Almost every RF test setup requires connectors and cables. Connectors will influence test
results in various and sometimes unpredictable ways. Test equipment can compensate for
some of these influences but often with detrimental effects. Knowing how the connections used
in a test setup will affect test results is important to create a valid test environment.
Connector and cable options
In RF testing there are various components, such as the antenna, testing equipment like a
network analyzer, device under test and cables, connectors and adapters. In this setup, there
are many options for the type of connectors as well as the type of coax and configurations. The
best setup would have the lowest attenuation across all frequencies and provide high accuracy
results. While the ideal setup is often not possible, many options exist to get close. One option
is to use fewer connectors with the highest quality and shortest length of coax. This may
require custom coax with the correct connectors on each end. Due to the flexible nature of
testing equipment, the coax and connectors in a test setup are often a secondary consideration.
Adapters such as gender changes and coax cables that are longer than required are often used.
It is important to understand how these equipment choices will influence a test and if those
influences are acceptable. Consequently, the connections should be given great consideration.
Types of connectors and coax
Many different kinds of RF connectors exist and each has its own characteristics of frequency range,
decibels of loss and power, as well as other aspects. BNC (Bayonet Neill-Concelman) connectors
are easy to connect due to a twist-lock system. These connectors work well up to about 4 GHz and 70 W
power and are standard on test equipment. N connectors are one of the most commonly used for
frequencies up to 11 GHz. They also have a high power range, around 150 W, where precision
N is good up to 18 GHz and 250 W. Subminiature version A (SMA) connectors work for
frequencies up to 26.5 GHz and 70 W, but also have other variations. The small size and high
frequency range make them useful for many applications. For instance, 3.5 mm and 2.9 mm
will mechanically mate with an SMA connector but with a different diameter corresponding
to the size. They also both offer a higher frequency range than SMA, where 3.5 mm typically
works up to 34 GHz and 45 W, and 2.9 mm works up to 40 GHz and 20 W. Another smaller
connector option is 2.4 mm, which has a frequency range up to 50 GHz and power of about
15 W. On the other end of the size spectrum, 7/16 DIN utilizes a measurement of 7 mm for the
inner contact with the outer contact having a 16 mm diameter. This is a larger connector compared to
the others, but it is popular in high-power uses. This connector goes up to about 7.5 GHz and 820 W.
In addition to connectors, different types of coax options also
exist. RG58 is used for low-power signals up to about 1 GHz,
and it is often used with BNC and other connectors. RG142
is a double-shielded, 50-ohm impedance cable with an upper
frequency range of 8 GHz. RG214 also has a double shield and
a typical upper frequency range of 4 GHz. Finally, RG223 is a
50-ohm cable that works up to 12.4 GHz and has some of the
lowest loss in the standard cable. Lower-loss, higher-frequency
range cables are also available. Cables such as SAC-18G-X or
SAC-40G-X have power ratings up to 1,900 W at 1 GHz and have
frequency ranges up to 40 GHz at low power ratings.
A.H. Systems makes all cables in custom lengths depending
on the test setup. Low-loss cables are useful when a more
accurate test is required and are a good option in general. The
combination of various cables and adapters will achieve varying
degrees of attenuation, depending on the test requirements.
Gender and connector adapters
When testing RF, it is often the case that the connectors on
various test equipment and the antenna are not the same type
or are the incompatible gender. A common way to deal with this
mismatch is to use adapters. Some adapters simply change the
connector gender, while others convert between two different
connectors. While utilizing adapters often creates a functioning
test, they can also have a detrimental impact. Each adapter and
connector introduces some attenuation. While this attenuation is
calibrated out, it will negatively influence the test. Attenuation
is not typically flat across frequencies; it will influence a test in
other ways such as by decreasing the dynamic range and test
Creating custom cables with the connectors to match the mating
equipment is an alternate way of adapting when different
connector types or genders are required. The fewer connectors
and adapters, the less attenuation and impact on a test.
Additionally, the high-frequency and power rated connectors
and cable will provide for data that is more detailed. If the
equipment performs the same test many times, then ensuring
that the correct cables and adapters are available is not that
difficult. If the type of test is not consistent, then it will require
an assortment of connectors. In this case, other constraints such
as time and money will factor more into the test setup where
adapters on hand are used instead of custom cables. While this
is a viable option, it does have consequences.
