# Antenna Engineering Blog

## Fractal Antennas, Explained

In previous posts, we discussed how antenna technology is constantly evolving and how one of the main focuses of antenna design is creating powerful antennas in low-profile form factors. This is especially true for applications such as personal communications, small satellite communications, UAV communications, SIGINT and ISR.

In addition to cylindrical dipoles and biconical antennas, fractal antennas can meet the requirements of utilizing the limited available space. Fractal antennas, as the name suggests, are based on the concept of a fractal, which is a recursively generated geometry that has fractional dimensions. While fractals antennas have many complexities that can be discussed at length, in this post, we will examine the basics of fractal antennas.

##### Fractal Antennas can be classified in two categories:deterministic and random.

Deterministic fractals are those that are generated of several scaled-down rotated copies o themselves. Such fractals can be generated using computer graphics that are repeatedly mapped out by a recursive algorithm. The Sierpinski gasket, for example, is a type of deterministic fractal.

Random fractals also contain elements of randomness that allow simulation of natural phenomena.

##### Fractal geometries can best be described and generated using an iterative process.

This process can be illustrated graphically, as shown in the illustrations of the Koch snowflake (or Koch fractal loop) and the Minkowski island fractal. The geometry generating process begins with a basic geometry known as the initiator (the triangle and the square, respectively). For Koch snowflake, the middle third of each triangle is replaced with a generator, and for the Minkowski island, each of the four sides of the square is replaced by a generator.

##### The trend of the fractal antenna geometry can be determined by observing several iterations of the process.

The final fractal geometry is a curve with an infinitely intricate underlying structure whose building blocks are scaled versions of the initiator.

##### Fractal antennas have space-filling properties that can be used to miniaturize classic antenna elements and overcome some of the limitations of small antennas.

Many fractal element antennas use the fractal structure as a virtual combination of capacitors and inductors. The antenna’s many resonances can vary according to the fractal design. The current on the structure has a complex arrangement caused by the inductance and self capacitance. Due to this reactive loading, the antenna elements can be compacted into a smaller space even if they are electrically long. Additionally, fractal antennas do not require additional components, assuming the structure happens to have the desired resonant input impedance.

##### Not all fractal antennas are ideal for certain applications.

The antenna’s suitability for a particular application can be determined by RF testing and computer search methods. Generally, the advantages of fractal antennas are good multiband performance, wide bandwidth, and small area.

##### While some researchers claim that fractal antennas have superior performance, some experts claim the contrary.

In 2003, Steven R. Best wrote in A Comparison of the Resonant Properties of Small Space-Filling Fractal Antennas, “that antenna geometry alone, fractal or otherwise, does not uniquely determine the electromagnetic properties of the small antenna.” However, in 2008 Constantine A. Balanis‘s research concluded that fractal antennas performed relatively similarly to the electrically small antennas they were compared against. More recently, in 2011, authors of Small Antenna Handbook Robert C. Hansen and Robert E. Collin reviewed many papers on fractal antennas. The pair concluded that they offer no advantage over fat dipoles, loaded dipoles, or simple loops, and that “nonfractals are always better.”

## Black History Month 2021

#### As a minority-owned business, JEM Engineering proudly celebrates Black History Month.

This month, we honor the impactful contributions made by African American engineers have made in STEM and American History. In this issue, we celebrate Granville Woods, Marie Van Brittan Brown, and Lewis Latimer by highlighting some of their many achievements.

#### Granville Woods

1856-1910

Woods was the first African American mechanical and electrical engineer after the Civil War. He held over 60 U.S. patents, including the steam boiler furnace, the automatic break, the safety dimmer and the egg incubator. Additionally, Woods improved numerous inventions, including the safety circuit, telegraph, telephone, and phonograph.

#### Marie Van Brittan Brown

1922-1999

Together with her husband, Albert, Brown invented the first closed-circuit television security system in 1966. This invention included a radio-controlled wireless system that could stream the video to any television in the house. Along with the video system, the Browns created a two-way microphone system that would allow for communication between the residents and the person at the door. Additionally, Marie created a system to unlock the door remotely. Her invention is the predecessor to the modern-day security systems, used in both residential and commercial spaces.

#### Lewis Latimer

1848-1928

Latimer was the patent draftsman for several inventions, including the incandescent light bulb. Patented the “Process of Manufacturing Carbons”, an improved method for the production of carbon filaments for lightbulb. He also developed a forerunner of the air conditioner called “Apparatus for cooling and disinfecting.” Latimer was the first African American to join the Edison Pioneers.

