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Radiofrequency Test Fixtures

RF 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.

RF Test Fixtures

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 Polarizations

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

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.

Elliptical Polarization

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

Microstrip Antennas

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

 

What are Genetic Antennas?


Genetic Antennas

First appearing in the 1990s, genetic antennas, also known as “evolved antennas,” resemble intricately bent paperclips attached to a radio frequency connector. They are designed entirely by an optimization algorithm, which can be a genetic algorithm or some other iterative method, having no input to their theory of operation by a human engineer.

Genetic antennas go through “Evolutions.”

 
Similar to manually designed custom antennas, it often takes multiple attempts and iterations to come up with the most optimal genetic antenna design for the specified application. However, genetic antennas are not existing designs modified by optimization. Rather, each iteration, designed entirely by a computer, is a new antenna altogether.
 
To design a genetic antenna, the computer program starts with rather simple shape. Using its own calculations and algorithms, the program modifies the shape, either by adding or reshaping elements. These new characteristics will have different effects on the antenna’s performance. After the said modifications, a new shape, thus, a new antenna, is formed. These new shapes are essentially different “evolutions” of the original antenna. 
 
After a good number of second-generation antennas are developed, each one is evaluated to determine how well they fulfill the design requirements; they are assigned a numerical score accordingly. The antennas with the worst performance scores are then discarded. This process of eliminating the poorest performing candidates and keeping the best performing candidates, mirrors Charles Darwin’s concept of “natural selection.” The computer then repeats the procedure of modifying elements, this time, using the best performing candidates from the second generation.
 
Essentially, the computer program develops multiple generation of antennas until it finds the best performing antenna. The resulting shapes are often very complicated and difficult to conceptualize by human engineers.

 

The Many Forms of a Flight Qualified Antenna

Flight-Qualified Antenna

In a previous post, we mentioned some of the environmental testing standards that JEM Engineering follows in order to classify and qualify our antennas. In this post, we explore what makes a “flight-qualified” antenna, as well as the many forms it can take.

In order for an antenna to be qualified for mounting onto a non-stationary ground plane, it has to meet stricter vibration testing standards than it would for stationary mounting. Additionally, atmospheric factors have to be considered. A stationary antenna, if mounted outdoors, will also have to withstand harsher temperatures and weather conditions, and therefore pass more rigorous temperature, wind, dust, humidity, and corrosion tests, to name a few.

Naturally, a vehicular antenna, especially one mounted onto the exterior of the vehicle, will need to withstand the aforementioned non-climate controlled conditions, in addition to increased shock and vibration. A flight-qualified antenna, however, has to meet even higher standards than a vehicular antenna. Additionally, it has to operate well in high altitudes.

A flight-qualified antenna can come in many different forms.

 

Conformal FPA | Flight-Qualified Antenna

 

 We are proud to offer a variety of flight-qualified antennas, ranging from flat panel, to box-type, to blades, and even flexible peel-and-stick antennas. These antennas are suitable for a various applications, including multi-band communications, EW, ISR, SIGINT, as well as satellite communications.

 
Our FPA product line features flight-qualified, multi-element antennas that operate within L-band frequencies (1 – 2 GHz). Our FPA-8296 is an ultra-high frequency (UHF) patch antenna, which features a curved design that allows for less conspicuous mounting.  

 

Among our many box-style antennas is the JEM-238MFC, which comes in an ultra-low-profile form factor designed for 1.2″ (3.18 cm) cavity depth. It is one of our Magnetic Flux Channel (MFC) antennas, which means it does not utilize electric charge to achieve propagation. That being said, it be placed directly on, or even within, conducting structure with no ill-effects on radiation or impedance match performance. JEM-238MFC | Flight-Qualified Antenna
 
All measuring less than 9″ (22.6 cm) long, and weighing 1lb (0.45kg) and under, our box-type wideband sector antennas (WSAs) are portable and multi-functional. They feature 90-degree angle brackets, which allow for easy mounting and dismounting.
UVW-0430A | Flight-Qualified Antenna

 

Specifically designed for aerodynamics, blade antennas are often mounted to the exterior surface of an aircraft. Ranging in size and frequency band, our UVW products used for ground-to-ground, air-to-ground, and ground-to-air communication systems. They are also optimal for unmanned aerial vehicle (UAV) communications. The UVW-0430A is suitable for sensor systems and low drag operations.

