Author: Jeffrey D. Paynter and Richard Smith
Company: Andrew Corporation
Part 1 With the performance improvements required for digital communication applications, use of the 7/16 DIN connector is becoming increasingly widespread. It offers many technical advantages over the type N interface.
Reprinted with permission from April 1995 Mobile Radio Techonology magazine. Copyright 1995, Intertech Publishing Corp.: all rights reserved. Telephone 913-341-1300.
The 7/16 DIN connector is becoming more widely used in the United States, especially for today's demanding wireless two-way radio systems. With the rapid evolution of wireless technology involving high transmit power levels and low receive noise thresholds, stringent requirements are increasingly being specified, especially regarding intermodulation (IM) performance. Coaxial radio frequency (RF) and microwave connectors, along with antennas and transmission lines, can be a source of intermodulation distortion in multichannel communication systems, although with proper design their effects can be minimized.
Operators and engineers must decide whether to use standard type N connector technology or to replace it with the 7/16 DIN interface in both new and existing installations. (See Photo 1 below.) The correct decision is important because an inappropriate selection could result in poor system performance, shorter service life or even component failure.
Historical perspective
Germany developed the 7/16 DIN interface during the 1960s for high-performance military applications; it was later adopted for commercial applications in analog cellular systems. Its use expanded to other European nations, and in 1975 the International Electrotechnic Commission (IEC) adopted the specification. With the introduction of digital technology in cellular and mobile radio systems, in particular the Global System for Mobile Communication (GSM, formerly Groupe Speciale Mobile), increased effects of IM distortion were noticed. DIN-style connectors theoretically are superior for IM performance and thus became the interface of choice.
Today, 7/16 DIN connectors are widely used in GSM systems. They are installed throughout the base station and on antenna feeder lines, as well as in the radio equipment shelter. (See Photo 2 on page 3.). Germany, as one would expect, is the largest user of the 7/16 DIN interface. Other European countries have been migrating rapidly toward the use of 7/16 DIN. For example, French Telecom recently chose 7/16 DIN connectors for its GSM base station installations.
In the United States, the type N interface has been widely used for many years. Though relatively new to the United States, use of the 7/16 DIN interface is on the rise. Both interfaces have advantages and disadvantages that must be considered in making an appropriate connector selection. How can one make the best decision? The following information is a concise technical comparison designed to simplify the decision process.
Electrical performance
Intermodulation generation - A leading cause of high bit-error rates in digital systems and unacceptable levels of noise in analog systems is a phenomenon known as intermodulation generation. In the United States, many large cellular carriers are testing for intermodulation generation in all passive components used in their digital cellular base stations. They have determined that replacing type N connectors with 7/16 DIN connectors reduces the level of IM generation. Radio original equipment manufacturers (OEMs) that have performed similar IM testing have claimed a measurable improvement using the 7/16 DIN connector. Cellular antenna manufacturers are beginning to convert antenna inputs on both panel and omnidirectional antennas to 7/16 DIN for the same reasons.
Intermodulation occurs in systems with multiple channels transmitted on the same media. In the presence of a non-linearity, any signal generates harmonics. When two signals are present, harmonics of both are produced. The harmonics of the two signals can intermix, resulting in further spurious signals that are known as intermodulation products. The result of an intermodulation signal can be devastating to reception if it falls in a receive channel. As the number of signals increases, the probability of an intermodulation signal causing noise in a receive channel grows.
Magnetic materials, such as nickel and steel, are known to be a prime cause of intermodulation generation. These materials are inherently non-linear and should be carefully avoided on current-carrying conductors or in regions of RF field distribution. Even a strike of nickel under a silver or gold plate has been implicated in unacceptable levels of intermodulation.
Electrical contact surfaces give rise to two additional non-linear effects: Microscopic arcing is caused by a small separation at the junction of electric conductors. Electron tunneling can occur when oxides form between contact surfaces. Together, these two effects represent the most challenging source of intermodulation in connectors.
The easiest way to minimize contact effects is simply to eliminate the junction, if possible. In a practical connector design, it is impossible to eliminate all contact surfaces; thus, further design techniques are used:
The greatest source of IM left in the connector is from the interface contact itself. The 7/16 DIN interface is superior to the type N regarding IM for several reasons. The coupling mechanism of the 7/16 DIN interface provides higher contact pressure. A special contact zone is provided at the tip of the inner contact, which ensures a reliable 360 degree contact; this full-circumference contact is not true with all type N connectors. The retractable coupling nut allows viewing of the inner contact mating to ensure that the conductors are properly engaged. The dimensions of the 7/16 DIN allow for easy cleaning of the contact surfaces to remove dirt and oxide buildup. The larger conductors are more rugged and durable, which improves long-term reliability.
Even if all of the theoretical guidelines are followed, a connector may not perform as expected. It is critical that a connector designer have IM testing facilities to verify that the steps taken to reduce intermodulation generation are effective in practice and under various conditions. A well-designed connector typically has IM levels of less than -125dBm with two +43dBm (20W) carrier signals. (See Figure 1 on page 3.)
As is a chain, a low-IM system is "only as strong as its weakest link." The connector design cannot solve IM problems entirely. For the lowest possible level of intermodulation products, use a cable with solid outer and inner conductors. In contrast, braided conductors are essentially a series of hundreds of sliding contact surfaces that can greatly increase the level of intermodulation generation.
Frequency - The lower maximum frequency of the 7/16 DIN interface allows it to be better optimized but limits its application. In contrast, the type N is more versatile but typically has reduced performance. The maximum frequency, one of the fundamental differences between these interfaces, is a function of the dimensions and dielectric. An equation for calculating the maximum frequency, Fmax, is:
The standard type N interface dimensions are A = 0.276" and B = 0.120". With an air dielectric, those dimensions give Fmax = 19GHz. In most connectors, an insulator is used to separate the conductors. With a common insulator, and allowing for a 10% safety factor, the maximum recommended frequency is reduced to 12GHz. This wide bandwidth allows the type N to be used in a wide variety of applications.
