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microstrip antenna was first proposed by G.A. Deschamps in 1953,
but didn't become practical until the 1970s when it was developed
further by researchers such as Robert E. Munson (now in our Microwave
Hall of Fame!) and others using low-loss soft substrate materials
that were just becoming available.
Also referred to as microstrip
antenna, or abbreviated MSA. For now we will only be discussing
rectangular, single-polarization microstrip antennas, there are
many other variations, enough to fill a book. A good volume on this
subject is Broadband Microstrip Antennas, by Kumar and Ray. Go to
our book section and we'll help you order
it from Amazon!
Advantages of microstrip antennas
- Low cost to fabricate
- Conformal structures are possible
(it's easy to form curved surfaces, as long as the curve is in
one direction only)
- Easy to form a large array,
spaced at half-wavelength or less
- Light weight
- Limited bandwidth (usually
1 to 5%, but much more is possible with increased complexity
- Low power handling
The size of a microstrip antenna
is inversely proportional to its frequency. At frequencies lower
than microwave, microstrip patches don't make sense because of the
sizes required. At X-band a microstrip antenna is on the order of
1 centimeter long (easy to realize on soft-board technology). If
you wanted to make a microstrip antenna to receive FM radio at 100
MHz it would be on the order of 1 meter long (which is a very large
circuit for any type of substrate!) For AM radio at 1000 KHz, the
microstrip patch would be the size of a football field, utterly
impractical. One everyday application where microstrip patches are
used is in satellite radio receivers (XM and Sirius). Here the antenna
is often mounted in a vehicle, where the angle in the X-Y plane
relative to the satellite is not fixed (like it is for the satellite
television dish mounted to your house.) Thus circular polarization
is employed for satellite radio, and the angle that the patch is
with respect to the satellite doesn't matter.
Rectangular, single polarization
This is by far the most popular
type of MSA. The figure below shows the geometry of the rectangular
microstrip antenna, not including the ground plane and dielectric
which would be underneath. The dimension L is universally taken
to mean the long dimension, which causes resonance at its half-wavelength
frequency. The radiating edges are at the ends of the L-dimension
of the rectangle, which sets up the single polarization. Radiation
that occurs at the ends of the W-dimension is far less and is referred
to as the cross-polarization.
The image below is a side view
which attempts to show a snapshot of the E-field under the patch.
Note that the fields under the L-edges are of opposite polarity
(due to the half-wave nature of the patch) and when the field lines
curve out and finally propagate out into the direction normal to
the substrate they are now in the same direction (both facing left).
In the far field perpendicular to the substrate, the radiation from
the two sides adds up because the fields are in phase and voila
you have a an antenna! As you look out in directions off of boresight,
the intensity drops off as the fields of the two edges become farther
and farther out of phase. At two angles the fields exactly cancel.
(We'll explain that more later). Thus the microstrip patch radiation
intensity depends on what direction you are facing it from (it has
gain and directivity).
For a microstrip antenna to work,
you want to think the opposite thoughts that you might want to think
if you were designing a microstrip MMIC. You want
the thing to radiate! The path toward this is threefold. First,
the structure needs to be a half-wavelength resonator. Second, use
a low dielectric constant under the patch. Third, use a thicker
dielectric than you normally would, but keep in mind the height
must still be just a fraction of a wavelength.
To use an audio analogy, a glockenspiel
uses half-wavelength resonators suspended at nodes placed a quarterwave
apart. Like the microstrip antenna, the width of the keys are significantly
less than their length. The primary mode is a resonance along L,
but by forcing W to be 1/4 L, if any mode is excited in the W direction
it is harmonically related and it doesn't hurt your ears!
The image below
is a depiction of the relative intensity (and direction) of the
E and H-fields along the L-dimension, ignoring the radiation that
occurs at the edges. The magnetic field is perpendicular to the
E-field according to Maxwell's equations
(it is in and out of your monitor). At the edge of the strip (X/L=0
and X/L=1) the H-field drops to zero, because there is no conductor
to carry the RF current, it is maximum in the center. The E-field
intensity is at maximum magnitude (and opposite polarity) at the
edges (X/L=0 and X/L=1) and zero at the center. The ratio of E to
H field is proportional to the impedance that you see when you feed
the patch. If you adjust the location of the feed point between
the center and the edge, you can get any impedance you'd like, including
Perhaps another intuitive way
to look at the input impedance to a microstrip patch is to think
about how far you are from an open circuit. If you feed it at the
center, you are looking at a short circuit in both directions, because
you are a quarter-wave from a short circuit. If you feed it at the
edge you see an open circuit, because you are a half-wave from another
The image below shows two ways
to feed the microstrip patch, on the left is a microstrip feed and
on the right is a coax feed.
What dielectric constant defines
the half wavelength?
The dielectric constant that controls
the resonance of the antenna is the effective dielectric constant
of the microstrip line. You can use our microstrip
calculator to come up with the value!
What is the best choice for
the dimension W?
The dimension helps maximize
efficiency. You need to pick W so that:
In other words, use the average
of the value for ER of the substrate and ER of air(=1) to obtain
What controls the bandwidth?
Bandwidth is proportional to
More to come!