“RF coaxial connectors and cables are common components that play a largely invisible but critical role in military applications. The job of this component is to transmit radio frequency signals at frequencies up to tens of gigahertz (GHz) from the antenna to the receiver. It must reliably handle sensitive radio frequency signals and maintain their integrity, while being rugged enough to withstand the rigors of the battlefield.
By Steven Keeping
RF coaxial connectors and cables are common components that play a largely invisible but critical role in military applications. The job of this component is to transmit radio frequency signals at frequencies up to tens of gigahertz (GHz) from the antenna to the receiver. It must reliably handle sensitive radio frequency signals and maintain their integrity, while being rugged enough to withstand the rigors of the battlefield.
For engineers who are looking for RF connectors, there are hundreds of choices. If they only meet some basic requirements, RF connectors are more or less the same, so everyone may tend to choose the cheapest one. But that would be a mistake. Many RF connectors are not manufactured to exacting engineering specifications, and the failure of one poorly designed component could render a multi-million dollar military installation inoperable at a critical time. This makes it crucial that the selected components are built to recognized international standards.
In addition to mechanical properties such as durability, moisture and dust resistance, and electrical properties such as impedance, frequency range, voltage standing wave ratio (VSWR) and insulation resistance, other factors affect the design. For example, in certain military applications, such as rocket launcher rails, interior surfaces of optics, and small arms applications, light reflections from standard stainless steel connectors can cause operational problems.
When looking for rugged and reliable RF connectors, choosing a reputable manufacturer is a good place to start because their RF connectors are field-tested and manufactured to relevant military performance specifications and standards.
This article explores key electrical and mechanical selection criteria for military RF coaxial connectors. And with the example device of Amphenol SV Microwave, their application is discussed. Particular emphasis is placed on the use of high-grade, durable, non-reflective surfaces for niche applications using black chrome RF connectors.
High Performance RF Coaxial Cables and Connectors
The key to ensuring that RF components can meet the stringent requirements of military applications is to purchase components that meet the relevant Military Performance Specifications (MIL-PRF). Only a few manufacturers can provide RF connectors that meet the requirements. One company that can do this is Amphenol SV Microwave. The company offers more than 400 RF connectors compliant to MIL-PRF M39012.
MIL-PRF-39012 is the core standard for RF connectors. It requires visual, mechanical and electrical testing at multiple stages of manufacturing to ensure compliance. The specification also requires that production facilities and processes meet quality standards. Coaxial connectors and adapters that meet the MIL-PRF-39012 specification are ideal for military (and other high-reliability) applications, including radar, SATCOM ground equipment, and aerospace RF/microwave assemblies.
A key standard of MIL-PRF-39012 connectors is that the center pin must be gold-plated to a standardized depth, adhesion, and finish. Gold plating on the mating contacts ensures minimal insertion loss and long-term good electrical contact in harsh environments (Figure 1).
Figure 1: A key criterion of MIL-PRF-39012 connectors is that the center pin must be gold-plated to a standardized depth, adhesion, and finish. (Image source: Amphenol)
RF Coaxial Cable Design Considerations
When designing RF coaxial solutions for military applications, the cables and connectors must be considered as a complete system. The performance of the system is highly dependent on the perfect cooperation of these two components.
Typical coaxial cables consist of a copper core or copper-coated steel wire. The copper core carries the high frequency input/output signals of the connected devices. At the same time a dielectric insulator, usually made of plastic, surrounds the copper core. The thickness of the insulator is fixed and ensures a constant gap between the copper core and the metal shield around the dielectric. This is important because any change in thickness will cause a change in impedance, which in turn affects signal integrity.
Metal shields are braided copper, aluminum, or other metals tightly wrapped around an insulator. Its role is to protect the inner core from external electromagnetic interference (EMI). The assembly is then wrapped in a rubber or plastic jacket to insulate and protect the inner conductors (Figure 2). Outdoor grade cables require extra insulation and a special jacket to protect the wires from sunlight and moisture.
The inner core transmits high-frequency signals, while the shield serves only as a return line. An electromagnetic field exists between two conductors, but not beyond the shield. This means that radio frequency signals passing through the cable will not affect nearby electrical and Electronic equipment.
Figure 2: Coaxial cable consists of four parts: an inner conductor, a plastic dielectric, a return wire/shield; and an outer (black) rubber or plastic jacket. (Image source: Amphenol)
Where the dielectric insulator meets the center conductor, it absorbs some of the electrical energy. For long cables, the attenuation of the signal can be significant. Efficient cables use a barrier that has minimal contact with the inner conductor and creates a consistent air gap between the inner and outer conductors, acting as an effective dielectric.
The Importance of Impedance
In high-frequency RF systems, transmission efficiency depends on matching the impedance of the antenna to that of the transmitter or receiver.
A significant mismatch can result in an ineffective antenna as it interferes with forward traveling power and creates a standing wave. A common measure of impedance equalization is the voltage standing wave ratio (VSWR). A VSWR of 1 indicates no impedance mismatch losses, while higher numbers indicate increased losses. For example, a VSWR of 3.0 means that approximately 25% of the transmitter power is lost in the antenna system.
