How to choose the right cable assembly?

How to choose the right cable assembly?

"Good softness, light weight, low loss, small standing wave" is the standard of perfect cable assembly in most people's eyes. However, due to the structural limitation of the cable itself, many indicators are contradictory. For example, if you want the loss of the component to be small, it will inevitably lead to a relative increase in the thickness of the cable, thus affecting the flexibility of the component. And can be close to the ideal of those cables because of high quality raw materials, special workmanship, the cost will be greatly increased. So, how to choose a cable assembly?

RF coaxial cable assembly is a transmission line composed of RF coaxial cable and RF coaxial connector, which are connected together by a certain connection method.. When selecting a cable assembly, it is necessary to have a necessary understanding of the RF coaxial cable and connector, including structural size, mechanical properties, frequency of use, attenuation, etc., and the understanding of the connector should include the connector structure, connector connection method, and frequency of use. Range, etc., and then choose the appropriate connection scheme according to actual needs.

1.Structure of RF coaxial cable

RF coaxial cable from the structure can be divided into four layers:

First layerRF coaxial cable inner conductor--The core wire has the following structures

• Single: It has the advantages of high frequency band and small insertion loss. The disadvantage is that the core wire has a certain strength and flexibility, which has a greater impact on the phase stability of the cable when the bending radius is small.
• Multiple stranded: in the use of frequency band and insertion loss is weaker than a single core wire, but the core wire strength is small, good flexibility, in the case of a small bending radius, the phase stability of the cable is better than a single core wire.
• Single (silver-plated) copper-clad steel wire: Due to the high strength of the core wire and its good bearing capacity, the core wire can be directly used as the pin of the connector, which simplifies the structural design of the connector and makes it lighter. The disadvantage is that the frequency band is not high, the attenuation is large, the heat dissipation is poor, and the flexibility is weaker than the copper core when bending.
• Single copper-clad aluminum wire: The advantage is light weight and low cost, but the disadvantage is that the heat dissipation and solderability are weaker than the copper core.
• Aluminum tube or corrugated copper tube: the physical size of the two is larger, made into a tubular is to reduce weight, save material, but in the same size, the attenuation of the aluminum tube is larger than the copper tube, carrying power is smaller than the copper tube.

Second layer: RF coaxial cable inner and outer conductor support medium--The insulating layer, which determines many of the electrical and mechanical properties of RF coaxial cables, is commonly used in the following materials

• Solid PTFE (PTFE): commonly known“The Plastic King”, Is one of the best engineering plastics, it has chemical stability, sealing, self-lubricating, non-stick, electrical insulation, good anti-aging endurance, high temperature resistance (can be in-180 ℃ ~ 250 ℃Long-term work), corrosion resistance. dielectric constant2.02, good uniformity, convenient processing, suitable for phase requirements of the cable, but to have a certain degree of hardness, especially in the size of the larger flexibility becomes poor, and it can not be processed by hot pressing molding method, its radiation resistance is particularly poor, in addition, his scrap and waste can not be recycled like other plastics.
• Solid core polyethylene (PE): It has the characteristics of corrosion resistance, self-lubrication, non-stick, electrical insulation, good anti-aging, and low price. The disadvantage is that the use temperature range is small (-40 ℃ ~ 80 ℃), dielectric constant2.2~2.4.
• High foaming polyethylene: the use of advanced extrusion and gas injection technology, foaming degree can reach80%Above, this makes the dielectric constant low, the media loss factor is small, to ensure that the attenuation is small, the structure is consistent, to ensure uniform impedance, large return loss, mechanical performance is stable to ensure long life, waterproof and moisture-proof.
• High-foaming PTFE: Compared with polyethylene foaming, it has the advantages of high temperature resistance and low dielectric constant, which determines that it has excellent electrical properties and the ability to cope with harsh environments.
• Lotus or skeleton type air mixed insulation support: low transmission loss, working frequency bandwidth, light weight It is easy to lay and install, and the electrical performance is stable and reliable.

Third LayerRF coaxial cable shielding layer--The outer conductor has the following structures

• Single-layer (silver-plated) copper wire weaving: Single-layer weaving mainly takes into account the low price, good flexibility, and small bending radius. The disadvantage is that the use of low frequency bands and low shielding efficiency can only meet the low-end transmission requirements.
• Double-layer multi-strand (silver-plated) copper wire braiding: It has better shielding and lower attenuation than single-layer braiding, the use frequency band is improved, and the cable has strong recovery ability when bent and deformed.
• Single-layer multi-strand (silver-plated) copper wire braided with aluminum foil: Compared with the double-layer multi-strand copper wire braided layer, aluminum foil replaces the first layer of braided layer, which has a larger coverage area than copper wire, better shielding, and weight Reduction, better flexibility, and improved performance.
• Copper tube: The outer conductor has good rigidity, high strength, very stable mechanical properties, excellent shielding, low attenuation, low standing wave ratio, high power and other advantages. The disadvantage is that it is difficult to bend and form once.
• Corrugated copper tube: two kinds of ring and spiral, the outer conductor adopts high precision copper tube, with low attenuation, low voltage standing wave ratio, low temperature coefficient, tensile strength, high power capacity and other characteristics. The cable with a spiral shape is an ultra-flexible cable with a small bending radius and many bending times.

Fourth layerOuter protective layer of RF coaxial cable--Sheath, optionally

• Polyethylene (PE)
• Poly (fluorine ethylene propylene) (FEP)
• Polyvinyl chloride (PVC)
• Low-smoke halogen-free (LSZH)
• Silicone rubber material
• Metal armored pipe sheath

II.RF coaxial connector

There are many types of connectors. According to different technical characteristics, they are divided into audio (20KHzThe following), video (30MHz~500MHz), RF (500MHz~300GHz), optical connector (167THz~375THz)

Among them, the radio frequency connector (RFConnectors) are classified according to their size and cable compatibility. The following models are mainly commonly used.

• NType Connector: StandardNType A connector according to American standard is11GHz, but the precise typeNThe type-connected connector adopts a non-slotted structure, and its working frequency can reach18GHz. The connector can usually be mated with an outer diameter3~12mmflexible, semi-rigid and semi-flexible cables.
• BNCType connector: called bayonet typeNType connectors, in addition to individual application cases,BNCThe frequency band used by the connector cannot exceed4GHz. Many applications are used2GHzthe following occasions.
• TNCConnector: its shape andBNCapproach, the standard operating frequency is11GHzThe working frequency of the precision type is18GHz. The biggest advantage of this type of connector is good shock resistance.
• SMAType connector: belongs to the small connector, its working frequency ratioNType of higher, standardSMAThe operating frequency of the connector is18GHz, and precisionSMAConnector working frequency up26.5GHz(using spread spectrum technology). The current application field is the most extensive.

• MCXType-type connector: a polar connector with a fast plug-in structure. Its operating frequency is available6GHz.
• 2.92mm（KType) connector: the highest frequency band of the connector is40GHz, using air medium.