Building a Coaxial Cable
That’s enough theory! Now it’s time to get your hands dirty and get started on this chapter’s project, which is building a coax antenna cable. Cable construction opens new freedom to creating wireless projects.With this skill, you can order the components you need and custom-build a cable that fits your application perfectly. And the cost of the components is usually lower than buying a pre-built cable. N-Male is the most commonly used connector for Wi-Fi cabling, because most antennas have N-Female connectors. And, as you know, N-Male mates to N-Female. So, these steps will assume you have chosen LMR-400 cable with the standard N-Male connector. Please adapt the steps to your application where needed. Figure 1-8 shows the necessary dimensions for a Times Microwave N-Male connector. Table 1-4 shows a list of connectors for use with LMR-400 cable. These connectors are solderless and each requires only two crimps. The connector types listed here are for hand-tightening. A myriad of other connector types are also available. With the right set of tools, building a cable is a step-by-step process:
1. Prepare the cable
2. Slide the crimp ring onto the cable
3. Strip off the outer jacket
4. Pull back the inner shield
5. Strip the dielectric foam core Chapter 1 — Building Your Own Wi-Fi Antenna Cable
6. Remove any shorting material on the foam core
7. Cut the center conductor to correct size
8. Place the center pin onto the center conductor
9. Crimp the center pin
10. Place the connecter body onto the cable
11. Replace shield over the connector body
12. Place the crimp ring over the shield and connector body
13. Crimp the crimp ring
14. Inspect your finished product Antennas
Step 1: Preparing the Cable The cable is treated as a bulk item until ready to assemble the end connectors. So the ends are often cut into an irregular shape. Use the cable cutters to square off the end of the cable, as shown in Figure 1-9. After cutting the cable, the dielectric foam will become elongated. Use a set of pliers to reform the foam into a rounded shape, as shown in Figure 1-10. This will make it easier to strip later. Don’t worry about the shape of the shield and outer jacket.
Step 2: Placing the Crimp Ring Before going any further, place the crimp ring onto the cable as shown in Figure 1-11. Slide it out of the way down the length of the cable. Later, you’ll pull the crimp ring into place on the back of the connector shell.
Step 3: Stripping and Removing the Outer Jacket There are special tools for stripping all types of cables, but a razor blade works well, costs less, and is more versatile. If you are very good at handling sharp objects, a pocket knife works too. Check the instructions that came with the connector for exact dimensions needed. Strip off about 1/2 inch more than necessary to leave room for trimming. When stripping a cable with a razor blade or sharp knife, try not to nick the underlying elements of the cable. By rocking the razor blade, you will score through the jacket without harming the shield underneath. Figure 1-12 shows a cut taking place around the entire circumference of the cable. It’s a little unclear at this angle, but my fingers are being kept well out of the way! Cut through the outer jacket just enough to be able to pull away the jacket without harming the shield. After stripping around the cable, make a groove along the length of the cable. Make three or four cuts with just enough force to cut a little deeper each time, as shown in Figure 1-13. The goal is to come as close to the shield as possible without cutting all the way through the cable. Now grab the end of the outer jacket with a set of long-nosed pliers and pull away the jacket. Tear along the grooves scored into the jacket and peel off the jacket with your fingers to reveal the shield mesh underneath, as shown in Figure 1-14.
Step 4: Pulling Back the Inner Shield To get the next cut ready, use your fingers to carefully fan out and pull back the shield mesh layer, as shown in Figure 1-15.
Step 5: Stripping the Dielectric Now strip off the foam dielectric core along with the solid aluminum wrapping. This requires much less force than the cable jacket. Be sure to apply light pressure and try not to nick the center conductor. (See Figure 1-16.) To remove the foam core from the center conductor, just twist and pull.
Step 6: Checking for Shorts At this stage, you need to inspect the cable for shorts along the dielectric. Remember that the dielectric material is a nonconductor of electricity. If there is an electrical short from the center conductor to the outer shield, the cable will not perform well, i.e. if it works at all. The easiest way to accomplish this is with a visual inspection. Check for any stray shielding strands or aluminum foil material. See Figure 1-17 for an example of foil shorting the center pin. Chapter 1 — Building Your Own Wi-Fi Antenna Cable 23 24 Part I — Building Antennas To remove a foil short, use small wire cutters to scrape away and cut the foam at an angle. You can also use a fingernail for any smaller, more elusive bits. The corrected foam should be white all the way around.
