Antenna Matching
within an Enclosure
Part 2:
Techniques & Guidelines
By: John Lienau
Antenna Matching Within an Enclosure Part II: Techniques & Guidelines
John Lienau, RF Design Engineer
Antenna selection and placement can be a difficult task, and the challenges of
implementing the antenna are not over once it’s placed on the board. As previously discussed
in Antenna Matching Within an Enclosure Part I: Theory and Principle, the enclosure affects the
antenna match. The process of matching an antenna can be a very complicated process. Not
only is an in-depth knowledge of RF principles and components needed, but proper technique
and understanding of the antenna properties is required. Any error introduced into the
measurement while matching the antenna will ultimately reduce the effectiveness of the
antenna and its performance. For this reason, as much care should be taken in the setup of
your measurements as the actual matching process itself.
The most important factors in antenna tuning are proper connection to the PCB,
calibration and port extension of the network analyzer, and knowledge of the antenna to
ground plane relationship. These three factors must be taken into account when attempting to
tune an antenna for an enclosure, and will be covered in this paper. Tuning an antenna is not
as easy as simply connecting it to a network analyzer and chasing the impedance around the
smith chart. Without doing the little steps right, it is easy to introduce error into the
measurement and improperly match the antenna.
Before matching can begin, a proper connection must be established to the PCB. There
are usually two methods for connecting to the PCB to match an antenna. The first is using a
U.FL connector. This involves finding an area big enough to place the connector down and an
area that allows for good grounding. It is often difficult to find enough room near the antenna
trace for this method; additionally care must be taken to make sure you don’t introduce
additional parasitic effects. The U.FL is typically only effective for tuning if the PCB was
designed with specific component pads for placement of the U.FL. The second, more common
method is to strip a thin coaxial cable and solder it directly to the trace line. This requires less
space on the PCB and is more flexible than placing a U.FL on the board.
When soldering a coaxial cable to the PCB, the inner conductor must be soldered to the
beginning of the trace and the outer shield well soldered to ground. Occasionally, it is not
possible to solder to the beginning of a trace (see Figure 1). This will give an inaccurate
measurement of the antenna’s impedance. The RF signal propagates down both ends of the
trace at the same time. The signal will reach the end of the stub, reflect back, and interfere at
the solder point to give false impedance readings. To eliminate this problem, the trace behind
the solder point has been cut. The RF signal will now propagate down the cable and only to the
antenna. Similarly, as shown in the third picture of Figure 1, make sure there are no extra
lengths of trace line after solder point as well. This will result in the same reflections problem.
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Cable soldered to feed trace. Trace must be cut to prevent reflections.
Trace is cut. RF signal will now flow down cable and straight to the antenna.
Figure 1: Soldering a Cable to an Antenna Feed Trace
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Small PCBs can be particularly difficult for tuning because of their ground plane size.
This is a common a problem with chip antennas because they are placed on very small boards.
With a small ground plane, the outer conductor of the coaxial cable can act as the antenna
ground. This causes currents to flow on the outer conductor of the cable. As a result, the cable
actually becomes part of the antenna and radiates. An easy way to test for this is by touching
the coaxial cable, if the S11 measurement on the network analyzer shifts significantly, this
indicates a problem. This can be rectified by making sure the cable is well connected to ground.
If this still does not solve the issue, other methods including adding a ferrite or routing the
coaxial cable different may help.
Figure 2: Trace F Antenna
After the cable has been properly attached to the antenna feed trace, the network
analyzer needs to be calibrated. The antenna in Figures 1 and 2 is a trace F. This type is
designed to be tuned without matching components. The F has been intentionally created too
long, and hence tunes too low in frequency. The advantage to doing this is that the engineer
can slowly trim the end of the antenna and increase the resonant frequency. This technique
can be used to tune a trace antenna in its final enclosure. Table 1 below documents the
trimming and tuning of the F antenna above. The desired frequency band is 2.4 GHz. This
represents the most basic form of tuning an antenna. The dimensions are being physically
altered, which is changing the impedance of the antenna, and consequently the load seen by
the transmission line delivering the RF signal.
This trace F antenna example was designed at LS Research. The overall dimensions for
the feed width, trace width, height, and feed location were determined through simulation.
The final step of tuning by trimming the overall length can be done on a bench, but
optimization of the antenna dimensions for bandwidth and Return Loss should be done by an
experienced antenna designer.