## What is a Smart Antenna?

The textbook definition of a smart antenna is “an antenna array with digital signal processing algorithms, which identify spatial signatures.” Using these spatial signatures, the smart antenna calculates beamforming vectors, which are then used to track and locate the antenna beam on a mobile or target.

Just as the phrase implies, signal processing algorithms are sequences of computer-implementable instructions for analyzing, modifying, and synthesizing signals (in this case, radiofrequency signals). A spatial signal signature is a response vector of an antenna to a mobile unit at a certain location. An example of a signal signature would be a signal’s direction of arrival (DOA). Calculating a DOA involves finding a spatial spectrum of antenna array and defining its peaks. Because the computations are very intensive, the smart antenna’s signal processing algorithms prove to be very useful.

Some of the most common smart antenna applications include acoustic signal processing, track and scan radar, and cellular systems such as 5G and LTE. The main difference between reconfigurable antennas and smart antennas is that reconfigurable antennas are single-element antennas instead of antenna arrays.

#### The two main types of smart antennas are switched-beam smart antennas and and adaptive arrays.

Switched-beam systems can choose from one of many predefined patterns in order to enhance the received signal. The overall goal of the switched-beam system is to increase the gain according to the location of the user.

In a previous post, we briefly explained that adaptive beamformers adjust to differing situations in order to maximize or minimize signal-to-interference-plus-noise ratio (SINR), which helps measure the quality of wireless communication.

#### Smart antennas provide many benefits.

The main benefit of smart antenna systems is its ability to simultaneously increase the useful receiving signal and lower the interference level, increasing the signal-to-interference ratio (SIR) in more densely populated areas. Smart antennas can essentially filter out the unwanted noise made by other users in the system so that important signals can be transmitted and received clearly.

Additionally, because smart antennas are more directional than omnidirectional and sectorized antennas, they can focus their energy toward the intended users, instead of wasting it by directing it in unnecessary directions. This means that base stations can be spaced further apart, as they would be in less-populated areas.

Smart antennas are also harder to tap into. In order to successfully tap into the connection, the intruder must be positioned in the same direction as their user as seen from the base station. Consequently, smart antennas provide more security, making them extremely vital in the present day, where organizations and individuals routinely transmit confidential information to one another.

Lastly, smart antennas’ spatial detection capabilities allow for geo-location services. For example, they can locate humans in emergency situations.

#### Smart antennas also have their limitations.

Smart antenna transceivers are much more complex than those of traditional base station transceivers in that smart antennas need separate transceiver chains for each array antenna element. This also means that that each transceiver needs to be accurately calibrated at all times. Additionally, smart antenna base stations must be equipped with very powerful digital signal processors to handle the computationally-intensive beamforming. This translates to higher costs in the short-term, however, the benefits will outweigh the costs over time.

## Advancements in Aerospace Antenna Design

With the latest advancements brought upon by more research and development, aerospace antenna design has continued to steer away from traditional manufacturing materials and methods.

According to an article featured by Military & Aerospace Electronics, the three main goals for selecting technologies for aerospace and defense are 1) ruggedness, 2) low-loss, and 3) cost-effectiveness.

Naturally, all flight-qualified antennas need to be rugged enough to withstand mechanical shock, vibration, and atmospheric factors. Also, it is generally important for any antenna to transmit and receive radio waves with as little energy loss (return loss) as possible. Higher loss may cause the antenna to overheat and can degrade its sensitivity.

Creating specialized aerospace antennas, especially those used for military and defense, must be a streamlined and repeatable process. Whenever mass production is involved, it is imperative to keep manufacturing costs as low as possible. While machined aluminum and hand-built thermoset radomes have made for very reliable antennas, fabricating such components often involve multiple steps—more labor and more machining means more cost.

#### Composite Substrates

Aside from yielding higher costs, another drawback of machined aluminum chassis is the added weight. Composite materials, on the other hand, are not only comparatively lighter, but also come in injection-moldable forms that allow for intricate design with higher repeatability.

Composite substrates can also be tailored to specific applications by using a variety of fillers, which have their own dielectric properties. These fillers range from carbon and glass fibers to foaming agents. Conductive-fiber fillers can absorb energy radiated in unwanted directions, increasing antenna gain by decreasing interference.

Most of JEM Engineering’s line of flight-qualified antennas are designed with carefully selected composite substrates, making them effective, rugged, yet extremely lightweight.

