounts are equal.
3. Apply a generous amount of flux around the joints.
4. Solder the element to the boom. To do this, first touch the iron to the joint, wait several seconds for the joint to heat-up, and then feed the solder onto the joint.

Warning: The elements will get hot...

The elements will get hot while soldering, so wear gloves and don't grab the elements near the solder joints. The boom also transfers heat quite rapidly, so avoid touching the boom near the solder joint until after it has cooled off.

Figure 17 shows a close-up of one of the solder joints. My soldering job in Figure 17 is poor. There is way more solder than is needed. However, I had a weak 25 W soldering iron that would not stay hot for more than a few seconds after contact with the boom/elements. I had to let the iron reheat for 15-30 seconds after each application, and thus the messy soldering job. It helps to have a more powerful iron or a propane torch.

After you've soldered all of the directors, sight down the boom to check that the directors are aligned (see Figure 18). Bend the directors as needed to correct for inevitable misalignments.

Figure 16: Place each element through its hole, and ensure that it's centered across the boom by using the 9-mm-away-from-midpoint marks drawn previously in Step 5.

Figure 17: Closeup of solder joints (they're not perfect, but they're quite solid).

Figure 18: Sight down the boom to ensure alignment of the elements; bend the elements as needed to correct.
 

The folded dipole needs to be insulated from the boom. This means no metal-to-metal contact and a vertical separation of at least half a boom diameter away from the boom (as a rule of thumb). Thus, to mount the folded dipole, we need some form of insulated standoff.

Figure 19 illustrates how the folded dipole will be positioned around the boom. Because our folded-dipole's bend diameter is 37 mm, and because the boom diameter is 18 mm, the required standoff height is 9.5 mm. A slight deviation from this height is OK (e.g., 10-11 mm instead of 9.5 mm); it will simply result in the folded dipole not being perfectly centered. I have done this in the past with no measurable effect on performance.

To use the PVC T-fitting as the standoff, do the following:

1. Slide the T-fitting onto the boom, measure 9.5 mm up from the top of the boom, and then mark and drill a hole from each side (see Figure 20).

2. Trim the ends of the "T" so that the fitting can slide into place without hitting the first driven element. (You really only need to trim one end of the "T"--the end that will face toward the first director.)

3. Slide the folded dipole through the drilled hole as shown in Figure 21 (you may need to temporarily unfold the dipole to get it around the curves).

4. Drill two holes for mounting screws, and then mount the fitting + folded dipole onto the boom using these two screws (see Figure 22). Ensure that the folded dipole lines up with your previous position mark for the driven element (you may need to rotate the T-fitting to see this mark).

5. Bend the folded dipole as needed to be as straight as possible, and then apply epoxy to the hole through which the folded dipole goes through the fitting (see Figure 22).

As you can imagine, there are numerous way to secure the folded dipole to the boom in an insulated fashion. Experiment on your own to determine the technique that you find best, given the parts that you have available.

A better alternative to the T-fitting would be to use some form of plastic box such as a plastic conduit body. This way, you can potentially enclose your coax connections inside the box.

Figure 20: Prepare the PVC T-fitting by trimming the ends and drilling two opposite-side holes through which the folded dipole will fit.

Figure 21: Slip the folded dipole through the holes (you might need to undo some bends temporarily).

Figure 22: Attach the assembly to the boom.

Following the same procedure described in Step 7, mount and solder the reflector to the boom. Figure 23 shows the result. (Again, the soldering job is not great.)

After mounting the reflector, sight down the boom to ensure it's aligned with the directors, and bend as needed to maximize alignment.

Note: The only reason the reflector...

The only reason the reflector must be mounted after mounting the other elements is to ensure that the PVC T-fitting can slide onto the boom from one end of the boom. If you mount the reflector prior to mounting the driven element, the reflector will prevent you from sliding the T-fitting onto the boom.

Of course, if you're using an insulated standoff that doesn't need to slide onto the boom, then you can certainly mount the reflector at the same time as you mount the directors during Step 7.

The Yagi Calculator software creates a picture of the balun and pigtail, which is shown in Figure 24 for RG59 coax with polyethylene insulation. Here, I will demonstrate using RG59 because I had a remnant piece with a pre-attached F-type connector.

Note: You can choose the coax type...

You can choose the coax type in the Yagi Calculator software. The only thing that changes when using different coax is the length of the balun's "U" loop. Different types of coax have different velocity factors, and it's the velocity factor and design frequency (787 MHz in the present example) that determine the length of the balun's "U" loop.

K7MEM's site has an excellent description and online calculator for the balun.

Regardless of which balun calculator you use, you'll need to know what type of dielectric (inner insulation) your particular coax uses. RG58 commonly uses polyethylene (PE), whereas RG6 commonly uses foam PE. As a rule of thumb, if the dielectric is translucent enough that you can see through to the center conductor, then it's most likely PE. If the dielectric is nearly opaque, then it's most likely foam PE.

For the RG59 used here, look closely at the dielectric in Figure 27. Notice that it's translucent and is thus PE.

To construct the balun and coax pigtail, first cut and strip the pieces as shown in Figure 25. The top piece will be used to create the balun, and the lower piece will be used for the pigtail.

Note: The length of the balun's loop...

The length of the balun's loop (126 mm in the current example) should be measured over the unshielded portion. For this reason, you should initially cut the coax for the balun to be longer than the loop's length so that you have sufficiently long center conductors and sufficiently long twisted outer conductors on both sides. In Figure 25, the total length of the top piece measured from the ends of the center conductors is approximately 180 mm.

Next, as shown in Figure 26, simply solder the connections to match Figure 24's diagram. If you're using RG6, you won't be able to solder the outer braid because it's most likely made of aluminum. In this case, just twist the outer braids together and secure them with a crimp fitting.

Finally, as shown in Figure 27, solder the center conductors of the balun to the folded dipole and use tie-wraps to secure the coax. If you plan to mount the antenna outdoors, be sure to cover the stripped portions of the coax with a thick coating of epoxy to prevent water ingress.

The construction of the antenna is now complete! Figure 28 shows a photo of the completed antenna.

Figure 25: Cut and strip the pieces for the balun (top) and coax pigtail (bottom).

Figure 26: Solder the connections to match the diagram.

Figure 27: Attach the balun and pigtail to the folded dipole.

Figure 28: Completed antenna.