In contrast, the interface dimensions of the 7/16 DIN are A = 7mm and B = 16mm, resulting in Fmax = 8.3GHz. A 7/16 DIN connector with a typical insulator has a recommended maximum frequency of 6GHz. This smaller bandwidth permits optimization of 7/16 DIN connectors for cellular, mobile radio and other lower-frequency applications.
Part 2--Along with the electrical differences described in part 1, the 7/16 DIN connector's power-handling capacity and mechanical differences may indicate its use in place of the type N for some installations.
The 7/16 DIN connector often is used with high-power RF transmission lines. Radio manufacturers are using 7/16 DIN in applications such as cellular base station transmitters, at the output of amplifiers and on combiners. On the other hand, type N interfaces often are used for receiving channels where average powers are lower. The 7/16 DIN also is being used for the input interface on the antennas used in high-power two-way radio applications such as paging and digital specialized mobile radio (SMR).
Historically, in the United States, LC connectors have been used when higher power was required, but the LC interface was not developed with intermodulation as a design factor and thus is not adequate for intermodulation-sensitive applications.
Peak power is the maximum power that can be used without the danger of energy arcing between conductors. The peak power rating is a function of the smallest air gap between inner and outer. A larger air gap provides a higher peak power rating, hence, the 7/16 DIN interface, with its larger air gap, has a rating of 40kW compared to only 10kW for the type N.
Average power is difficult to specify absolutely because of the complexity of the thermal problem to be solved. There are a number of factors, such as ambient temperature, solar radiation, wind effects and variations in contact resistance that cannot be totally controlled by the design of the connector. A typical average power limit for the type N interface at 800MHz is 600W, compared to a limit for the 7/16 DIN interface of 3,000W. This five-fold increase in average power handling for the 7/16 DIN interface results from higher contact pressure, lower contact resistance and better heat dissipation provided by larger conductor surfaces.
Compensation - Another advantage of the 7/16 DIN is better VSWR performance. This performance is related to several factors. First, as mentioned previously, the pin depth tolerance is tighter, reducing a primary source of reflection from an interface. Second, because the bandwidth is narrower, the design can be optimized for the lower frequency range. Compromises at the lower frequencies often must be made to provide adequate performance over the wider bandwidth of type N connectors. Third, the larger physical dimensions once again provide an advantage, because proportionally, 7/16 DIN connectors are longer than type N connectors, allowing the transition from the cable to the interface to be more gradual. A VSWR of 1.04 at 1GHz and 1.10 at 2GHz is common for a 7/16 DIN connector. Table 1 below lists some characteristics of several connector styles.
Mechanical performance
Dimensions - The larger interface size of the 7/16 DIN compared to the type N is responsible for many of its mechanical and electrical performance advantages. It has an outer and inner contact diameter of 0.630" (16mm) and 0.275" (7mm), respectively, compared to 0.276" and 0.120" for the type N. (See Figure 1 on page 5.)
Contact pressure - The 7/16 DIN joint ensures a more stable electrical and mechanical connection than the type N. It has less chance of loosening over time, longer service life, lower contact resistance and improved intermodulation performance. Its interface dimensions and coupling nut torque are about 10 times greater than those of the type N. This torque provides almost three times greater contact pressure at the outer body connection. Although the female inner contact geometry is not defined by either interface standard, the 7/16 DIN interface normally features a high contact pressure zone at the end of the inner contact pin that is not typical of the type N interface.
Pin depth requirement - The tighter pin depth specification for the 7/16 DIN produces a more consistent VSWR performance and normally an improved VSWR compared to the Type N connector. The total tolerance range for the 7/16 DIN male and female is 0.011"; for the type N male and female, the range is 0.020". As a result, with extreme tolerances, the interface gap between the mating pins can vary from 0.001" to 0.023" for the 7/16 DIN and 0.003" to 0.053" for the type N.
Environmental - If properly designed and sealed at the cable connection, both types of connectors can be used outdoors because they use interface gaskets. At the interface, the 7/16 DIN connector is almost double the size of the Type N. The interface gasket diameter difference is approximately 0.875" vs. 0.438". This difference, combined with the larger coupling nut on the 7/16 DIN male connector, makes it less likely that the connection will loosen over time because of environmental stresses and vibration. (See Photo 1 above .) Regardless, for the hostile conditions that commonly occur on an antenna tower, it is good practice for either connector type to use weatherproofing systems such as butyl rubber tape or shrink tube. (See Photo 2 on page 7.)
Mating - Assembly of the 7/16 DIN connector to the mating connector is easier with less chance of cross-threading because the larger coupling nut has a larger thread engagement. Furthermore, with the addition of a retractable coupling nut design, which is standard on most 7/16 DIN connectors, positive conductor engagement can be ensured, improving reliability. (See Photo 3 below.)
Summary
With the performance improvements required for digital communication applications, use of the 7/16 DIN connector is becoming increasingly widespread. It enjoys many technical advantages over the type N interface. Intermodulation performance is more stable because of its higher contact pressure, greater coupling torque and robust design. Mating of the 7/16 DIN is easier and more reliable and, once mated, it has greater resistance to environmental forces. The 7/16 DIN can be used at higher power levels without degrading connector performance. By taking advantage of the narrower frequency range and tighter pin depth requirement, the VSWR performance of the 7/16 DIN can be optimized for cellular and mobile radio applications. The 7/16 DIN connector gives system engineers a technically superior, high-performance alternative to use when designing today's more challenging and demanding wireless systems.