#### Conductive Coatings

Conductive coatings metallize materials such as plastics, chemically resistant composites, glass, and ceramics, in order to create conformal antennas on nearly any shape at minimal cost. High-quality conductive coatings meet flight-qualification standards.

#### 3D Printing

In another post, we described how additive manufacturing is used in prototyping antennas. Mainly, it allows for reduced material waste, lower manufacturing costs, faster turnaround, and more design complexity.

#### Magnetic Flux Channel Technology

Developed by JEM Engineering, MFC antennas do not utilize electric charge to achieve radiation. Instead, the magnetic flux that resides in the MFC core shapes and supports an efficient form of magnetic polarization current in a specially designed flux channel. Unlike conventional antennas, the MFC may be placed directly on, or even within, conducting structure with no ill-effects on radiation or impedance match performance. For communications in the VHF/UHF bands (30 MHz to 3 GHz) MFC antennas can provide equivalent performance to conventional. To learn more, read our blog post on our proprietary MFC technology.

## Large Bandwidths in Small Packages: Spiral Antennas

Frequency independent antennas theoretically have no bandwidth limitations. Polarization, radiation pattern and impedance of such antennas remain unchanged over large bandwidth. Spiral antennas are a type of frequency independent antenna first introduced in the 1950s. Their spiral design allows them to be relatively small.

Characteristically, spiral antennas have low gain, but an array of spirals can produce higher gain. They transmit circularly polarized radio waves and receive linearly polarized waves in any orientation. They also attenuate circularly polarized signals received with the opposite-rotation. They can reject circularly polarized waves of one type, while being able to receive those with a different polarization.

Spiral antennas usually have two conductive spiral arms, which extend from the center outwards. The direction of rotation of the spiral defines the direction of the antenna polarization. More spirals may be added to form a multi-spiral structure. Spiral antennas can take the form of a flat disc or a three-dimensional shape like a screw thread. As with microstrip antennas, the spiral can be printed or etched over a specifically chosen dielectric medium. While spiral antennas have several classifications, the Archimedean spiral and logarithmic spiral are the two most common.

Spiral antennas, such as several of those featured in JEM Engineering’s HSA product line, are widely used in the defense industry for sensing applications, where there is a need for very wideband antennas that can fit in smaller spaces. Spiral antenna arrays are used in military aircraft in the 1 to 18 GHz range. Other applications include direction-finding, frequency spectrum monitoring, as well as GPS, where RHCP antennas are common.

## STEM Day

Every year, JEM Engineering celebrates STEM Day. Because November 8 falls on a Sunday this year, we decided to celebrate it all week!

STEM stands for science, technology, engineering, art, and mathematics skills. These subjects push society forward and these educational programs help to find fun and engaging ways to teach them to students, which is all worth commemorating (NationalToday.com).

JEM Engineering fully support students and professionals in pursuit of careers in STEM, for our company is built on the passions of such individuals.

## Radiofrequency Test Fixtures

JEM Engineering offers both on-site and remote RF testing services at our facility in Laurel, Maryland. Our Spherical Near-Field Chamber (SNF) and Tapered Antenna Testing Facility (TATF) can measure a number of antenna performance factors.

While many tests do not require the use of a test fixture, tests that involve a full 3-dimensional measurements in our TATF chamber will require some type of interfacing with our roll arm.

#### What materials are used for RF test fixtures?

Dielectric (non-conductive) materials, such as foam and wood, for example, are ideal for test fixturing. Such materials can support antennas and radomes without affecting their RF performance characteristics during a test.

#### Does JEM Engineering provide its own test fixtures?

We house a variety of simple test fixtures, which we use to test our own antennas, as well as our customers’. Our existing structures allow us to fixture multiple antennas during the allotted test time.

#### Customers may also provide their own test fixture(s).

What about custom fixtures?If the weight of the antenna under test is off-center by more than 36”, or if the antenna requires a more specific configuration, a custom fixture may be necessary.

We have the in-house capability to build custom fixtures. While customers are encouraged to provide their own fixturing specifications (for example, with drawings and dimensions), we are generally able to  fabricate the appropriate fixtures based on a unit’s design and specific test requirements.

## Intro to Antenna Polarization

In an earlier post, we touched upon different antenna’s intrinsic characteristics, such as gain, radiation patterns, directivity and VSWR. In this post, we touch upon polarization and how it affects an antenna’s performance.

