Field-proven magnetic base Antennas with strong holding force

COJOT’s omnidirectional mag-mount antennas are the preferred choice for applications and circumstances where permanent installations are not possible and a rapid deployment and removal is preferred or even required.

We provide a wide range of antennas covering VHF, UHF, GSM, LTE, 5G, WiFi and other bands. Each antenna features a strong, rugged magnet base for temporary mounting on suitable magnetic surfaces such as a vehicle roof or engine hood. The mag-mount grips the car very well, even at high speeds, with a typical vertical holding force of 280N.

 

Magnetic Mount Accessories

Standard types of vehicle (NATO/US base flange) and portable antennas can be installed with our optionally available mag-mounts, allowing antennas to be used for a quick and easy temporary set-up, including retro-fitting, providing a cost effective solution to overcome restrictions with permanent installations.

 

 

This is our range of field-proven magnetic mount antennas and accessories:

WB25512W (25 – 512 MHz)

WB30108W (30-108 MHz)

WB30175W (30-175MHz)

WBCX512W (118-512MHz)

WB460W (420 – 6000 MHz)

WB525W (500-6000MHz)

MG512R

MG3091

 

COJOT beam steering antennas with support for dual-polarized MIMO 

COJOT supplies Switched Beam Antennas (SBAs) for different frequency bands with support for MIMO (multiple input multiple output) – featuring dual RF inputs/outputs that are connected to a different polarization of the radiator elements. MIMO is an essential requirement as modern Radio Systems are often relying on this to enhance the capacity or coverage of a radio network.

COJOT SBAs are currently in use at major global players in multiple applications – fielded to Tactical Communication networks both on land and at sea. These contain fixed Point-to-Multipoint as well as Mobile-Ad-Hoc-Networks (MANET). SBAs fit extremely well to these kind of adapting networks like MESH – where on top of higher layer routing, physical RF link routing can also be utilized.

2x2MIMO System

Spectrum monitoring and management are key factors in these kind of modern communication networks and with different beam modes and fast beam steering capabilities, COJOT SBAs provide an ideal solution for these – offering enhanced LPI/LPD resistance.

SBA Enhances LPI/LPD Resistance

Automatic and quick network setup and management 

In addition to superior performance features and enhancements, COJOT beam steering antennas can also improve and facilitate common basic routines in a wireless network. One such example is when you add a new node to a mobile network, it is often problematic and time-consuming to manually allocate the precise direction of the best signal with directional antennas. However, using a SBA in omni-mode, the new node can join the network immediately and the link can be optimized afterwards by searching for the strongest signal using the SBA’s narrow beam. Similarly, the wide beam mode provides faster scanning time. The possibility to utilize sharp beams within a frame rate over the 360-degree area opens many possibilities for network management and usage of advanced network routing protocols like OSPF (Open Shortest Path First).

Products

Coming Soon

  • BAND L/III+ (1350-2700 MHz): SBA1327B-DP

Introducing New 4G/5G Antennas for Counter-RCIED/Jamming Applications

COJOT introduces new wideband antennas that have been designed to meet the challenging requirements of mobile Counter-RCIED/Jamming applications for mobile networks.

In addition to high power handling capabilities and a rugged mechanical design, these new portable and vehicle antennas provide enhanced omnidirectional radiation performance to cover 4G / 5G frequencies.

View the new range:

WD730Q:

  • Wideband dipole antenna
  • 690-3000MHz
  • 50W power rating

HD730Q:

  • High gain dipole antenna
  • 690-3000MHz
  • Up to 75W power rating

HD727XM:

  • High gain/high power dipole antenna
  • 700-2700MHz
  • Up to 175W power rating

WD28P:

  • Compact wideband dipole antenna
  • 2-8 GHz
  • 20W power rating

Magnetic Mount Antennas for Rapid Deployment

COJOT prides itself on understanding client needs. Where a quick setup and removal is required or where the permanent installation of an antenna on a vehicle is not possible, our magnetic mount antennas are unparalleled.

Here are three of our most popular magnetic mount antennas:

WB460W (420 – 6000 MHz)

WB25512W (25 – 512 MHz)

WB30108W (30-108 MHz)

New Extended Beam Steering Antennas from COJOT

In a world where drone strikes are a near-daily occurrence, counter-drone measures are becoming increasingly important. At COJOT, we are working tirelessly on our beam steering antennas and proudly introduce two new versions with enhanced counter-UAS antenna functionality.

The new extended dual band SBA2456DB (10W) and high power SBA2456XDB (100W) are now covering the full 2 and 5GHz WiFi frequency bands (2.4 to 2.5 and 5 to 6 GHz), while providing the same excellent performance with up to 24 sharp beams and a minimum of 20-degree beamwidth to cover the full 360-degree horizontal area. The SBA2456 models are equipped with the latest updates of the COJOT SBA product family, including support for fast RS-485/422 and Ethernet interfaces as well as thoroughly environmentally tested mechanics – according to MIL-STD-810.