Step 7: Clipping the Core Clip off the center core to the proper length for the connector being used. The connector packaging or data sheet should have this specific measurement. In the case of this connector, we clipped it to 3/16 of an inch. If the center conductor is too short or too long, the connector shell will not seat correctly. Figure 1-18 shows the relative length for an N-Male connector. After trimming back the core, remove any ridges or burrs around the cut edge. This will allow the pin to seat properly.
Step 8: Inserting the Center Pin Place the center pin onto the conductor as shown in Figure 1-19. Ensure the center conductor bottoms out at the first stop of the pin. Also, ensure the pin rests within 1–2 mm of the foam dielectric.
Step 9. Crimping the Core This is the first of two crimps for the connector. Ensure your crimp tool has the correct die for the type of cable and connector being used. For LMR-400, the crimp tool should have a die with hex sizes of 0.429 for the outer ring, and 0.128 and 0.100 for the center pins. The center pin for an N-Male connector is crimped using size 0.128. When you crimp coax cables, press all the way down once only. The hex design of the crimp tool die ensures the pin will grip the core properly in six places. If another crimp is applied to “make it tighter,” it could misshape the pin. Place the pin into the crimp tool as shown in Figure 1-20. The bottom edge of the pin usually will have a small ridge to help line it up and keep the pin seated on the core. Crimp down with even, strong pressure. If your crimp tool has the ratcheting feature, it will apply only the necessary amount of pressure before releasing.
Step 10: Placing the Connector Body It’s time to place the connector body over the pin. Figure 1-21 shows the connector about to slide onto the pin. Before continuing, be very sure that the crimp ring from Step 2 is still waiting for you down the cable behind the splayed out shield before you place the connector body onto the cable. Ensure the crimp ring is on the cable before snapping the connector shell into place. Once the shell is snapped into place, it will be difficult to remove. Also, removing and replacing the shell would degrade the cable performance. If the ring is not in place, you’ll need to cut the connector off and rebuild the cable with a new connector. Line up the connector, and begin to slide the connector over the pin, over the dielectric foam, and butt it up against the shield strands. If all goes well, there may be an audible click when the Your Own Wi-Fi Antenna Cable 27 connector mates with the pin. This mating is meant to hold the connector on the pin until the last crimp. Tug lightly at the connector like you are going to pull it back off. It should stay in place under light pressure. If forced or yanked, it may come off, so be gentle. Figure 1-22 shows the connector fully inserted with the shield still pulled back. Notice that the center pin does not extend past the inner ring of the connector.
Step 11: Shields Up! Fan out the shield strands and trim down with scissors, as shown in Figure 1-23.To help cleanup, hold the cable over a wastebasket. The goal is to trim down the shield but still have enough to fit under the crimp ring.Trim the shield down to about a quarter of an inch. Shield strands are made of steel. The thin wires can pierce the skin like a needle in some circumstances. Make sure to handle the waste strands with care, and clean up the area to minimize the chance of accidents. Antennas Chapter 1 — Building Your Own Wi-Fi Antenna Cable 29
Step 12: Placing the Crimp Ring Now pull up the crimp ring you placed in Step 2. The shield will slip under the crimp ring and should be splayed out evenly around the connector body, as shown in Figure 1-24. If the shield is still too long, move the crimp ring out of the way and trim a little more of the shield with the scissors.Try to get just enough shield under the crimp ring, but not sticking out past the ring.
Step 13: Crimping the Ring Finally, it’s time to crimp the crimp ring onto the cable. This is the second of the two crimps needed to make the cable. As in Step 9, use the crimping tool. But this time crimp with the larger diameter hex size of 0.429. Place the tool at the upper edge of the crimp ring, butted against the connector body as shown in Figure 1-25. Crimp with strong, even pressure, and only crimp once, just like in Step 9.
