Download here - Magnolia Intertie

Repeater Workshop
Presented on Behalf of the Magnolia Intertie
By: Thomas Gandy (N5WDG)
Return Loss and VSWR Measurement Return loss (RL) and VSWR measurements are at the core of cable and antenna
measurements. These measurements allow the user to determine if the system in question is
working the way it should. If problems show up during this test, chances are that the system’s
overall performance is being impacted. These measurements are based on the principle that
some parts of a signal are reflected due to mismatches in impedance between cables,
antennas, or connectors. The ratio of the input signal to the reflected signal is called the
voltage standing wave ratio or VSWR. This ratio can also be measured in dB, and expressed
as return loss (RL).
Benefits
Return loss (RL) and VSWR can reveal significant
problems. For instance, a poorly matched antenna
will reflect costly RF energy which will not be
available for transmission and will instead end up in
the transmitter. This extra energy returned to the
transmitter can distort the signal and affect the
efficiency of the transmitted power, reducing
coverage area.
Return Loss
The return loss (RL) is the ratio of reflected power to reference power in dB. The return
loss view is usually preferred because of the benefits with logarithmic displays – it’s easier
to compare a small and large number on a logarithmic scale.
For reference, a 20 dB system return loss measurement is considered very efficient as only
1 percent of the power is returned and 99 percent of the power is transmitted. If the return
loss is 10 dB, 10 percent of the power is returned. While different systems have different
acceptable return loss limits, 15 dB or better is a common system limit for a cable and
antenna system.
Voltage Standing Wave Ratio
In contrast to return loss, VSWR displays the impedance match of the system linearly,
measuring the ratio of voltage peaks and valleys. If the match isn’t perfect, the reflected
signal will add and subtract from the transmitted signal. The greater this number, the worse
the match. A perfect or ideal match in VSWR terms would be 1:1. A more realistic match for
a cable and antenna system is in the order of 1.43 (15 dB return loss). Antenna
manufacturers typically specify the match in VSWR based on a certain operating frequency
and characteristic impedance. Higher VSWR's indicate a greater degree of impedance
mismatch and can be viewed as having less efficient power transfer.
VSWR, RL and Reflection Coef.
The relationship between RL and VSWR is easy to grasp and its importance in an antenna
system does not require an engineering degree to understand. Reflection Coefficient (ρ)
Voltage Standing Wave Radio (VSWR)
Return Loss (RL)
or
VSWR and RL Relationship
Return loss can be translated into
Reflection Coefficient, SWR, or into the
percent of power reflected by the device
under test (DUT). This table shows those
values for various levels of return loss, as
well as two resistance values that would
generate that return loss.
Scattering Parameters One of the most fundamental concepts of high-frequency
network analysis involves incident, reflected and
transmitted waves traveling along a transmission line. It is
helpful to think of traveling waves along a transmission
line in terms of a light wave analogy. In the diagram to the
right we can imagine incident light striking some optical
component such as a clear lens. Some of the light is
reflected off the surface of the lens, but most of the light
continues on through the lens. If the les were made of
some lossy material, then a portion of light would be
absorbed within the lens. If the lens had mirrored surfaces
then most of the light would be reflected and little or none
would be transmitted through the lens. This concept is
valid for RF signals as well, except the electromagnetic
energy is in the RF range instead of the optical range, and
the components are electrical devices and networks instead
of mirrors and lenses.
S‐Parameters & The 2 Port Network
Transmitted
S21
Incident
a1
S11 - Reflected
Port 1
b2
Device Under Test
S22 - Reflected
Port 2
a2
b1
Transmitted
Reflection Coefficient ( )
Insertion Loss
VSWR
S12
Incident
Return Loss
Cable Loss & Masking Effect
Cable Loss - Cable loss is the total insertion loss of your transmission
cable system. This will typically include insertion loss of the
transmission cable, jumper cables, connectors and lightning
protection. Note, loss of other components (e.g. VSWR/power
monitor, duplexer, combiner or filter) may also come into play.
Masking Effect - Cable loss that causes an error offset when measuring
antenna VSWR or return loss levels. This error offset may be corrected
with the following equation:
• RL at Antenna = RL at Transmitter + (2 x CL)
• RL = Return Loss
• CL = Cable Loss
VSWR levels may be corrected by converting to/from equivalent
return loss levels.
Masking Effect Example
3dB Cable Loss
Transmitter
Pforward = 100w
Preverse = 15w
Pforward = 50w
Preverse = 30w
Let’s say that there is 3 dB of attenuation along the length of a cable. If we send 100 watts forward power into the
cable, only 50 watts makes it to the termination. Let’s say that the termination reflects 30 watts back. When the
reflected signal makes it back to the amp, the same 3dB of cable loss will reduce the reflected power to 15 watts. The
amp would see a VSWR of 2.26. However, using 50 watts forward power and 30 watts reverse power to calculate
VSWR, we end up with a VSWR of 7.9! The amp sees a return loss of 8.2dB, but at the termination the return loss is
2.2dB, or exactly 6dB difference.
Distance to Fault (DTF)
Distance to Fault (DTF) is a measurement that shows how far from the end of the cable faults occur. A DTF test sees
impedance disturbances, determines where they are located, in feet or meters, and displays the fault location information to
the operator. Today's analyzers can and do identify bad connectors, pinched cables, or even a faulty grounding kit. Faults,
in many cases, are isolated to within 1 percent of the total length of the cable.
Examples of faults include:
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Bad connectors
Bent (impedance affecting) cable
Bad Lightning protectors
Moisture in Coax
Bad antennas
Bad jumpers
Subtle changes at connections that are not found otherwise
Improperly installed antennas (too close to the tower)
Antennas spacing too close to other antennas
Improperly installed Ground Kits on transmission lines
Calculators & Tools Resources
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Understanding Line Sweeping, Gary A. Minker – Radio Guide; August 2005
Effects of Cable Loss in VSWR and Return Loss Measurements, Bird Systems/Application Engineering Understanding Cable and Antenna Analysis, Anritsu Application Note 70