Fields of applications: Counter-UAS, Counter-Drone, EW, ISR applications

COJOT SBA product family:

C4i Band IV/C: SBA4450B and SBA4450XB

C4i Band III+/L: SBA1327B

5G: SBA3438B-DP

Direction Finding Antennas and Accuracy Part 2: Characterization of DF antennas

By Johan Fuhri, Product Owner

In the previous blog post we considered some of the ways in which we can express the performance of an DF antenna, and we noted that simply stating the ‘accuracy’ of a DF antenna is not quite that simple.  We also noted that there are various algorithms that can be used along with a typical DF antenna, and that the accuracy and sensitivity of DF system is dependent on both the quality of the antenna and the direction-finding algorithm.

In this blog post we will discuss the process of ‘characterizing’ a DF antenna array to generate a table of values which can be used in correlative algorithms. We will also look at a few real-life problems that cause inaccuracies in DF systems, specifically when using basic correlative algorithms.

What is a correlative algorithm?

Correlation is a mathematical and statistical tool that can be used to quantitively express how similar two or more sets of data are to each other.  It is basically a fancy way of playing ‘pick the closest match’.

To illustrate the process, we are going to use colour swatches (the type that adorns the walls of the paint department at your favourite hardware chain store) and create a simple direction-finding system.  Consider the image below, which is a circle divided into six sectors and a colour assigned to each sector.  Let’s suppose we have a DF antenna right in the center of this circle, and the lines/sectors represent the various directions from which a signal can arrive.  This, however, is a very special antenna, in the sense that it will tell you the colour of an incoming signal.

If we are clever, we can make sure that we install the antenna in such a way that a colour of our choosing, yellow in this case, is always pointing due north.  If we then receive a signal from our antenna (which will be a color, remember), we can compare the color that we received from the antenna with the image above to determine which direction the signal came from!  That’s not too hard now, is it?

On a very basic level, this is what correlative DF algorithms do.  The DF antenna is spun through 360⁰ and the incoming signal from each direction, on each of the elements on the antenna, is sampled and stored in a table.  This table is called the ‘characterization table’ (although different people might name it something else), and it effectively allows a DF system to calculate the exact direction from which a signal arrives by comparing the signal with those stored in the characterization table, just like we did with the colours.  The circle indicating which colour is in which direction is effectively our own ‘characterization table’.

 

Alaris Antennas offer characterization of our DF antennas and systems as a service to our clients.  These characterization measurements are most often done at an outdoor reflection range at the National Antenna Test Range just north of Pretoria, which we affectionately simply refer to as ‘Paardefontein’ (https://www.paardefontein.co.za/).

The measurements entail mounting the antenna on one of the turntable towers and setting up a signal source some distance from the tower.  The DF antenna is then turned through 360 degrees in 4⁰ steps (depending on client requirements) and a sample captured from each of the elements of the antenna at each step.  This data is then compiled into a characterization table which we make available to our clients.

When life hands you green lemons…

As you might expect though, in real life things are not quite as simple.  There are various factors that can influence the accuracy of a DF system in real life scenarios, and in this section we are going to consider just a few of these, using our colour-wheel to illustrate why they cause problems.

Resolution of the characterization table

In our colour wheel example, we divided the circle into 12 sections, each representing an angle of 30⁰.  It is thus (nearly) impossible for us to determine the direction of an incoming signal to anything more accurate than about 30⁰.  Note that the signal to noise ratio, or the strength of the electric field, or the amount of noise has very little influence in the accuracy we can achieve, since we are fundamentally limited by the number of colours we have in our characterization table.

We can significantly increase our accuracy by dividing the circle into 36 smaller sectors, giving us many more options to compare our incoming signal to!  We can now determine the direction of our incoming signal to within a 10⁰ sector.

 

The original orange signal we received is most similar to the third sector, indicating that our signal arrived from a direction of 25⁰±5⁰.

 

 

 

 

 

 

 

The same is true for DF systems in practice as well.  When the antenna is characterized, the angle steps, or angle resolution, can be freely chosen by the person doing the measurement.  While it is tempting to choose super small steps (let’s measure a sample every 0.1⁰), this implies that the DF processor has do deal with a table that has 3600 entries, which can potentially slow things down to a crawl when limited processing power is available.  In practice, steps of between 2⁰ and 5⁰ are typically used, depending on the specific implementation of the client.

It is also worth mentioning that certain more sophisticated algorithms can achieve resolutions smaller than the characterization table resolution by employing all kinds of clever mathematical techniques.  It is the equivalent of being able to look at a greenish-yellow signal, and inferring that it must be somewhere between the green and yellow sectors, and thus giving a better guess than simply choosing the green or yellow sectors purely by similarity.  The details of these ‘super-resolution techniques’ are, however, often part of the proprietary magic that system integrators implement as their own competitive advantage.

The effects of noise

Up to now we assumed that the incoming signal that we are evaluating is perfectly clear, and that our accuracy is only limited by the resolution of the table against which we can compare our incoming signal.  In antenna terms this is called the ‘large signal accuracy’ of the antenna, as it assumes that no noise is present in the measurements.