RF connectors and cables form part of the antenna impedance, so it is imperative for the designer to understand the impedance values of the components in order to maximize the efficiency of the RF system. The characteristic impedance of a coaxial cable is proportional to the ratio of the outer diameter of the inner conductor to the inner diameter of the outer conductor. The main function of a dielectric insulator is to set and maintain this isolation.
In most cases, cable manufacturers supply RF cables with impedances of 50, 75, or 95 ohms (Ω), although other impedances are available. In addition to a range of fixed impedances, cables are available in different dielectric types, capacitances, outer diameters, attenuation characteristics and shielding materials.
Versions are generally divided according to two systems, LMR (the meaning of the letters is historical: it is important to know that these cables are a low-loss type) and Radio Guide (RG). There are more options for the RG type, however not all of them meet military specifications. Reputable suppliers can assist with military device selection.
Precautions for RF Coaxial Connectors
RF coaxial connectors allow cables to be securely connected to radio equipment. No matter how good the cable is, poorly fitting connectors will ruin electrical performance. A mechanically secure and electrically secure fit between the cable and connector will ensure good signal integrity and consistent impedance, even at high frequencies. One way to provide integrity is to assemble the cable and connector together, delivered by the manufacturer as a complete assembly. Manufacturers are in the best position to ensure high-quality connections and also test the integrity of components prior to delivery.
Standard polarized male connectors are threaded on the inside of the housing and have a center pin, while standard polarized female connectors are threaded on the inside of the housing but have no center pin. It is not good design practice to connect male and female headers together, as this will increase the inherent signal loss of the cable. It is better to use one continuous long cable as this will ensure low signal loss and consistent impedance. If you want to connect a male connector to another male connector, or a female connector to a female connector, the correct way is to use a coaxial cable adapter. While this can be used, it is not recommended in high specification applications as it will also increase signal loss.
Like cables, RF connectors come in various types and have fixed impedance values. A common example is the Subminiature A version (SMA). The connectors feature a robust threaded connection mechanism with a standard impedance of 50 Ω and are designed for frequencies from dc (0 hertz (Hz)) to 18 GHz. Applications include microwave systems and handheld radios (Figure 3).
Figure 3: SMA connectors provide a threaded connection, have an impedance of 50 Ω, and can handle RF signals up to 18 GHz. (Image source: Amphenol)
Another common type is the Subminiature B version (SMB). This product is smaller than an SMA and features a snap-fit coupling design. They are available in 50 and 75 Ω impedance and operate up to 4 GHz. Compared to SMA products, this connector is not robust enough to be used in harsh environments.
Other types of RF connectors include; Type F, used in applications such as cable modems and cable TV; Type N, used in commercial applications using thick cables; Type FME, used in cellular communication equipment; Type TNC, used in outdoor cellular applications and the UHF type for amateur and marine radios.
RF connectors attached to RF coaxial cables are typically screwed into reverse polarity connectors attached to bulkheads or directly to devices that receive or transmit RF signals. Bulkhead types have external threads that mate with the internal threads of the cable connector (Figure 4). When choosing a through-board connector, it is best to find a manufacturer that manufactures cable RF connectors. Although standardization ensures that the same type of connector from any manufacturer can be used, there are always some unexpected factors.
Figure 4: The SMA bulkhead connector has external threads that mate with the internal threads of the cable connector. (Image source: Amphenol)
Non-reflective RF connector options
Reputable manufacturers offer a range of straight, angled, and male and female connector options. But some vendors, such as Amphenol SV Microwave, also include specialty components in their offerings. An important addition to the company’s product line is the non-reflective SMA RF connector for both cable and bulkhead applications.
The standard stainless steel or brass finish of industrial RF connectors can cause problems in many military applications. For example, if used on small arms, such RF connectors can reflect natural light and reveal a combatant’s position. Or the use of RF connectors in the optics of an aircraft or helicopter can create light artifacts that can destroy the accuracy of the targeting system.
To meet the demand for non-reflective RF connectors, Amphenol has introduced the black chrome-plated SMA male cable connector 2911-61008 (Figure 5, left), and the matching black chrome-plated SMA female bulkhead connector 2921-61689 (Fig. 5, right). These products have an impedance of 50 Ω and can support RF signals up to 18 GHz with up to 500 connection cycles. The center contact material for each of the above example connectors is gold-plated beryllium copper. Designed for use with RG-405 (0.085 inch), semi-rigid RF coaxial cable, this RF connector meets Mil-Spec requirements and is popular in defense applications.
Figure 5: Amphenol Mil-Spec SMA male cable connectors (left) and female bulkhead connectors (right) are available with a non-reflective black chrome finish. (Image source: Amphenol)
The body material of this RF connector is stainless steel with a black chrome finish per MIL-C-14538. The black chrome layer involved in this specification is unique to the military and is hard, strong, heat-resistant, and completely non-reflective. The connector strip is provided with a non-reflective SMA male dust cap 2911-61009 that caps the female bulkhead connector after disconnecting the cable.
Military applications are subject to dust, grease, heat and vibration. Connectors and cables for these applications are expected to transmit high frequency signals reliably for many years. Some variants of these connectors require non-reflective coatings that are tough and durable enough to withstand high temperatures and shock without cracking or wearing out.
Designers of RF military applications can ensure that their designs meet the required requirements by seeking support from vendors like Amphenol SV Microwave, which can provide products that meet military performance specifications and standards.
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