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Coax Selection

The coaxial cable running from your radio to the antenna is unbelievably important. Everything you transmit and receive must travel along its length. All too often the coax is ignored and performance suffers because of the lack of attention it receives. The fact remains, when it comes to buying coaxial cable, bulk or in assemblies, quality is everything!

Although there are many types of communications grade coaxial cables on the market, this discussion will primarily focus on those used in mobile communications applications. Before starting, a few terms and/or phrases need to be clarified.

Coaxial: two conductors sharing the same center
Center Conductor: the wire at the very center of the cable
Insulator: the material surrounding the center conductor
Shield: the outer conductor surrounding the insulator
Jacket: the outer covering of the cable
Propagation velocity: speed of signal traveling in coax

For mobile installations, there are three (3) primary types of coaxial cable used to build the assemblies.

RG-58 type: This type of coax is used for single antenna installations and for jumper wires that go between the radio and a test meter (SWR meter). The RF resistance of this type of coax is 50-ohms. Within this group you will find coax labeled with RG-58 or RG-58/U. These cables have a solid center conductor. The second type is RG-58A/U and they have a center conductor made up of many thin wire strands (normally about 17) that are twisted together to form the center conductor. The common outer diameter for this type of cable is about 0.20".

RG-8X: This type of coax is also used for single antenna installations or jumpers between pieces of equipment. Like the RG-58 type coax, RG-8X also provides 50-ohms of resistance. In short, this cable could be called hi-performance 50-ohm cable. It always has a stranded center conductor and a high shielding percentage. It will also handle higher power (wattage) and has a higher propagation velocity. For the general user, it is more than what is required. However, if you are using amplifiers or just like to get the absolute most from your set-up, it will deliver. The outer diameter of RG-8X is typically about 0.24".

RG-59 type: This type of coax is used for dual antenna installations only. The RF resistance of this type of coax is 72-ohms. Within this group you will find coax labeled with RG-59 or RG-59/U. These cables have a solid center conductor. The second type is RG-59A/U and they have a center conductor made up of many thin wire strands (normally about 19) that are twisted together to form the center conductor. The common outer diameter for this type of cable is about 0.22".

About the center conductor: We strongly believe that mobile installation should always use stranded center conductors. The reason behind this is due to potential breakage of solid conductors due to vibration and/or repetitive flexing. If you have ever picked up a piece of wire and bent it repeatedly until it broke, you would fully understand our reason for recommending stranded center conductors on mobile installations.

About insulation: There are two common types of insulating material used in coax. First there are the foam (polyfoam) types. Although most specifications sheets show that polyfoam insulated coax has a faster propagation velocity, we do not recommend it for mobile installations. We prefer the plastic types (polyvinyl, polypropelene, etc.) because they are just plain tougher. The small loss in velocity, for all intents and purposes, is insignificant insofar as low power, low frequency communications are concerned. The properties of coax cable changes if the center conductor is not in the physical center. Polyfoam insulation deteriorates faster than the plastic types and also tends to collapse easier than the plastics when pinched or sharply bent

About shielding: The shield surrounds the center conductor and prevents internal leakage and external interference. The typical shield used on two-way radio communication cables is a woven braid. For the most part, it is formed with either bare or tinned copper wire and is a very important consideration when trying to determine the quality of the coax. Low percent coax has a loose braid and exposes more of the center conductor to leakage. Unfortunately, the cable industry did not invent a coding system that designates the type and percentage of shielding used to build the coax. Shielding percentage is the most abused part of the coaxial cable manufacturing process. There was a time when 70% coverage was considered the absolute minimum, but we have seen cheap cable with as little as 58% shielding being sold in recent years. Because it is readily available, we do not recommend coax cables with shielding less than 90%. Using anything less is the equivalent of watering your lawn with a hose that is full of holes and has a restricted opening. You must exercise caution when it comes to shielding.

About the length: This is a testy subject with many engineering types. They have argued with us on many occasions regarding this matter. They say that if your system is set-up properly that the length of the coax is irrelevant. We agree! However, mobile installations have so many variables that a perfect set-up is the exception, not the rule. One guy has a pick-up and another has a fiberglass motorhome. One wants the antenna on the bumper, another on the hood and a third on the roof. Few people want to drill holes in their vehicle so quality grounds are always a consideration. Because of the imperfect world, we almost always recommend 18' (5.5m) when our products are used. We do so with good reason too! At 18' the voltage curve has dropped back to the zero voltage point where the cable meets the antenna which reduces the reactance within the cable itself (a null cable if you would). It has been our experience that if the antenna location makes it somewhat out of sync with its surroundings, cable lengths that are not multiples of our 18' suggestion adds to the problem. To that, our complaining experts say, "Then the antenna should be moved!" to which we say, "You tell the guy with the $30,000 vehicle that he must drill a hole in his roof so he can use a 9' cable". We solve problems in the best way we can given the boundaries that the customer establishes.