The polarization of an antenna is loosely defined as the direction of the electromagnetic fields produced by the antenna as energy radiates away from it. These directional fields determine the direction  in which the energy moves away from or is received by an antenna.

### Linear polarization is the most common polarization.

Vertical polarization refers to the oscillation of an antenna’s electrical field on the vertical plane, whereas horizontal polarization refers to the oscillation on the horizontal plane.

Slant polarization refers to an electrical field that oscillates at a 45-degree angle to a reference plane. JEM Engineering’s LPA-069, for example, is a handheld, direction finding, log periodic antenna that can tilt to a 45-degree angle to receive slant, horizontal, or vertically-polarized signals.

Circular polarization (CP) refers to a radio wave that rotates as the signal propagates. When it rotates to the right, the polarization is referred to as Right-Hand Circular Polarization (RHCP); when it rotates to the left, it’s referred to as Left-Hand Circular Polarization (LHCP). Many of JEM’s multi-band antenna products are circularly polarized. For example, the standard HSA-218 is RHCP, but JEM also makes an LHCP version of the antenna.

Not to be confused with circular polarization, elliptical polarization refers to an electrical field that is propagated in an elliptical helix. Similar to circular polarization, elliptical polarizaton can either be right-hand, or left-hand.

### The way an antenna is mounted affects its polarization.

A straight dipole antenna will have different polarizations when mounted either horizontally or vertically. Thus, a horizontally polarized antenna will perform better when mounted near a ceiling, whereas a vertically polarized antenna will perform better when mounted near a side wall.

### Omnidirectional antennas can be dual polarized.

Omnidirectional antennas, such as the JEM-0221A, radiate radio frequency energy in all directions perpendicular to the antenna’s axis in a doughnut-shaped pattern. Although omnidirectional antennas are usually vertically polarized, they can also be circularly polarized or dual polarized. Circular polarization helps mitigate the reflections that are common in omnidirectional antennas.

A circularly polarized omnidirectional antenna is insensitive to wave orientation. Thus, they provide particularly effective performance and gain. In receiving antennas, a dual polarized CP omnidirectional antenna offers optimal transfer of electromagnetic energy. Ideal for field-sensing and signal environment monitoring applications, the WSA-0520, is able to receive vertical, horizontal, RHCP and LHCP polarized signals equally.

## Microstrip Antennas: The Basics

In a previous post, we introduced microstrip antennas. In this post, we explore their basic characteristics, benefits, and drawbacks.

Microstrip antennas were patented in 1955, although their origins trace back to 1953. They became more commonplace in the 1970s. The antennas consist of a very thin metallic radiating element, or a “patch,” placed small fraction of a wavelength above a ground plane.

There are various substrates used in designing microstrip antennas, Thicker substrates with lower dialectric constants deliver larger bandwidth and better efficiency, while thinner substrates with higher dialectric constants are better or microwave circuitry.

### Microstrip antennas have many benefits.

The most notable benefit of microstrip antennas is their versatility. Firstly, they are small and lightweight, as well as easily conformable to planar and nonplanar surfaces. Additionally, they can be are mechanically robust when mounted onto rigid surfaces. Thus, they can be used in an various applications, including aircraft, spacecraft, satellite, missile, mobile radio, and wireless communications.

Another benefit of microstrip antennas is that they are simple and inexpensive to manufacture. These low-profile antennas can be printed directly onto a circuit board.

### But they have their drawbacks.

Micostrip antennas do have their disadvantages, one of which is their low efficiency. They also have low power, poor polarization purity, poor scan performance and faulty feed radiation. Additionally, these antennas have very narrow frequency bandwidth, which may be a benefit for some government security systems.

While there are ways to correct some flaws, doing so can negatively affect the antenna’s performance in other ways. For example, increasing the height of the substrate can extend the antenna’s efficiency. However, as the height increases, more surface waves will travel through the substrate, scattering at bends and surface discontinuities, degrading the antenna’s pattern and polarization characteristics. Similarly, while there are ways to increase the bandwidth, in large arrays, there would be a trade-off between bandwidth and scan volume.

### Shapes do matter.

Patch elements of microstrip antennas come in various shapes and modes (field configurations), which affect the antennas’ resonant frequency, polarization, radiation pattern, and impedance. These factors are further influenced by adding loads, such as pins and varactor diodes, between the patch and the ground plane.