Real life signals, however, rarely qualify as large signals and often has noise superimposed on the signal of interest.  Let’s look at an example in our colour wheel example.

Adding noise to the signal makes it harder and harder to find the colour that best matches the characterization table.  If there is noise present on the signal, invariably the DF processor will look at the ‘colours’ it can choose from and make mistakes.  At best it can tell you that the signal belongs to one of these four sectors, and this reduces the RMS accuracy of the overall system.

Adding even more noise, at some point it becomes practically impossible for the DF processor to make any sense of the incoming signal, and it may start taking ‘random’ guesses.  These wild guesses can be in any direction whatsoever, and are called appropriately called ‘wild bearings’.

The main source of noise in a DF system is the internal electronic components of the system.  Every amplifier, transistor and resistor contribute a bit of noise, and all this noise is added to the incoming signal before the DF processor has the opportunity to compare this signal to the characterization table. We can thus see that it is important to ensure that the received signal is as strong as possible while keeping the noise as small as possible to get a good signal-to-noise ratio.

The design of the DF antenna, specifically the gain of the antenna elements, directly contributes to keeping the SNR as high as possible by ensuring that the incoming signal is converted to a large as possible voltage for the receiver to work with.  For a more detailed discussion on this topic, refer to the previous blog post in this series.

Multipath interference

In certain scenarios it might be possible for a signal to bounce off objects, typically large buildings, and the signal that arrives at the DF antenna may end up being a combination of copies of the same signal, arriving from different directions.

These multipath signals, which we will illustrate as a mix of two colours, are very difficult for DF processors to deal with, and is very likely to a reduction in accuracy at best, or wild bearings at worst.

 

Using our colour wheel example, we can see that a pure green signal leaves the source that we are trying to find, but along the way a signal bouncing off a building also makes its way into the DF antenna.  The ‘colour’ making its way into the DF processor is mostly green, with a dash of blue mixed in.  This mixed signal will certainly cause problems for most simple correlative algorithms, as the received signal does not match any of the signals in the correlation table, causing inaccuracies and wild bearings.

Having said that, there are a few clever algorithms, like MUSIC, which can actually use these strange signals to its advantage and can figure out that there are multiple paths in play.  How these clever algorithms work is beyond the scope of this blog post (and the authors current intellectual grasp…).

Conclusion

This blog post attempted to explain the principle behind most correlative algorithms while trying to stay away from the complicated maths that make it work.  We also looked at a few real-world effects that influence the accuracy of DF systems in general, and specifically the mechanism that causes problems in correlative algorithms.  Most of these inaccuracies are caused by external factors, i.e. they are not so much caused by the design of the DF antenna, but by the environments in which they are employed.

In the last post of the series we will discuss a few of the intricacies of DF antenna design and how they can cause inherent inaccuracies (i.e. non-environmental causes) in poor designs.  This will be a rare glimpse into the everyday challenges that antenna engineers at Alaris Antennas deal with on a daily basis!

Direction Finding Antennas and Accuracy

By Johan Fuhri, Product Owner

Part 1: Accuracy and Sensitivity

It is sometimes said that: “If you can’t convince them, confuse them!”.  Nowhere is this more apparent than the minefield which is antenna datasheets and brochures.

Direction finding antennas are meant to be instrumental in accurately determining the direction of an incoming signal, that much we all agree on.  One would thus think that a graph of accuracy vs frequency would be very obvious and clear measure of how well an antenna will do its job, and you would be very wrong.  Welcome to the world of specmanship.

Specmanship is the nefarious use of data to present to the customer what he wants or expects to see, rather than what is true.  It is quite possible to present antenna performance figures, like ‘accuracy’ in this case, in a way that looks fantastic on paper, but will fail to live up to the customer expectations in real life applications.  This blog post will attempt to delve into the world of DF antenna accuracy to explain how it should be interpreted in the context of real-world applications.

What is ‘accuracy’?

To generate the accuracy plot of a DF antenna, we generate a mathematical model of the antenna, and transmit a hypothetical signal to the antenna from a specific angle of incidence.  The antenna (and its associated DF algorithm) will then receive this signal and calculate the angle from which it believes the signal is coming.  If we do this for each direction while moving around the antenna, we can calculate an average (or RMS) error, taking into account incoming signals from all directions.  This RMS error, plotted over frequency, is the typical ‘accuracy’ plot that you will find in many DF antenna datasheets.  Simple enough, right?

Well, not quite.  The accuracy with which a DF antenna can estimate the direction of an incoming signal is dependent on a whole bunch of different factors.  An accuracy plot without stating the underlying assumptions really provides very little information indeed.  Let us consider a few factors that will influence the accuracy of a DF antenna.

Noise and signal level

The first, and most obvious parameter, is the signal level (or more accurately, the strength of the electric field) of the incoming signal.  The incoming E-field needs to be large compared to the noise level, else the antenna/algorithm will have no way of extracting the signal of interest from the background noise.

As an analogy, suppose someone in the room is trying to get your attention at a dinner party.  The person will need to shout louder than the average ‘noisiness’ created by all the other people at the party in order for you to differentiate that person’s voice above the others.

