Most two-way radio communications technicians cringe at the mere mention of the phrase. Intermodulation interference (intermod) combines two or more signals that cause one or more undesired signals to impair reception. Transmitters, receivers, transmitting combiners, receiver multi coupler, antenna systems, and dissimilar metal junctions near transmitters are generally the source of the problem. Few technicians consider inspecting the system’s coaxial connections. The development of 800MHz and 900MHz trunked and cellular communications systems, which integrate numerous transmitters onto a single antenna, has created the possibility for a broad inter-mod issue, which should be resolved before a system is installed.
Intermod that can be produced in an active circuit, like a receiver, may also be made in passive system components. Any non-linear passive component, such as a simple coaxial transmission line, might exacerbate the issue. Examining a transmission line’s electrical and mechanical characteristics helps understand how intermod might be created inside it. A well-designed connection is nothing more than a transmission line extension. It has the same electrical characteristics as the antenna, combiner, and transmission line. It might be just as essential in the original system design as the antenna, combiner, and transmission line. Radio energy moves over the surface of a conductor when it is carried by it.
The skin impact becomes more prominent as the frequency rises. A variety of variables, including conductivity and permeability, determine the skin depth for radio energy carried by a conductor, in addition to the radio frequency. When a metal is susceptible to magnetism, such as iron-based materials often referred to as ferromagnetic materials, its conductivity remains constant, but its permeability fluctuates. The magnetic field associated with an ac signal applied to a ferromagnetic wire change in proportion to the current. The conductor’s skin depth changes as the magnetic field changes.
The variance results in a non-linear circuit capable of generating intermod. This feature may appear trivial because the current coaxial cable comprises non-ferromagnetic materials such as copper, silver, or aluminium. However, have a look at how connections are made. Economic factors, as well as mechanical and electrical performance considerations, influence the production of connectors. The majority of contemporary connectors are constructed of a base material that is subsequently plated. This design provides a good balance of mechanical and electrical characteristics. Brass is the ideal base material, which is a copper alloy made up of 65 to 70% copper and the rest zinc. A variety of materials can be used to plate the base metal. Nickel is widely utilised because metal is inexpensive, tarnish-resistant, and long-lasting.
Nickel, on the other hand, is a ferromagnetic material that is vulnerable to variations in skin depth and the resulting nonlinear behaviour. Intermod produced by nickel-plated coted connections has been identified as such an issue that a navy research laboratory study addressing passive component non-linearity difficulties in satellite communications systems was published in 1975.
Because the alternative nonferromagnetic plating material, silver, is more costly, challenging to the package to avoid tarnishing while on the shelf, and prone to tarnishing once installed, many connection makers continue to sell nickel-plated connectors. The somewhat higher cost of silver-plated connections, as well as the possibility of tarnish, should not discourage communications system designers and technicians from specifying them in their installations. When nonferromagnetic connections are utilised, they do not have to be the weak link in a communications system, generating destructive and difficult-to-find intermod interference.