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

Expecting the Unexpected: High Power Transmit Antennas on Vehicles

High Power Transmit Antennas on Vehicles Written by Johan Fuhri Introduction Many of the Alaris Antennas products are used in vehicle-based high power transmit systems, a typical application being the protection of convoys or VIP transport vehicles against remotely triggered improvised explosive devices (IED)s. Since the performance of the antenna will directly influence the effectivity of the high power transmit system, choosing the correct antenna and placing it in the correct position is vital.  Unfortunately, antennas tend to be heavily influenced by their environment, and vehicles are especially antenna-unfriendly installation locations. Let’s discuss a couple of issues that a Systems Engineer might unexpectedly stumble into when placing antennas on a vehicle. Dipoles and Monopoles There are two major types of antennas that are commonly used on vehicles.  The first is a dipole-type antenna that consists of two similar elements which are fed by connecting the “positive” and “negative” terminals to the two respective elements.  These antennas do not require a ground plane to work, and they are often said to be “ground-plane independent”, since the ability of the antenna to radiate energy into space is generally unaffected by the presence (or lack thereof) metallic or conducting surfaces below it.
Figure 1: The dipole antenna, on the left, consists of two symmetrical conductors attached to the radio, whereas the monopole antenna (right) has only one conductor over a ground plane that acts as a “mirror”.
The second antenna type used in high power transmit systems is the monopole antenna.  These are highly ground plane dependent, as they are effectively half a dipole with the conducting mounting surface acting like a “mirror” of sorts.  These antennas are mostly used at low frequencies where it is not practical to use full sized dipole antennas. Monopole-type antennas on a vehicle Monopole antennas, like the MONO-A0062, are often used at HF frequencies where the wavelengths are 10m or longer, making the vehicle electrically small.  By electrically small, we imply that the metallic parts and panels of the vehicle are much smaller compared to the wavelength of the signal, and they can no longer act as a proper ground plane “mirror” for the monopole antenna. While the too-small ground plane issue is problematic, most System Engineers will already be aware of this and will already consider the higher than advertised VSWR that comes with using a monopole antenna on a small ground plane.  The VSWR, however, is only one part of the equation.  Hidden beyond our ability to easily measure, lurks a potentially problematic change to the shape of the radiation pattern. Monopole antennas will typically have an omni-directional radiation pattern under ideal conditions, and a vehicle installation by no means qualifies as an ideal environment.  Below is an example of a monopole antenna that was mounted on the front corner of a large vehicle, and the radiation pattern measured.  Instead of the beautiful circular pattern you would expect to measure, there are certain directions that have gain holes as deep as -18dB.
High Power Transmit
Figure 2: A vehicle with two whip antennas (MONO-A0062) on the front corners of a vehicle being measured.  The other antennas on the roof (OMNI-A0281, OMNI-A0266) are all dipole-type antennas.
High Power Transmit
Figure 3: Measured radiation patterns of the MONO-A0062 mounted on the front corner of a large passenger vehicle.  Notice the deep gain holes.
To make things even worse, moving the antenna by even a small amount will significantly change the resulting radiation pattern.  Even mounting two antennas exactly symmetrical on two sides of the vehicle will not guarantee symmetrical radiation patterns, as it turns out that the quality of the electrical continuity between the metallic panels of the vehicle (among other things) will also influence the final radiation pattern. This leaves the System Engineer with a major problem in practical applications regarding predicting the range and, consequently, the effectivity of a high power transmit system.  Unfortunately, short of installing and measuring the performance of the system, predicting real-life performance is dubious at best for monopole antennas in the HF band. Dipole-type antennas on a vehicle Fortunately, the ability to predict performance improves drastically as the frequency of operation increases.  Above roughly 100MHz we can start making use of dipole antennas, such as the OMNI-A0266 or OMNI-A0281, which are not as dependent on the vehicle.  This does not, however, imply that the vehicle has no influence on the radiation pattern.  Below, is an example of the measured radiation pattern of an OMNI-‑A0281 antenna installed on a roof-rack of a Toyota Land-Cruiser (see Figure 2).
High Power Transmit
Figure 4: Measured radiation pattern of an OMNI-A0281 antenna on the roof-rack of a Land-Cruiser vehicle.
The radiation pattern is significantly more omni-directional than when compared to the monopole antennas discussed in the previous section, but there can still be significant variations.  At higher frequencies, the gain holes can be as deep as -12 dBi or more and change rapidly as you move around the vehicle.  It is important to note that this fast-changing pattern is purely due to the signal interacting with the edges of the roof-rack. Other physically large objects on the roof (e.g. gun turrets on military vehicles) will also have a significant influence on the radiation pattern. On a moving vehicle, a rapidly changing radiation pattern will present itself as a rapidly changing E-field at a particular position around the vehicle, for instance, the target to be jammed. The System Engineers will need to take this into account in the design of the system. On a more positive note, it was found that simulations of complete systems (antennas on the vehicle) tend to agree relatively well with real-life measurements, and simulation studies could assist in identifying major potential problems with antenna placement before building expensive prototypes. Conclusion When placing antennas on a vehicle, keep in mind that the physics and mechanics of the vehicle/antenna system could present you with nasty surprises in the shape of deep nulls and dead zones in the radiation patterns.  In many situations, a simulation study could assist in identifying potential problem areas and give insight into the performance of the system. Relevant Products Alaris Antennas offers a number of vehicle mount antennas which are suitable for use in most applications. Antennas such as the MONO-A0062, OMNI-A0266 and OMNI-A0081 are high power antennas which are typically used in counter-RCIED applications. When combined into a system, they cover the entire 20 MHz to 6000 MHz band. Alaris also offers a complete range of communications antennas and spring bases which are designed for use in the harsh vehicular environment. To view the YouTube video, click here