When trying to attract someone’s attention at a rock concert, however, you will have to shout much, much louder to get the same effect.  Trying to determine your friend’s location by listening for his voice at a rock concert is much harder to do than at a dinner party.  The same principle applies in DF systems.

 

In the example above, we used a hypothetical 8-element DF antenna and calculated the DF accuracy for incoming signals with a Signal-to-Noise ratio (SNR) of 5dB (loud rock concert) and 20dB (dinner party) respectively.  As we suspected, when our signal of interest is barely stronger than the noise floor (SNR=5dB), the DF antenna/algorithm will make many estimation errors and will only be accurate to within about 20⁰ or so.  The very same antenna, given a nice strong signal (SNR=20dB), is however capable of achieving accuracies of way better than 1⁰.

Antenna manufacturers will sometimes publish accuracy plots like these labeled as “large-signal accuracy”, indicating that the plot assumes zero (or very low) noise levels.  This accuracy provides information about the array layout, size and a couple other more esoteric features of the antenna.  While this information is extremely useful for the antenna engineer during the design process, it really does not say much about how well the antenna will perform in the real world, mainly because of the following two factors that are cleverly (and sneakily) ‘omitted’.

  1. The accuracy calculated from the SNR excludes the gain of the antenna elements. SNR is a measurement taken at the radio interface, assuming the signal was already received on the antenna.  You can achieve a good SNR on a poor antenna by blasting it with an extremely strong signal.  Likewise, and desirably, a good antenna element should be able to provide a decent SNR given a relatively weak signal.
  2. The accuracy is also dependent on the algorithm used for direction finding. Certain algorithms (like pure Watson-Watt or interferometry) can introduce ‘bias’ errors, which is caused by applying a mathematical model that assumes a perfect antenna pattern to a ‘real-life’ antenna which has patterns that may deviate from the ideal.  The plots above make use of a correlative algorithm which has pre-knowledge of the antenna imperfections and can compensate for these during the estimation calculations.  This topic will be explored in more detail in part 3 of this series.  Accuracy plots should thus always be interpreted in the context of the algorithm used and should explicitly declare the algorithm used to generate the plot.
DF Antenna Sensitivity

To address problem 1 from the previous section, the performance of a DF antenna can be expressed in terms of its sensitivity to electric fields.  Given that a typical DF antenna, using a correlative algorithm, is capable of sub-one-degree accuracy when the SNR is high, we can choose to calculate the incoming E-field strength that will generate a given SNR, or alternatively, the E-field strength that will produce a given RMS accuracy.

The advantage of using a sensitivity plot as a performance metric for a DF antenna is that it includes the gain of the antenna elements, and thus provides information about how good the individual antenna elements are, in addition to the overall layout of the array.  Expressing the performance of a DF antenna in terms of its sensitivity also provides a much clearer indication of how well the antenna is expected to perform in real-life applications.  With all other things being equal (same receiver system, same incoming signal), a DF antenna with better sensitivity will achieve better real-life accuracies than an antenna with lower sensitivity.  The large-signal accuracy does not provide this straightforward equivalence to real-life performance without significant interpretation.

As an example, consider the calculated E-field sensitivity for the same antenna considered in the previous section.

This plot indicates the electric field strength required to achieve a SNR of 15dB (blue line) and the E-field strength required to achieve an RMS error of 2⁰ or better.  This plot allows the customer to directly evaluate whether this antenna is appropriate for the type of signals expected in their specific application.  Notice however, that these plots also require several assumptions (like the channel bandwidth and noise figure of the system), and these should be stated explicitly.

As is the case for large-signal accuracy, this plot is only valid for correlative algorithms and will differ significantly when used with other types of algorithms.

Given these obvious advantages in using the sensitivity plots as a metric of DF antenna performance, most Alaris Antenna brochures will publish sensitivity plots rather than accuracy plots.

Part 2 of this series will consider the factors that influence the accuracy and sensitivity of DF antennas, while part 3 will delve more deeply into the nuances of characterization measurements as it applies to correlative algorithms.

An Introduction to Radio Direction Finding

By Eugene Botha and Kalinka Faul

Humans do audio direction finding to a remarkable precision of less than two degrees.  Shortly after the development of radio transmitters and receivers, radio direction finding (RDF or just DF) evolved for much the same reasons as human audio direction finding: firstly, for the location of possible threats and secondly for spatial awareness.

RF direction finding is used in several applications:

  • Military: such as the direction of a threat, the location and movement of enemy transmitters and the direction of enemy jammers.
  • Search and Rescue: the location of RF search and rescue beacons.
  • Science: the tracking of animals in their environment.
  • Radio monitoring: the location of sources of interference and of illicit transmitters
Terminology and abbreviations

RF: Radio Frequency

DF: Direction Finding

RDF: Radio Direction Finding

AOA: Angle of Arrival

 

Single Receiver DF Systems

Directional Antenna Technique

The simplest RF direction finding system consists of a directive antenna and a single receiver.  The antenna is pointed in different directions while the receiver indicates the received signal strength. Only the magnitude of the signal is used to determine the direction of a transmitter.