Step 14: Inspecting the Finished Product Now that the cable is complete, it’s time for a visual inspection. Check the back of the connector at the seam of the crimp ring. If there are any shield strands sticking out, cut them off with the razor blade, as shown in Figure 1-26. Chapter 1 — Building Your Own Wi-Fi Antenna Cable 31 Clipping off the loose strands at the back of the connector reduces the chance of injury when you’re screwing on the cable. Loose strands are like splinters that may pierce the skin of unwary fingers. Visually check the front of the connector for any loose bits of metal that may have dropped into the connector during construction. If you find any, remove them to prevent shorts. That’s it! Now repeat steps 1 to 12 for the other end of the cable. After doing a few of these, it will become second nature.With practice, building a cable connector can take just a few minutes.
Choosing a Wi-Fi Pigtail A pigtail acts as a converter between large diameter cables and small connecters commonly used on Wi-Fi cards. Because of the very small connector sizes, pigtails are difficult to build and require highly skilled soldering techniques.We recommend that you purchase pigtails for use in your projects. Several online stores sell pre-built pigtails in specific lengths. To purchase a pigtail, the vendor will need to know a few things:
Length of pigtail: should be less than 2 feet to keep signal loss low
Cable connector : the type of connector to plug into the larger cable (usually male)
Device connector: the type of connector to plug into the Wi-Fi device (usually female)
The device connector is specific to the type of Wi-Fi card or access point being used.
Building Antennas Connector Types for Wi-Fi Cards
There are almost as many connector types as there are Wi-Fi device manufacturers.Table 1-5 lists some of the most popular connectors. The MMCX, MC-Card, RP-SMA, and RP-TNC male connectors and some of the female devices to which they attach are shown in Figure 1-27.
Finding Pigtails
Pigtails are not available in stores. They must be purchased from vendors that construct them on a regular basis. Sometimes you can find them locally at swap meets or user group meetings. Usually it’s easier to buy them online. Here are some popular sites: www.ecwest.com
www.fab-corp.com
www.hyperlinktech.com
www.wlanparts.com
www.ydi.com
These stores generally sell antennas, wireless devices, and cables as well as pigtails.
Cheap Cable Testing
When a transmission problem arises in a Wi-Fi system, the first place to look is at the cables and connectors.
Connectors generally take the most physical stress in a system, and also can be the first piece to break down while operating in poor conditions. The middle of the cable or the inside of an antenna is less likely to sustain damage if stressed when compared to the cable ends and connectors. This is where simple cable testing can be of great value to troubleshoot a system.To check for continuity and for shorts, use the ohm-meter function on a multimeter.Test the entire length of the cable through each connector. 1. Check for continuity from center pin to center pin. This should be a short or zero ohms.
2. Check from connector body to connector body. This should be a short or zero ohms.
3. Check from center pin to connector body.
This should be open or infinite ohms. Often when you’re testing a cable, it’s already installed on-site, which limits access to the cable ends. To get around this, disconnect both ends of the cable and short the center pin to the connector body on one end only. Then measure resistance of the pin to the body on the other end. The resistance should still be zero ohms (or very close). For the unlimited budget, products like a time domain reflectometer (TDR), spectrum analyzer, RF Power meter, and network analyzer can be used to test entire transmission systems, including the cable. These usually cost several thousand dollars to buy and hundreds to rent. If the connector is presumed bad, replacing it is often much less costly than extensive testing. And very often, the only way to fix a bad connector is to replace it and start over.
Summary
Wi-Fi is radio at microwave frequencies.
Transmission lines at 2.4 GHz are more prone to signal loss and must therefore be considered an important part of the entire Wi-Fi system. A low-loss, large diameter cable and a pigtail adapter makes it easy to position the antenna for the best radiation pattern and signal strength. Building connectors on-the-fly opens up a new realm of independence. By obtaining the cable in bulk, and the various types of connectors, it becomes a simple process to build your own custom cables tailored to each application. And the cable will be exactly as long as necessary. Read on to the next chapter to explore antennas: how antennas are defined, antenna radiation patterns, choosing an antenna, and pros and cons of high-gain antennas. You’ll even see how to build a simple omni antenna that will boost your range by up to 200 feet.
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