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Frequency
Return Loss
2.4 GHz
-3.5 dB
2.44 GHz
-2.7 dB
2.48 GHz
-2.1 dB
Without Trimming
Frequency
Return Loss
2.4 GHz
-10.0 dB
2.44 GHz
-7.2 dB
2.48 GHz
-5.4 dB
After 1st Cut
Frequency
Return Loss
2.4 GHz
-16.5 dB
2.44 GHz
-39.0 dB
2.48 GHz
-14.9 dB
After Final Cut
Figure 3: Trace Antenna Tuning
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While the previous example relied on looking at an S11 measurement on a network
analyzer, most commonly an engineer will have to match an antenna via a discrete network of
inductors and capacitors. Antennas that are purchased ‘off the shelf’ will require matching, the
antenna dimensions cannot be altered as with the F example above. In order to match an
antenna with components, an in depth knowledge of the Smith chart is needed to properly
determine the matching network.
When employing the use of matching components, the coaxial cable for measurement
should be soldered on before the matching network. In the F antenna example, since it was
being tuned without components, the coaxial cable had to be a directly connected to the feed.
As shown below in Figure 4 and Figure 5, the cable is now placed before the matching network.
In addition to properly placing the cable, the port extension feature of the network analyzer is
needed. Since the Smith Chart will be used to determine the impedance of the antenna, the
reference plane for the measurement must be correctly set. Since the cable is soldered directly
to the board, you cannot calibrate to the end of it. A port extension must be applied to account
for the phase change of the additional length of cable not included in the calibration. This will
essentially move the calibration plane past the cable and to the end of the trace. If this is not
done, impedance measurements on the Smith Chart will not be correct.
Inner conductor is soldered
to start of antenna match
Calibration plane is set to
here via port extension
Observe that the trace is
cut to prevent reflections.
Outer shield of cable is well
soldered to the PCB ground
Figure 4: View of Matching Pads and Cable Under a Microscope
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With the port extension properly set, the antenna impedance must be measured so that
a matching network can now be designed. An RF short should be used to bridge the gap in the
trace lines. However, whatever component is used to bridge the gap will inherently add some
of its own capacitance or inductance to the measurement. At frequencies under 1 GHz, a 0
Ohm jumper is may be used as a short. However, at higher frequencies, the 0 Ohm jumper
becomes inductive and will introduce error into the measurement. This is especially true for
anything above 1 GHz. The best thing to use to bridge the gap is a capacitor that is resonant
near your frequency of operation. This will eliminate most of its inductive or capacitive
qualities. For example, an 8.2pF Murata GRM 0402 capacitor is resonant at approximately 2.4
GHz. While it’s not a perfect RF short, it is much better than a 0 Ohm jumper or a glob of
solder. Both of the later alternatives will be inductive at higher frequencies. Also, the resonant
frequency of the capacitor will vary depending on the manufacturer. A 10pF Johanson 0402 LSeries capacitor is a good RF short at 2.4 GHz. Make sure to have a thorough understanding of
the components being used.
Without Matching
With Matching
Figure 5: Puck Antenna Board
Another key component that significantly affects the tuning and also radiated
performance of an antenna is the ground plane. Many antennas, particularly traces and chips,
are dependent on the shape and size of the PC B ground plane. The chip (or trace) is only half
of the antenna, the other half is the ground plane. Recalling the dipole diagram and current
distribution from the previous paper (Antenna Matching Within an Enclosure – Part 1), there is
positive and negative charge buildup on the antenna. This buildup of charge is ultimately what
causes current flow. In a similar fashion, charge builds up on the trace antenna and on its
corresponding ground plane. There is current flow on the antenna due to this charge, but since
the ground plane is now partially responsible for the charge buildup it will also directly affects
the current distribution and ultimately the impedance and radiation of the antenna.
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Manufacturers for chip antennas commonly give recommended matching circuits, but
these are only valid if the PCB is the same size as the one the manufacturer tested with.
Additionally, testing at the manufacturer would have been done in free space, using the chip
antenna within an enclosure forces it to be re-tuned. Essentially, the recommended matching
circuit is not usable for most implementations and the antenna must be re-tuned. Additionally,
the orientation of the chip antenna with respect to the ground plane will affect tuning and
performance. The datasheet for the chip antenna will depict an orientation and keep out area.
This is should followed as closely as possible!
The key to successfully matching an antenna is maintaining accuracy in the
measurement. Good grounding of the coaxial shield, proper solder location on the feed line,
and knowledge of your component are all very important. Small inaccuracies can easily result
in an engineer designing the wrong matching network for an antenna. This paper has covered
some common mistakes, but overall a good understanding of RF principles is the best tool for
matching an antenna.
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