The accuracy of this technique is dependent on the width of the antenna radiation pattern. A narrow beam will improve the accuracy but will increase the time spent scanning all possible directions.  If the beam is too narrow the target may even be missed especially with intermittent transmission sources.  A broad beam will decrease the bearing accuracy.

Scientists use handheld RDF devices to follow and accurately locate wild animals once their general location is known.  Radio beacons can be made very small and consume very little power. This is ideal for long duration tracing of even very small animals.  The relative signal strength is used to determine the direction of the radio beacon and the absolute signal strength shows some measure of the distance to the radio beacon.

In military applications this RDF is called a Spinning DF system and is commonly used for broad band high frequency DF.  These systems are fully motorized and use knowledge of the antenna radiation pattern to improve accuracy.

The benefit of a single receiver RDF system is that they are cost effective, compact and have low power consumption.

The image below shows both a wide and narrow beam as they scan for signals transmitted by the target. The received signal level of both beams can be seen on the left. Note how the signal levels increase the closer the beams are to the direction of the target.

Doppler and Pseudo-Doppler Technique

The Doppler DF system uses a single receiver connected to an omni directional antenna that is physically rotated on the circumference of a circle.  As the antenna moves toward the radio source, doppler shift will increase the received frequency and the received frequency will decrease as the antenna moves away from the source.  The change in frequency (obtained by FM demodulation) is used to determine the direction of the radio source.

The modern approach is to successively sample each antenna in a circular array of antennas, removing the need for any moving parts.  This is referred to as Pseudo-Doppler DF.

The image in the left of the figure below shows the circular trajectory of a doppler DF antenna. The received signal is incident from the right.

The received frequency at various positions along the trajectory of the antenna is shown in the right image of the figure. As the antenna moves away from the incident signal, the frequency increases (red graph) and as the antenna moves towards the incident signal, the frequency increases (blue graph). The black curve shows the received frequency response at the position indicated by black circle on the antenna diagram.

 

Two Receiver Systems

Mono-pulse Technique

The mono-pulse or sum-difference RDF technique uses two antennas.  The antennas are connected to a four-port combiner 180° hybrid that generates a sum and difference signal.  Such sum and difference patterns are generated by means of closely spaced overlapping radiation patterns at boresight.  These signals form sum and difference radiation patterns.  The ratio of the sum and difference signals and knowledge of the sum and difference patterns are used to determine the direction of the transmitter.  Phase information is used to determine on which side of the sum pattern the transmitter is.  An advantage of this system is in its capability to determine the direction of a transmitter after receiving one pulse. Such pulse could be a mere few microseconds. Accuracies of 10meter over a 100Km distance has been reported.

 

 

Interferometer

The relative difference in the phase of the signal received by two omni directional antennas spaced a set distance apart can be used to determine direction or angle of arrival of a RF signal.  In the omni directional case, the interferometer does not have a way to determine if the signal arrives from the front or the back of the antennas.

As the frequency increases (or electrical separation increases) ambiguities appear as the phase differences wrap.  If the antennas are too close (electrical separation very small) then the resulting phase difference will be very small and the system will not be able to determine the AOA.  The frequency range of use is thus determined by the separation of the antennas and the noise figure of the receivers.

The image below shows two omni directional antennas with the incident signal circulating around them. The relative phase of the incident signal between the two antennas is shown in the image on the right. This phase difference is used to determine the direction of the incident signal.

 

Adcock Antenna, Watson-Watt Method

An Adcock antenna uses two crossed loop antennas. The bearing of the RF signal is determined using the level of the signals received at each antenna.  The method to process the information from a Adcock array is referred to as Watson-Watt. This is the best-known method of radio direction finding.

In a more general application four closely spaced omni directional antennas positioned in a square can be used to form an Adcock array.  The opposing antennas are combined using a 180° combiner to form figure-of-8 patterns, which creates a unique set of magnitudes for any bearing direction.  A Watson-Watt antenna cannot determine if a signal comes from the front or the back without the use of a third omni directional antenna to resolve the 180° ambiguity.

The figure below shows the typical radiation pattern of an Adcock array. The received signal rotates around the array. The relative amplitude of the signal received by the two crossed loop antennas (or combined omni directional antennas) is shown on the right. The + and – at the top indicates the relative phase of the two crossed loops with respect to the omni. It is this relative phase that resolves the ambiguity.

N Receiver Systems – Correlative RDF

With the technological improvements in receivers and digital processors, all the information produced by multiple antenna elements can be used to improve the performance of RDF systems.  Typically, the bearing is calculated using the phase differences of the signals received at the various antennas in the correlative array.  The correlative algorithm compares the phase differences of the incoming RF signals at each antenna to a set of calibration phases stored in the processor to determine the most likely AOA.  The correction function correlates the relative phases (and magnitudes for some correlators) of the received signals with the correlation table over all possible angles; the maximum of the correlation function indicates the AOA.  The calibration information can be obtained by calculation or by measurement of the antenna.