On that note, when you have 18' of coax going to a radio that is only 8' away, what should you do with the other 10'? We recommend that you serpentine it like a skein of yarn so that it is 10-14" long and tie it in the center with a wire tie then tuck it away. Do not roll it up in a tight circle as this can cause it to act like an RF choke, which often times will cause system problems.

Other ways to wreck your coax: Wear holes through it, slam it in the door a few dozen times, attempt to splice it as you would a wire going to your taillight, tie it in knots or make real sharp bends in it.

For your information, all of our Fire-Flex coaxial cables have stranded center conductors, polyvinyl insulation and bare copper shielding in the 95% range. Even though we offer some cables that are not 18', we do so for the knowledgeable installer, who knows that if a problem shows up, he must exercise the 18' recommendation. All of our mini-kits and complete kits are packed with 18' cables.

If you have ever installed an antenna and needed coaxial cable, there is a good chance that you paid a premium price for less than standard grade cable. Unless you have knowledge regarding issues related to coaxial cables or have a very trusting relationship with the organization where you are planning to purchase the cable, it is wise to proceed with extreme caution. Over the last 20 years vicious competition in the coaxial cable industry, especially in the pre-made assembly sector, has allowed the market place to be flooded with low-grade coax and you may never know it.

The current cable designation numbers are at best severely flawed. How are coaxial cables identified? Well technically, only cables that are made to U.S. Government specifications can be marked with the "RG" designation. But in reality, many manufacturers/importers pay little or no attention to the requirements. In those situations, it is up to the purchaser to demand that specific standards be fulfilled. However, as sadly as it is, most of the energy goes into bickering over the cost instead of the quality. And even if the cable carries the designated number, there are often cost cutting activities going on right under your nose. Following find the designated characters legend for cables.

R - Means Radio Frequency

G - Means Government

8 - The number in a government assigned approval number

/U - Means it is a universal application.

Letters that appear before the /U characters (i.e. A, B or C) means a specification modification or revision. For instance, it is common in the CB industry to see the designation RG-58A/U. The original RG-58/U coaxial cable had a solid enter conduct. The "A" modification replaced the solid center conductor with a more flexible stranded center conductor (that is highly recommended for use in mobile installations).

So where is the flaw in the numbering system? The numbering system flaw is that there are no hard character designators that indicate the type or quality of the extremely important shield. Typically, the minimum shielding coverage on "RG" cables used for CB and amateur radio is 78%. That is, 78% of the insulator that encapsulates the center conductor is covered (or shielded) with the over-laid copper braid. Firestik, by choice sets the shielding specification on all their cables to 95%. In contrast, we have inspected cables that end users were using on troubled set-ups that had coverage as low as 52% and their problems were solved by simply upgrading the quality of the coaxial cable.

Shielding is very important when it comes to keeping unwanted RF interference from entering the cable. Furthermore, when it comes to delivering energy to the antenna, even under standard 4 watt CB outputs, holes in the shielding creates openings for energy losses. There is barely a day that goes by that our technical staff doesn't fix a user radio performance problem by doing nothing more than having them upgrade their coax. In our business, whenever someone is experiencing poor performance they go right to the antenna. Little or no attention is given to the coaxial cable or the antenna stud mount whose responsibility is to deliver energy to or from the antenna. Our standard story is, "If you bought a twenty-five dollar nozzle and connected it to a garden hose that was full of pin holes, would you blame the nozzle or the hose?" Unfortunately, you can't see RF leaking out of the coaxial cable but it certainly does from low shielded cables.

Pay attention to your coaxial cable! If you want your antenna to perform to its potential, make sure that your feedline and antenna stud is capable of delivering the goods. And once installed, avoid sharp bends, pinching conditions, exposure to extreme heat (which could melt the outer covering) as well as situations where rubbing against a sharp surface could wear a hole in the covering. And under no circumstances should you ever splice the coaxial cable together as you might a common wire. Do not settle for anything other than the best.