Square, rectangular, dipole,and circular are very common because they are the easiest to manufacture and analyze. They also have favorable radiation characteristics, especially cross-polarization radiation. Dipoles, in particular, inherently have large bandwidth and occupy less space, making them attractive for arrays.

Linear and circular polarizations are achievable with either single elements or arrays of microstrip antennas. Arrays with either single or multiple feeds may also improve scanning capabilities and directivities

## Frequency Bands and Applications

The textbook definition of a frequency band is an interval in the frequency domain, delimited by a lower frequency and upper frequency. The International Telecommunication Union has assigned designations to these intervals.

Beginning with the lowest and ending with the highest, we will enumerate the ITU-designated frequency bands and provide examples of their corresponding applications.

Firstly, the Extremely Low Frequency (ELF) band is ideal for underwater communication. Transmitters in the 22 Hz range of this band are useful in pigging, also known as pipeline transportation. The Super Low Frequency (SLF) band is also suitable for submarine communication.

The waves within Ultra Low Frequency (ULF) band are able to penetrate through dirt and rock. Through-the-earth signal transmission is especially useful in secure communications, making it suitable for military applications. TTE is also used in mining. Similarly, the Very Low Frequency (VLF) band can also penetrate dirt and rock for some distance. Thus, geophysicists use VLF-electromagnetic receivers to measure conductivity in the near surface of the earth. VLF frequencies benefit from their long range and stable phase characteristics, allowing them to be quite versatile. Like ELF and SLF, VLF can also penetrate seawater to some extent; the military can use VLF to communicate with submarines near the surface of the water. Historically, VLF has been used for navigation beacons.

The Low Frequency (LF) band is mostly used for AM broadcasting in Europe as well as in areas of Northern Africa and Asia. Similar to VLF, LF can also be used for navigational radio beacons. It can also be used for maritime ship-to-shore communication, as well as transoceanic air traffic control. Like the LF band, the Medium Frequency (MF) band is also mostly used for AM radio broadcasting.

The High Frequency (HF) band is most useful in shortwave radio applications, as well as aviation air-to-ground communications. Dipole antennas, such as the Yagi, quad, and log-periodic antennas, operate within the higher frequencies of the HF band. Because its wavelengths range from one to ten decametres (10 to 100 meters), the HF band is also known as the decametre band. The Very High Frequency band is suitable for similar applications as the HF band. Additionally, whereas AM radio operates within the LF and MF bands, FM radio operates within the VHF band

The Ultra High Frequency (UHF) band is perhaps most closely integrated into modern civilian life. In addition to military applications, the UHF band is used in satellite television, mobile phones, Wi-Fi, walkie-talkies, and GPS.

Falling within the microwave band, the Super High Frequency (SHF) band is also optimized for wireless communications. Because the relatively smaller wavelengths of microwaves allow them to be directed in narrow beams, the SHF band is optimal for point-to-point communication using parabolic dishes and horn antennas, for example. Patch antennas typically operate within the SHF band as well. Aside from microwave heating, the SHF band is optimal for satellite links and radar transmitters. The SHF band is also known as the centimetre band because its wavelengths range from one to ten centimeres.

Lastly, the Extremely High Frequency (EHF) band is the highest band on our list. It is also known as the millimetre band, because its wavelengths measure between one to ten millimetres. Because its radio waves are able to be absorbed by the gases in the atmosphere, they only have a short range and can only be used for terrestrial communication over about a kilometer. While certain frequency ranges near the bottom of the band are currently used in 5G cellphone networks, the EHF band is most commonly used in astronomy and remote sensing.

 Frequency Band Name Acronym Frequency Range Wavelength (Meters) Extremely Low Frequency ELF 3 to 30 Hz 10,000 to 100,000 km Super Low Frequency SLF 30 to 300 Hz 1,000 to  10,000 km Ultra Low Frequency ULF 300 to 3000 Hz 100 to 1,000 km Very Low Frequency VLF 3 to 30 kHz 10 to 100 km Low Frequency LF 30 to 300 kHz 1 to 10 km Medium Frequency MF 300 to 3000 kHz 100  to 1,000 m High Frequency HF 3 to 30 MHz 10 to 100 m Very High Frequency VHF 30 to 300 MHz 1 to 10 m Ultra High Frequency UHF 300 to 3000 MHz 10 to 100 cm Super High Frequency SHF 3 to 30 GHz 1 to 10 cm Extremely High Frequency EHF 30 to 300 GHz 1 to 10 mm