The most common implementation of a correlative array is to have several omni directional antennas (typically four to nine antennas) in a circular pattern.  In this configuration the differences in the phase of the incoming RF signal at each element is used by the correlative algorithm to determine the AOA.

Cutting edge systems can use both magnitude and phase of an arbitrary number of antennas arbitrarily positioned to determine the AOA in 3D, not just in a horizontal plane.  The spatial positioning of the antennas in the array is of critical importance to achieve good performance without introducing ambiguities.  3D calibration data (not just in azimuth but also in elevation) with elements spatially positioned in atypical configuration is used to determine the AOA in 3D.  In many practical applications the patterns of the antennas will not be omni directional, clever algorithms can incorporate the magnitude information to improve RDF performance.

Even an Adcock antenna can be characterized and used in conjunction with a corelative estimator to improve performance of the Watson-Watt method.

 

RDF Performance

The two critical performance measures for DF systems are accuracy and sensitivity.

Accuracy is the measure of how accurate the bearing direction can be determined.  The accuracy of a DF system is dependent on the DF processor, the specific design, the quality of the antenna elements used and the installation environment of the antenna.

Sensitivity is the measure of how well the DF system will perform in the presence of a small signal in a specified noise level.  The sensitivity is dependent on the receiver noise, losses in the antenna and even the topology of the antenna elements in the array.

A discussion of these performance measures will be the topic of a follow up blog.

High-Gain Omni Antennas in Mobile Applications

By Johan Fuhri Antenna Principles High-Gain Omni-directional antennas are used in situations where the direction of the receiving antenna is not static or known.  A typical situation is where antennas are mounted on a mobile platform like a vehicle or vessel. In any communication system, the quality and throughput that can be attained through a link is highly dependent on the signal strength and quality of the signal between the receiving and transmitting antennas. Antennas play a crucial role in the performance of any wireless communication link.  High-gain antennas can be used to provide additional power in the link budget in general, due to the fact that antennas effectively amplify the signal being fed into the antenna. It should be noted however, that antennas provide the additional amplification in a completely different way than a powered amplifier.  Antennas, being passive devices, can only amplify the input signal by focussing the beam in a particular direction.  Therefore, there will always be a trade-off between the gain and the beam-width over which this gain can be achieved.  For example, a narrower beam will provide more gain and vice-versa.  This trade-off also exists in omni-directional antennas.  Since omni-directional antennas are required to provide signal over a 360o beam in the azimuth plane, the additional gain can only be achieved by reducing the beam-width in the elevation plane.
Looking at the above radiation patterns, it should be noted that a single dipole has a radiation pattern with a wide beam and gain of about 2.2dBi.  The second figure shows an array of four dipoles identical to the first.  By stacking four of these dipoles and feeding each dipole in phase, the gain of the array increases to 7.7dBi, but over a much narrower elevation beam.  In addition to the narrower beam, additional side-lobes are created that are separated by deep nulls in the pattern.  These nulls imply that, at certain angles of incidence, the antenna will receive practically no signal at all.  It is therefore important to note the positions of these nulls when designing a system where an omni antenna is required. The beam-widths of antennas are normally expressed as the angle over which the gain is within 3dB of the maximum gain in the main lobe. It is often useful to also find the angle of the first null in the pattern. Comparison of a 5dBi vs 8dBi Omni-Directional Antenna The gain of an antenna comes from stacking a number of similar elements and feeding them in-phase. The radiation pattern below shows the first-null and 3dB beam-widths of a simulated antenna:
The below radiation pattern is that of an antenna of about half the length of the above:
The second antenna would use half the number of antenna elements and will have a gain of nominally 3dB less than the first. The table below summarize the gain and beam-widths of the two antennas:
AntennaNominal Gain3dB BeamwidthFirst-null Beamwidth
Dipole 8-Stack8dBi20°
Dipole 4-Stack5dBi18°41°
It should be noted that both the 3dB and the first-nulls’ beams are doubled when the gain is halved (i.e. 3dB lower), exactly in line with expectations. Practical Applications Vehicle parked on a sidewalk curb A typical sidewalk curb in South Africa is about 150mm in height and it is not uncommon for bigger vehicles to park with two wheels on the sidewalk.  Assume that the vehicle is about 2m wide.
8dBi Antenna5dBi Antenna
Antenna @ 4.3° incline4.9dBi4.05dBi
At an incline of 4.3o, the 8dBi antenna already lost 2.73dB relative to the maximum gain, while the 5dBi antenna only lost 0.85dB. Vehicle driving on a typical urban road
8dBi Antenna5dBi Antenna
Antenna @ 5.71° incline2.34dBi3.51dBi
At an incline of 5.71o, or a 10% slope, the 8dBi antenna already has less gain in the direction of the receiving antenna compared to the 5dBi antenna.  Due to the narrow beam of the high-gain omni, the gain rolls off much quicker as the elevation angle increases.  In this scenario, the lower-gain omni will outperform the high-gain omni. Vehicle driving in off-road conditions
8dBi Antenna5dBi Antenna
Antenna @ 10° incline-24.5dBi0.92dBi
At an incline of 10o the 8dBi reaches its first null and the effective gain of the antenna towards the receiver is effectively non-existent.  A vehicle parked in this orientation will completely lose connectivity.  While it is not typical for a vehicle to be parked in this orientation, it is not unusual for a vehicle to reach an incline of 10o or more for short instances during off-road excursions.  The signal strength seen at the receiver will vary wildly over short periods (more than 20dB in less than a second).  Some communication systems will attempt to switch to lower modulation schemes, and if not designed to deal with these quick changing signal levels, keep switching continuously to the point where throughput is significantly reduced. In the case of the lower gain omni, the antenna still presents a gain of almost 1dB towards the receiver, which is usually well within the designed safety margin of most communication systems.  Since the change over time is much less (about 4-5dB) compared to the high-gain omni, the communication system will be presented with a more time-stable signal as well. Conclusions High-gain omni-directional antennas achieve additional gain at the expense of elevation beam width. In static installations, the reduced beam-width is generally acceptable, but one needs to practice extreme caution when using them in mobile applications. The shorter antenna will, in addition to the electrical advantages of a wider beam, also have significant mechanical advantages.
  • Shorter antennas are less likely to hit over-head obstructions.
  • Shorter antenna will be lighter and more stable on the same spring mount.
  • Shorter antenna will be mechanically more robust, even when experiencing significant impacts like an oak-beam test.
For more information on our range of High-Gain Omni Antennas, visit our Product Section here To view the YouTube video, click here

