Anritsu HFE0104 Moyher.

20 High Frequency Electronics

High Frequency Design

RF CONNECTORS

RF Connector Selection for
Higher Frequencies

By Kevin Moyher
Times Microwave Systems

W

hen designing
an RF system,

an engineer will
frequently be very careful
in the selection of the
coaxial cable, basing any
decision on the cable’s
ability to meet system
requirements such as

return loss or VSWR, insertion loss, shielding
effectiveness, velocity factor, passive inter­modulation, power handling capability, bend
radius, bending moment, diameter and other
characteristics. It’s a wise decision to spend
this “up front” time on cable selection, because
choosing the optimal cable for the application
will help to insure that system design param­eters are met.

Unfortunately, the design engineer will fre­quently pay much less attention to the selec­tion of the RF connector, even though the
selection of an appropriate connector and
ensuring proper attachment of that connector
to the cable are equally critical to achieving
required performance. More often than not,
transmission line problems can be traced back
to improper design or installation of the cable
connectors. The focus of this article is the
effect of connector design and termination on
voltage standing wave ratio (VSWR) and
insertion loss (IL).

Selecting the Right Cable Construction

The primary consideration in selecting a
coaxial cable is usually the loss budget for the
run. Times Microwave Systems’ LMR family
of cables offers a wide range of sizes and con­structions that can satisfy the requirements of
a broad range of systems and is generally very

cost effective, so we will consider termination
issues with respect to this range of cables.

The construction of the basic LMR cable
consists of a copper or copper-clad aluminum
center conductor or copper tube that is coated
with an adhesive over which a closed cell
polyethylene foam dielectric is extruded.
Bonded adhesively to the outside of the
dielectric is an aluminum-mylar-aluminum
composite tape that serves as the outer con­ductor of the cable. Covering the tape is a
tinned copper, round wire braid. A heavy-wall
black, UV-protected polyethylene jacket is
extruded over the braid. This construction is
low-loss, flexible and cost effective, suitable
for many different applications.

There are many variations on this con­struction in the LMR family that may be used
with the same standard connectors. Each is
optimized for specific requirements or applica­tions. A few of these constructions are shown
in Figure 1.

Selection of the proper

RF connector, and proper

attachment of connectors,

can affect transmission line

performance as much or

more than choosing the

right coaxial cable

Figure 1 · Different types of coaxial cable
construction.

From January 2004 High Frequency Electronics

Copyright © Summit Technical Media, LLC

22 High Frequency Electronics

High Frequency Design

RF CONNECTORS

How Connector Quality Impacts Performance

The preparation of the cable can greatly affect the
overall performance of the assembly or jumper. Improper
workmanship can readily result in poor performance of
the finished cable assembly.

The efficiency of a transmission line is partly a func­tion of impedance uniformity. Impedance is a function of
the center conductor (“d” in Figure 2), the outer conductor
(“D” in Figure 2) and the dielectric constant (ε) or veloci­ty of propagation (V

g

), where ε = 1/Vg.

An ideal RF transmission line has uniform impedance
along its length and matches the impedance of the system
itself. In practice, however, this will never be the case. But
over the years cable manufacturing processes have
improved to the point that the cable is seldom the culprit
when impedance non-uniformities are detected. Due to
the line size transitions that are taking place and the
mechanical techniques that are required to secure the
connector to the cable, the connector and the junction
between the connector and the cable will exhibit
impedances that are different than the cable impedance.

In a well-designed connector, proper design of the
transition between the different line sizes of the cable and
the connector interface will minimize the deviation of the
impedance from the nominal value. However, many con­nector designs have less than optimal design of these
transition sections. We will look at a few examples below.
Another contributor to impedance non-uniformity is the
termination process. There are many opportunities to
alter the impedance constant during the termination pro­cess. Figure 3 shows the relationship between impedance
non-uniformity and the VSWR. The VSWR value can be
used to express the level of impedance non-uniformity
within a cable or cable assembly. A cable assembly having
perfect impedance uniformity will have a VSWR of 1.00:1.
Increased levels of impedance non-uniformity will be rep­resented by increasingly higher levels of VSWR.

Mismatch loss (MML) is an often overlooked factor in
system planning. The formula of Figure 4 shows how the

MML value is a component of the overall IL . MML is the
additional loss experienced by reflected waves as they
travel through the cable/connector system and, therefore,
is a function of both matched loss and VSWR. Figure 5 is
a table showing the MML number for a range of VSWR
values in a system with a fairly high matched loss (not
unusual at microwave frequencies). It also lists the reduc­tion in transmission efficiency that is a function of the
impedance mismatches in the system. In this example,
you will lose an additional 25 percent of your incident sig­nal with a 3.0:1 VSWR.

How the Termination Process Can Impact VSWR

Many steps in the termination process can impact the
VSWR. Figure 6 shows a properly prepped and soldered
cable end; in Figure 7 the cable end is properly crimped.
The length of the various strip backs from the end of the
cable should be in accordance with the manufacturer’s
recommendations. In addition, care must be taken to cut
the dielectric and outer conductor square. It should be cut
with a sharp instrument so as not to form an indentation
in (or deforming of) the dielectric or to produce a jagged
outer conductor. Commercially available cable stripping
tools are great for obtaining the proper strip length, as
well as assuring a square cut of the dielectric. Make sure
that the tool is sharp.

The soldering of the pin is another step in the termi­nation process that can have a great impact on the final
performance of the transmission line. Aside from cold sol­der joints and the risk of opens, the pitfalls that are pre­sent during the pin soldering process are (1) excess solder
and flux, (2) improper pin-to-core gap, (3) melting of the
dielectric and (4) the actual pushing of the solder cup of
the pin into the dielectric material. All of these mistakes
will create impedance mismatches and higher overall val­ues of (IL).

The jacket strip back and the crimping of the connec­tor can also have ramifications in terms of VSWR. The
length of the jacket strip back must be in accordance with

Figure 2 · Impedance is a function
of the conductor diameters and the
propagation velocity (V

g

).

Figure 3 · The relationship between
impedance non-uniformity and the
VSWR.

Figure 4 · IL is the sum of all con­tributing losses—cable loss, con­nector loss and mismatch loss.