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).
Figure 3 shows a screenshot of the datasheet for 4G LTE with the element positions highlighted in yellow.
Figure 4 shows an illustration of the relative position ("Spaced") and absolute positions ("Boom position") of the first four elements.
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 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.
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.
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.
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:
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:
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).
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.)
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).
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).
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.
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.
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.
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.
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