Spacing Considerations of Multiple LPDAs on a Mast

Written by Johan Fuhri

Introduction

One of the questions we often get asked at Alaris Antennas, is the question of spacing between multiple antennas on the same mast. Often, this question is related to situations where the space on the mast is limited (mobile or tactical application) and where high-power transmitters are involved. Unfortunately, there are no easy formulas that will work in all circumstances, so the answer typically starts off with something along the lines of “it depends”. In addition to distance, there are also several other parameters that complicate the discussion.  This answer, obviously, is not all that useful to most clients, so this article will attempt to discuss some guiding principles that Systems Engineers can use to address this problem of spacing antennas on a mast.

What is the worst that could happen?

When two antennas are placed close to each other, they will inevitably couple to each other to some degree. ‘Coupling to each other’ is just a fancy way of saying that whenever one antenna transmits, some of that signal will be received by the other antenna. This stray signal may or may not be a problem, depending on the specific hardware, power of the signal being transmitted, gain of the two antennas, orientation, spacing and many other factors. A typical example may be where a high power transmit system is on the same mast or shelter as a monitoring system, or where multiple high power transmit antennas (with their associated transmitters) are co-located on the same mast. Typically, any receiver (or transmitter) can only handle so much power coming in from the antenna before being permanently damaged. When dealing with kilowatt-capable amplifiers somewhere in the system, other transmitters or receivers could easily be damaged if the design of the system does not specifically make provision to prevent this. The first and most important question is thus to determine how much isolation (the inverse of coupling) is really required between the various devices in the system. This will determine, in turn, how much isolation is required between the antennas.

Factors determining the isolation between co-located antennas

Let us consider some of the factors that will improve or deteriorate the isolation between two antennas. Since LPDAs are often used in wideband high power transmit systems, we will specifically use them to illustrate the principles, but they equally apply to most other types of antennas as well.

Polarisation

The one parameter that has a much larger impact on isolation than spacing, is the polarization of the two antennas.  As the name suggests, Log-Periodic Dipole Antennas consist of several dipoles that are fed in specific way to create forward gain.  Dipole elements, or any sort of “skinny” element, always have a null directly above or below the element when mounted vertically. When they are mounted horizontally, there is no longer a null in the radiation pattern aiming up or down where the other antenna may be located.
Figure 1: All LPDA antennas have a null in the radiation pattern above and below the antenna when mounted vertically (image at top), but still have significant gain towards the sides (image at bottom).
If two LPDAs are mounted on a mast, one on top of the other, the coupling between them will be much higher when they are both horizontally polarised (HP) compared to when they are vertically polarized (VP). In a simulation of the LPDA-A0102 using the same spacing between the two antennas, we can see that the isolation between the two VP antennas at 100MHz is about 40.7dB, whereas the isolation between the two HP antennas is only 22.5dB. To put this into perspective, a 5kW (67dBm) signal transmitted from one of the antennas will induce only about 0.43W (26.3dBm) of power into the other antenna when both are VP. When both antennas are HP, a massive 28.2W (44.5dBm) signal is picked up by the second antenna.
Figure 2: Two antennas mounted in vertical polarisation (left) vs horizontal polarisation (right).
Figure 3: The coupling (S21) between the two VP antennas (blue) is much lower (-40.7dB) compared to that of the two HP antennas (green) (-22.5dB).