 

In addition to readily available antenna products, JEM Engineering offers custom antenna development. We often create special antenna designs to meet our customers’ unique requirements. Our blog post, The Makings of a Reliable Antenna, briefly explains some of our development processes. 

Remote Radio Frequency Testing

JEM Engineering offers all of our customers the added convenience of performing remote radio frequency testing

 

We find that most of our customers prefer to visit our facilities for their scheduled radio frequency tests. However, JEM also offers the option of remote testing, which may be preferable, depending on the specific requirements and circumstances.

 

The main purpose of our remote testing is to provide customers with the same quality rf tests without the need to travel (and the expenses associated with travel). Remote testing also comes at no extra cost. Our invoicing only takes into account our current rates and how much chamber time is spent on each test, no more, no less. 

 

The process of scheduling a remote test is as simple as scheduling an onsite test and involves no additional paperwork.

 

1. Call or email us to begin the quoting process. Whenever a customer inquires after our in-house or remote radio frequency testing services, we walk them through the same steps to determine how much chamber time is needed to complete the test. The customer fills out a form, which outlines the passive test requirements. Once we receive the form (which we commonly refer to as the “Test Plan”), we start generating the quote. Depending on the requirements, we can then determine whether or not the test is suitable for remote testing and most tests are. 

 

2. We schedule the test together. If a test date has not been specified prior to quoting, our team will consult with the customer to determine the best available date(s) to perform the test.

 

3. The customer ships the unit(s) and any additional equipment necessary to complete the test. The customer is responsible for any applicable fees associated with shipping and insurance.

 

4. Upon receipt of the components, we begin the test as scheduled. Throughout the testing process, our experts remain in communication with our customers to eliminate any confusion and ensure accurate results.

 

5. When the test is complete, we ship everything back, carefully packaged. We work with the recipient to send every component back via their preferred courier.

 

6. We invoice the customer. Our standard invoicing schedule is Net 10. Any deviation from this payment method will have been discussed prior to scheduling the test.

 

Our company takes pride in our flexibility when working with customers. 

We understand that sometimes an onsite test becomes a remote test. In the event that travel is either inconvenient or no longer feasible, our technicians will perform the test(s) as scheduled, provided that we have received all the necessary components. If there is any delay on the sender’s part, we will also hold customer property for a specified amount of time, as outlined by our Terms and Conditions.*

 

In summation, almost any and every test performed onsite can be done completely remotely. As long as we have all the applicable components in-house, our experts can perform tests on even the largest units and with the most complex pieces of equipment.

 

* First-time customers receive a copy of our Terms & Conditions.

Design Overview: Loop Antennas

Loop antennas come in many forms, but their overarching distinction is that they are relatively simply constructed, yet very versatile. 

Loop antennas are generally classified under two categories: electrically small and electrically large. If the loop’s overall length (circumference) is less than about one-tenth of a wavelength (C < λ/10), it is usually considered an electrically small antenna. On the other hand, an electrically large loop’s circumference is about a free-space wavelength (C ~ λ). 

Electrically small antennas have proportionately small radiation resistances that are usually smaller than their loss resistances, rendering them ineffective for radio communication. They are better suited as probes for field measurements and as directional antennas for radiowave navigation.

Conversely, electrically large loop antennas are primarily used in directional arrays, including helical antennas, Yagi-Uda arrays, and quad arrays. These applications require the maximum radiation of the loop to be directed towards the axis of the loop to form an end-fire antenna. The proper phasing between turns enhances the overall directional properties.

A loop antenna could take virtually any shape, flexible or rigid. Due to its convenient geometrical arrangement the most popular configuration is the circular loop, particularly the small circular loop. 

The loop’s mounting orientation will determine its radiation characteristics relative to its ground plane. placing multiple loops side by side on the same plane is one way to form an array.

Most applications of loop antennas are within the high-frequency (HF), very high-frequency (VHF), and ultra-high-frequency (UHF) bands.