Accounting for attenuation
It is possible to account for and calculate what the expected
loss from various connector and cable configurations will be.
Each connector, adapter and cable will have specifications that
document the expected loss at various frequencies. It is possible
to arrive at an expected loss for a cable, connector or adapter
system. Another way is to measure the loss of a system, which
involves transmitting and receiving through a system and then
adding the unknown cable and connectors, where the difference
equals the loss of the added connections. Often, one connector
or cable choice creates a bottleneck that lowers the maximum
frequency, power or both. It is important to note the highest
frequencies and power that various tests will require, as well
as the capabilities of the connections. Other factors such as
cable length or any damaged equipment will also play a role
in overall attenuation. Online calculators
such as this one
from A.H. Systems, can help with this calculation.
Frequency range vs. attenuation
Attenuation from cables and connectors is rarely flat across the
spectrum and is often nonlinear as well. This can make loss
difficult to account for as it becomes necessary to know the
testing frequencies. If multiple frequencies are going to be used
in testing, then understanding how the attenuation will affect
each frequency is important. More often, low-frequency tests
have less issue with loss than high-frequency tests. Tests that
span both low and high frequencies may have more detail in the
lower range but less at higher frequencies. In some instances,
unusual frequency dips may even occur due to equipment
characteristics. Test equipment can often compensate for this
unevenness with some compromises. Knowing the spectral
response of any given test setup will help understand the results.
Measuring and adjusting for attenuation
While all these different connection aspects in a test make
it more complicated, it is not that difficult to measure and
adjust for the attenuation of a system, even without a bunch of
calculations. Often, various types of measurement equipment
can establish a baseline for calibrating the connection factors.
Even if the equipment cannot directly account for the attenuation,
once the attenuation is measured, the difference between the
baseline and test data can be calculated. However, even though
the attenuation is accounted for, it does not mean that it will
negatively affect the test.
The availability of different adapters, connectors and cables
does not mean that attenuation can be factored into the test
and therefore ignored. It may seem like it does not matter what
connection options are chosen, so long as the connections are
made and the equipment can account for them. Sometimes
users will even stack multiple adapters to get the conversion
they need. Yet, haphazard use of adapters and connecters has
consequences. The cumulative losses in a test system will
influence the quality and validity of a test in the form of limiting
its dynamic range. Dynamic range is the difference between the
maximum source output levels to the smallest value that can be
measured on the receiver input. It is essentially a measurement
of how small a signal level can be observed.
Any losses in the system reduce the range of measurement.
While the idea that having fewer signals might suggest a lowquality
test, adding adapters that reduce the signal level is
often an overlooked solution. Another overlooked factor is the
frequency-dependent characteristics of attenuation. For testing,
it is ideal to have the widest dynamic range possible. This means
reducing the length of cables and number of connectors, and
using low-loss cables and connectors that work well over the
entire testing range. Generally, low-loss cable is more lab-grade
than standard cable, which is a more cost-effective alternative.
If budget cable is used, it will reduce the dynamic range and cut
off the upper-frequency range, even after compensation.
In special cases, connection issues observed in testing can yield
confusing results. One such example is a frayed center pin that
begins to act as a small antenna at resonant frequencies. Test
results may show a dip at certain frequencies. In other cases,
certain adapters have nonlinear frequency responses inherent
in the design. Damage from cables that are repetitively flexed
or twisted, or that have been kinked, can be impacted in ways
that allow them to work, but with a reduced bandwidth. Dirt and
debris can also affect connections, which should be inspected,
cleaned or replaced if necessary.
Connectors that are connected and disconnected often may
become worn or physically damaged and create more loss. In
instances where cables are bent at tight angles due to equipment,
mounting a 90 degree adapter may be a better approach than bending
the cable. Ideally, cable connections should be parallel with
minimum movement and stress, connectors should be free
of all damage and protective caps should be used when not
connected. Anytime a cable or connector is suspicious, it should
be inspected or replaced to ensure the best testing environment.
At first thought, cables, connectors and adapters may seem like
they are trivial aspects to a test setup and can be mixed and
matched as needed. In practice, it is important to carefully select
a configuration to achieve useful and repeatable test results.
For help selecting a testing antenna, calculating antenna beamwidth or beamwidth measurement services,
contact A.H. Systems, inc.