The worst isolation will (almost) always be at the lowest frequency

From Figure 3 it can be seen that the highest coupling (lowest isolation) occurs at the lowest frequency point of the plot.  This will almost always be the case, with a few notable exceptions (like LPDAs with less efficient reduced length elements at the rear). It is important to keep in mind that many antennas are designed to be wide-band and LPDA’s are no exception. The isolation will be determined by both the distance between antennas and the frequency of the signal. For a given spacing, a higher frequency does not transfer as much power as a lower frequency signal, with the reduction in power proportional to the frequency squared! Hence with all other things equal (e.g. antenna gain), the lower frequencies will typically be where the problems lie. In addition to the propagation effect, it is also worth noting that all the other structures on the shelter, mast or building will be much closer (in terms of wavelengths) and is more likely to cause secondary effects that may (or may not) influence the isolation between the two antennas.  System Engineers will do well to tread carefully when trying to estimate the isolation between two antennas in the HF and even VHF frequency bands.

Physical spacing between antennas

Thus far we have seen that the spacing between the two antennas is only one of the many factors that determine the isolation between two antennas. Unfortunately, it is also one of the parameters that turns out to have a relatively small influence compared to things like polarisation. To illustrate, let us consider three scenarios where two identical antennas (LPDA-A0102) are horizontally polarised and spaced 2m, 4m and 8m apart.  While we are using them both in HP, the same principle and relative difference will apply for the VP case.
Figure 4: Coupling (S21) between antennas that are spaced 2m, 4m and 8m apart. Every time the spacing doubles, another 6dB of isolation is achieved.
Doubling the distance from 2m to 4m only increases the isolation by 6dB (and even less at 100MHz), which is not all that much since an additional 2m spacing on a mast is a practical impossibility under most situations.  As a rule of thumb, every time the spacing between two antennas is doubled (or halved), the isolation between them increases (or decreases) by about 6dB. This is true unless the antennas are really close to each other (in terms of wavelengths) and is true for both the VP and HP scenarios. It is thus clear that an increase in spacing between antennas is unlikely to be the magic bullet that will solve isolation issues in most practical scenarios!

Different type/models of antennas

While we have used two identical antennas to illustrate the various parameters that influence isolation between two antennas, we should not forget that most of these also apply to antennas that are not identical. It would be easy to assume that antennas that are not working in the same frequency bands will not couple to each other, and we would be very wrong. To make matters even worse, because of the physical smaller size, one would typically be tempted to space them closer to each other. Typically, antennas are designed to work well over a specific target bandwidth.  Often the performance of the antenna at other frequencies is considered not to be important but some antennas could have unintended resonant frequencies way outside of their specified bands of operation. For antennas like LPDAs (and many other wideband antennas), the performance outside of the specified bands of operation may roll off relatively slowly. When two LPDAs of different bands are mounted near each other, there may still be significant coupling between them, especially if they have partly overlapping frequency bands. To illustrate the potential pitfall, consider the scenario where an LPDA-A0102 (100-500MHz) is mounted close (2m) to another LPDA designed to work from 200-1000MHz.
Figure 5: An LPDA (200-1000MHz) antenna mounted 2m above an LPDA-A0102 (100-500MHz).
It can be seen that there is significant coupling between the two antennas from 500-750MHz, which is well outside of the frequency band where the LPDA-A0102 is designed to work.  The same is true for the frequencies below 200MHz, which is below the designed operating frequency of the smaller LPDA. Thus, just because two antennas do not have overlapping frequencies of operation, do not assume that the coupling will be negligible.

If you must know…

In some scenarios it might not be good enough to simply “do your best”, and a relatively accurate estimate of the isolation between antennas on a structure may be required.  Unfortunately this cannot easily be done on the back of an envelope but there are ways of getting a better estimate.

…measure the isolation

There is simply no better way of knowing than doing an actual measurement. One should still be careful in setting up the measurement as it is quite easy to get wrong answers if one is not careful. It is important to recreate the final setup as closely as possible and remove as many objects that may cause reflections (thereby falsely reducing isolation) or direct paths between the antennas. In many scenarios, measurements may not be practical or possible.

…simulate the system

With the advancement in computing power over the past decades it has become possible to simulate relatively complex and sophisticated systems involving multiple antennas and structures. Such simulation studies are extremely useful to point out unexpected problems and allow System Engineers to rapidly explore multiple configuration options while getting relatively accurate, or at least indicative isolation values for the various combinations. All this can be done without the investment in hardware and equipment that comes with measurements. But, as is the case with measurements, simulation studies have their own potential pitfalls and it is important to have a good understanding of the potential inaccuracies that may arise from assuming a ‘perfect’ world on a computer so as to avoid disappointment with the results. As is often the case, a combination of simulation-based studies and measurements are generally the best approach.
Figure 7: Simulations offer a relatively inexpensive option to investigate the coupling between antennas that are co-located on structures like vehicles or masts. An added advantage is that the effect of the structure itself can also be investigated.

Conclusion

When considering the placement of multiple antennas on a mast or some other structure, it is important to understand the principles that contribute to the isolation between various antennas. While physical spacing between antennas is often the first and obvious parameter that is considered, it is by no means the only one, or even most significant contributor to the isolation between two antennas. Understanding the underlying principles will allow System Engineers to make educated decisions on how to co-locate antennas on a structure. To view the YouTube video, click here