Small Loop Antenna

Antenna Development & Environmental Testing

 

Antenna Development and Environmental Testing

Environmental testing is a crucial part of antenna qualification. Depending on which application(s) an antenna is to be used for, the environmental qualification standards for which it has to pass will vary. 

 

JEM Engineering qualifies both our own and our customers’ antenna products in a number of ways, including testing them in at least one of our in-house chambers. Both the Tapered Antenna Test Facility (TATF) and the Spherical Near-Field (SNF) chambers perform a number of measurements at varying frequencies. While these measurements indicate an antenna’s RF capabilities, the antenna is not fully qualified until it is rugged enough for long-term practical use.

 

Each of JEM’s qualified antennas is a product of a collaborative effort between both electrical and mechanical engineers. For example, the electrical engineer designs the printed circuit boards (PCBs) and various radio frequency components, whereas the mechanical engineer designs the product’s housing, as well as additional internal components that would allow the unit to withstand various environmental conditions. Additionally, the RF test technicians collect quantitative information pertaining to the antenna’s electrical design, while the mechanical engineer must put the unit through rigorous environmental testing, including shock, vibration, heat, immersion, humidity, chemicals, wind, and frost. 

 

There are a number of different standards by which an environmental test may be performed, but because JEM Engineering is a contractor for the United States Department of Defense, we adhere by defense (or “military”) standard, also known as “MIL-SPEC.” While JEM Engineering does not have the capability to perform environmental testing in-house, we work closely with our trusted partners, who handle our required MIL-SPEC tests once we’ve designed and built any necessary fixturing for them.

 

MIL-SPEC environmental tests are classified by codes. For example, MIL-STD-810 Method 516 measures shock, at values ranging from as low as 20G’s to as high as 75G’s. Similarly,  MIL-STD-810 Method 514 denotes a vibration test. Some tests take into account a combination of atmospheric factors. An example of this is MIL-STD-810 Method 520, which involves quantifying the temperature, altitude, humidity, and vibration a product can withstand. 

 

Lastly, for a general overview on what processes are involved in qualifying antennas, you may refer to our blog post, The Makings of a Reliable Antenna.

What is Beamforming?


Beamforming Waves

The term beamforming refers to a method of directing a wireless signal towards a specific receiving device, whereas the alternative would be allowing the signal to spread in all directions from a transmitter the way it naturally would.

By focusing a signal in a specific direction, beamforming delivers higher signal quality to a receiver. This means information is transmitted faster and more accurately. Furthermore, this accuracy can be reached without boosting broadcast power.

Beamforming | Unfocused Signal
Example of Radiation Pattern with
a Fixed Beamformer
Beamforming | Focused Signal
Example of Radiation Pattern with
an Adaptive Beamformer
 

An antenna array is comprised of multiple radiating elements, each of which contributes an element pattern to the array’s radiation pattern. Each element pattern is a spatial distribution of RF power arising from the amplitude and phase of the RF signal at the element’s RF feed point. The array’s radiation pattern is determined by the coherent sum of all element fields, each which may be “weighted” by an additional amplitude and phase. Such weighted patterns exemplify beamforming in the array, whereby sidelobe levels and nulls are produced and controlled by adjusting the element weights.

Beamforming techniques loosely fall into two categories: conventional and adaptive. 

Fixed beamforming generally describes a conventional technique where the antenna array pattern is obtained from fixed element weights that do not depend on the signal environment. Conversely, adaptive beamforming element weights that do depend on and can adapt to the signal environment via some feedback mechanism. 

Adaptive beamforming, which was initially developed in the 1960s, uses a digital signal processor (DSP) to compute the complex weights using an adaptive algorithm, which then generates an array factor for an optimal signal-to-interference-plus-noise ratio (SINR). Basically, adaptive beamformers are designed to adjust to differing situations in order to maximize or minimize SINR, which helps measure the quality of wireless communication.

The average civilian experiences adaptive beamforming technology in their every day life. In fact, wireless carriers use adaptive beamforming to provide next-generation wireless communications (5G) and long-term evolution (LTE) services.