 

UNGROUNDED ANTENNA MOUNTS

On standard mobile antenna systems (those NOT designated as no-ground-plane), it is imperative that the antenna mount be grounded to the vehicles chassis. Ungrounded mounts will cause high SWR readings and/or low output power readings. There are three common situations when the mount may be inadvertently affixed to an ungrounded fixture.

1. Swing-away SUV spare tire racks mounted on nylon bushings.
2. Luggage racks or RV ladders mounted with rubberized bushings.
3. Mirror arms on fiberglass trucks or motorhomes.

For proper performance the mount must be grounded. It should be a standard part of the installation procedure to verify that the mount is grounded. It is an easy test to perform with either a VOM meter set in the Resistance (Ohms) mode or in the audible diode testing mode. Another method of checking is with a self-powered continuity tester. These test need to be made with the coax disconnected from the antenna mount or the radio. If the coax is fully connected and the radio is grounded you will get the false sense that you have a sufficient ground at the mount. Grounding the mount via the coax cable to the radio is not good enough. It requires a direct ground. Either the mount has a good direct chassis ground or it is grounded via the object it is mounted to.

The SWR readings on an ungrounded mount will almost always be 2.0:1 or higher on all channels. You may actually see the SWR dip during the testing procedure (i.e. 3.5:1 on channel 1, 2.5:1 on channel 20, 3.5:1 on channel 40). But, anytime it stays high you can be 90% sure that the mount is ungrounded, there is insufficient ground plane and/or you have low quality or the incorrect coax.

Also, keep this in mind. If it becomes necessary to run a ground wire or braid to a chassis ground point, use a wire of at least 12 gauge. We have reviewed installations where the installer used a small gauge wire (20ga) and when the SWR was tested it fell from over 3:1 to below 1.5:1. However, the power out put was very low due to the undersized path to ground provided by the thin wire. The path to ground must be sufficient enough to carry the power being applied to the antenna.

MAGNETIC MOUNTS FOR ANTENNAS

The primary purpose of the magnet antenna mount is to allow the CB’er the means to gain temporary or emergency communications capability. The fact that the user doesn’t need to drill holes in the vehicle should be of secondary importance. Magnetic mounts should always be treated as a temporary solution to an immediate or short-term communications need. If you don’t plan to use your CB on a regular basis, don’t want to drill holes in your vehicle for permanent mounts, or don’t expect the maximum performance from your equipment, than a magnetic mount may be all you need. But remember, in spite of their convenience, a magnetic mount antenna will rarely meet the performance that is realized from a properly installed permanent antenna. However, for short-range caravan type communications or emergency use, magnetic mount antennas are sufficient.

GENERAL CONSIDERATIONS

1. Like all other transmit antennas, those mounted on magnet mounts must be tuned on the vehicle in the location that it will always be used. If you tune the antenna in the middle of the roof then decide to use it on the hood or truck, or a different vehicle, it will require re-tuning at its new location.
2. Always place your magnet on the flattest surface available.
3. Do not abuse the coax cable. It is extremely important. Sharp bends, tight pinches and holes rubbed through outer jacket will affect performance.
4. Always place your magnet on a clean, dry surface.
5. If avoidable, do not use on a vinyl roof. (Adds capacitance and diminishes holding strength).
6. Never leave the mount in one location for extended periods of time. (The vehicle paint will fade at various rates and moisture under mount can cause rust to form under the painted surface.)
7. If you drop a ferrite magnet on a hard surface the magnetic strength may be reduced, or the material could shatter into many pieces.
8. Magnet mounts rely on high resistance, capacitance grounding. If you use power amplification, there is a good chance that heat will discolor the vehicles paint. We do not recommend the use of amplifiers with any magnet-mounted antenna.
9. At highway speeds there are considerable forces acting upon the mount. Even if the antenna is holding fast, a side burst of air from a passing 18-wheeler can hit the antenna with a force from another direction causing it to loose its grip. For that reason, it is always a good idea to have a spring between the antenna and the magnet mount.
10. When removing the magnetic mount from the vehicle, do not slide it to the edge to make it more convenient. Dirt between the magnet and the vehicle paint will surely leave scratches.
11. Never place your magnet mount near your audio and/or video tapes, or computer disks of any type. The strong magnetic field will destroy them. And, if you want to wreck the magnetic strip on your credit card, set them on the magnet for 1 second or more.