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Coaxial Cables

Products Explained Coaxial Cables

If you're going to be installing any kind of two-way radio in your vehicle or at your house you will be using coaxial cable. Here are a few frequently asked questions about coaxial cable.

Q) What is so special about CO-AXIAL cables and connectors?

A) For ordinary electrical wiring we typically use two parallel wires (typically red and black) in DC (Direct Current) systems such as cars, and three parallel or twisted wires (typically Red, Blue, and Green/Yellow) in AC (Alternating Current) systems found in homes and buildings. In DC systems the positive wire is always positive and the negative is always negative, but in AC systems the live and neutral alternate from positive to negative about 50 times per second. Radio waves (RF – Radio Frequency) also have a "live" and a "neutral" component and they can also travel along a conductor. However, the "live" and "neutral" states alternate MUCH faster than ordinary house electricity. For example, the old style CB radios on 27MHz (Megahertz) propagated a wave that switches state 27 million times per second. Your mobile telephone generates a signal that oscillates at around 900 million times per second. At these frequencies radio waves start to behave quite differently from ordinary electricity. They induce current in nearby conductors, and they tend to want to try to "leave" the wire and propagate into space. In fact, an antenna is simply a conductor that is designed to allow the waves to "escape" easily into space. RF also introduces the concept of "impedance" to cables. "Impedance" is similar to "resistance" in that it is a force that resists the propagation of electricity along a conductor, and it might be convenient to think of "impedance" as "RF resistance". Using ordinary parallel wires at radio frequencies RF is possible, but they can't be easily bent, and they must be clear of nearby conductors, so for practical reasons, coaxial cable is preferred.

Coaxial cables are designed in such a way that the radio waves traveling through them are "trapped" inside the cable, and are transported to the antenna efficiently. Coaxial cables (coax) are typically designed using an insulated centre conductor, surrounded by a braided shield. The entire cable is then insulated again. The ratio of the diameter of the inner core to the outer shield is CRITICAL, as is the composition of the insulating material. Poorly designed or poorly manufactured coax will cause losses of power, not only on transmit, but also on receive. Coaxial connectors are also designed with the same parameters, which is why they come in so many strange shapes.

Q) Which coax should I use?

A) For most two-way radio work there are two types of cable commonly used. Both are designed to have a characteristic impedance of 50Ω (Ohms). RG-58 is 5mm in diameter and RG-213 is 10.3mm in diameter. RG-58 is more "lossy" than RG-213, but it is more flexible and thin enough to go under carpets in vehicles etc. The loss through a short length of RG-58 (under 10m) is negligible.

Coax quality is of the utmost importance. Good quality coax is less "lossy" than poor quality coax. A good indication of the quality is to see if the cable has any military specifications. Printed on the outer insulation should be the description MIL-STD-C-17 or similar. This is still no absolute guarantee of quality, but it gives a good idea. The best way to judge cable is to trim a bit off the end and look at the braid. In a good cable you won't be able to see the inner core through the braid.

Annotated Coaxial Cables good and bad.JPG
Good vs. bad quality coax

For most uses it is important to use a coax with a multi-strand centre conductor. The solid conductor will snap if you install the cable under a carpet, or where there is movement in the cable.

Q) Why are there so many different connectors, and which one should I use?

UHF Family.JPG

A) There are about ten different connectors that are commonly used in radio applications. They're mostly from one of two families of connector:

The PL-259 family (sometimes inappropriately known as "UHF" connectors) are most commonly used on CB radios. They're actually probably the worst connectors to use from a performance point of view, but they are very easy to install and they're cheap. They are not waterproof.

Annotated PL-259 good and bad.JPG

The "N type" family of connectors include the original "N-type" (Navy type), the BNC (Bayonet Navy Type), and the TNC (Threaded Navy Type). All of these have the same basic design, but with slightly different fastening methods. A good quality, properly installed "N-type" connector is waterproof. All "N-type" connectors are good to use, even up to microwave frequencies.

N-types.jpg

Coaxial connectors are supplied as either "solder type" or "crimp type". Crimped connectors are usually used in bulk-fitted commercial applications, and they require specialized crimping tools. The average radio user would use solder type connectors. A well installed, soldered connector is always preferable.

Q) How do install a coaxial connector?

A) Like anything else, they're quite easy to install once you have done a few, but your first few are likely to be a complete bodge. Start by identifying how much of the outer conductor you need to trim away. Using a sharp knife, trim through the insulator, but be careful not to nick the braiding. If you damage the braid, cut the end off and start again. Once you've assembled the connector, solder it where necessary using only resin cored solder. Use a soldering iron that has enough power to heat the whole pin and allow solder to be sucked into the pin by capillary action. Don't hold the connector with the pin upwards to allow gravity to get the solder into the pin. Rather, hold the connector at a slight downward angle. When the solder and the pin get to the correct temperature, the solder will flow UPHILL into the pin. That will be a decent connection. You can tell if you've soldered correctly if the connection looks like it has flowed, rather than "blobbed". Have a look at the picture for an example of good and poor soldering.

With the PL-259 family there are two schools of thought about dealing with the braid. The first (and the method that is far easier) is to cut the insulation back, then fold the braid backwards over the insulation. This is then screwed into the body of the connector, and a good connection is made through the thread on the back of the connector. The other method is to trim the insulator away, then insert the cable into the connector. The braid is then soldered through the little holes in the connector body. This is a better way to do the job, but the amount of heat needed to melt the solder usually melts the cable too. It's a tricky method, so I prefer the other way.

Q) My coax is too short to reach the radio. Can I lengthen it?

A) Yes, you can lengthen coaxial cable, but don't just solder the cable to another piece of cable. As I said earlier, the dimensions of coax are critical to it's impedance. If you solder pieces together and use insulating tape you will change the characteristic impedance and you will lose power through the coax.

RG213 connected to RG58.jpg

To lengthen a piece of coax you should install a good quality female connector on the one piece, and a good quality male connector on the other piece, and join the two. You can then wrap self-vulcanising tape around the two connectors to keep them waterproof and tight.
Author
Marc Lurie
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