We will talk about several common coaxial cables and why they should be used. The RG is an old term meaning “Radio Grade” The U means Universal and dates back to the MIL-HDBK-216 published in the early 1960’s. The commercial version would be RG-174U and the quality would be subject to the specific manufacturer. Certain brand name manufacturers make an excellent version of the RG-174U they are also the ones making the military version most likely on the same production line.  The Military Version would be M17/173-00001 which is QPL and certified to meet the specifications of MIL-C-17 which is the current Military Standard. The manufacturer must have the production line certified by the US Government to meet those requirements. 

What Frequency Is the cable used?

When choosing a cable the 1^st question one should ask is what frequency is the signal that I am trying to send over the cable? Is the signal digital or analog? Digital signals are summations of sine waves with the highest frequency ones giving the digital square wave its edges. If the system is sensitive to sharp edges or fast rise times this should be taken into account. We have found a general formula to use: is three times the clock speed for a digital signal. For a high performance digital signal use 5-7 times the clock speed. Example: if the clock speed is 1 GHz use a minimum of 3GHz to 7 GHz for the transmission line. The reason for this is the high frequency signals are attenuated to a greater extent within the transmission line. This causes rounding of the signal and a loss of the sharp edges.

What are the different parameters for a coaxial cable?

Frequency response:

How high does the cable go before moding? Size matters, the smaller the size the higher the frequency, but the cost is higher loss. All things being equal, larger cables have lower loss.

Loss Per Foot or Meter:

In general the larger the cable the lower the loss. Another thing that matters is the dielectric material that the cable is made from. Foam dielectric is lower loss.  The reason foam is lower in loss is the dielectric properties are much closer to air. The drawback of foam cables is water adsorption. Further most foams cannot tolerate soldering temperatures so the connector must be crimped for attachment.

Type	RG-316	RG-174	RG-188
Temp	-70C-+200	-50C+85C	-200C +250C
Jacket	FEP	PVC	PTFE
Shield	20-40dB	20-40dB	20-40dB
Dielectric	PTFE	Polyethylene	PTFE
Loss at 1 GHz	26	27	27
Loss at 18 GHz	90	95	100

Dielectrics- The magic of the electromagnetic wave.

Cables are bound by Maxwell’s equations and all coaxial cable must follow a set of parameters and ratios for the proper impedance to be set. With that said the lowest loss dielectric would be air which is a wave guide. To mimic the waveguide and be more practical a dielectric was developed. The dielectrics vary with the cables. Common dielectrics are PTFE Dielectric constant 2.2. Foam Polyelethlyene.

Shielding and its construction: Shielding is how much cross talk you are going to get from outside signals and how much the cable will radiate. Coaxial cables can vary from 15 dB of shielding to over 100 dB of shielding. Construction of the shielding can be very different. Example: there a braid shield, foil shield woven wires etc. The foil shield will provide high quality shielding but is subject to breakage at the crimp. Braids are not as good for shielding but are much better at the crimp attachment. Some manufacturers combine both for greater shielding. The drawback of this combination is cost.

Temperature

The materials the cable is made of determine the temperature. PTFE –200 to +250C has the largest temperature range. PVC has a very modest range –50 to +85C, but PVC is very inexpensive and that temperature range is suitable for indoor applications.

Sunlight and Cables UV Light Resistance

PVC Jacket will last about ½ as long as Polyethylene with a black carbon black jacket. That is why one popular manufacturer makes their cables  black jacketed with polyethylene, they are designed for the outdoor cellular tower.

Fire and Cables

The materials used will determine how susceptible the cable is to fire. PVC is a common cable material most.  Should be flame resistant which means if the fire is removed the jacket should stop burring. This is not always the case with some brands of cables. A simple test is a burn test. Hold a lighter to the end, of a cable (out doors is a good idea) and after the flame is removed will the cable continue to burn? Our findings is about 1 in 4 cables will continue to burn even though claimed otherwise. Do a burn test of your own to be safe especially if the cable is PVC plenum rated. If you are using a unrecognized brand do a burn test.

Major Groups

Flexible coaxial – Charteristics of these cables is the outer jacket is made of a polymer such as PVC, PTFE, Polyethylene,

Semi Rigid- Characteristics of these cables is they are solid tubes made of copper or Aluminum and offer the best shielding. Their major drawback is the flexibility. The cable can be very rigid Copper jacketed 0.250 to very flexible Applied Interconnects Polycoated 0.141.

Conformable Coaxial- These cables are more flexible then semi rigid and less flexible than flexiable coax.

Helix- is like semi rigid but usually much larger outside diameter such as 0.25 inch and 0.50 inch. These cables were developed for the cellular industry for the outside towers.

Groupings of flexible coaxial cables:

The most common cables are grouped in 4 categories

0.07 inches OD cables RG-178

0.1   Inches OD cables RG-174, RG-316, RG-188 LMR-100, SS-405, T-Flex 405

0.2   Inches OD cables RG-142, RG-400, RG-58, RG-59, RG-223, LMR-195,

0.4   Inches OD cables RG-393, RG214, RG213, LMR-400

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