Exploring Antennas With Optenni Lab

Posted by Olli Pekonen | 28 November 2023

Introduction

With the blog article of September 2023, we touched the main design philosophy of Optenni Lab, where the user may Explore the antenna, Accelerate the design of antenna matching, and Maximise the impact of matching.

In this blog article, let me focus on the first aspect of this philosophy, Explore.

Before matching circuits are synthesized and time is spent to gain adequate performance, it is very beneficial to get an overview of the theoretical limits of the antenna’s radiator. If the radiator has very little chance of surviving the communication task at hand, it is better to go back to the drawing board and try to redesign the radiator. After all, if a failure is to happen in engineering, it always pays off to fail fast and in early stages of the design, and then move on.

Optenni Lab Preassessment tools

There are three main preassessment tools in Optenni Lab: Bandwidth potential, electromagnetic isolation and total scan pattern.

Electromagnetic isolation can reveal that the isolation between two antenna ports is, in fact, caused by poor matching. If we take away the effect of poor matching, it may become evident that the two antenna ports are quite strongly coupled. This can become a problem for antenna systems with many radiators.

Total scan pattern is an important feature in assessing the ability of array antennas to direct radiation over the solid angle enclosing the array.

Preassessment Workhorse – The Bandwidth Potential

Bandwidth potential is likely the most frequently used preassessment tool in Optenni Lab. It gives a quick overview on the following: What is the width of the frequency band at each of the frequencies available in the antenna impedance data (Touchstone file) if we wish to reach a certain matching level (using an ideal two-port matching circuit).

A typical bandwidth potential graph is given in Figure 1 below. It shows that the antenna in question will be difficult to match to -10 dB return loss level at frequencies below 1.3 GHz. At approximately 1.8 GHz a massive bandwidth of 700 MHz is easily achievable. Again, at around 3.6 GHz a broadband matching of more than 100 MHz (twice the indicated potential of 50 MHz for a two-port matching) will likely be hard. And finally, around 4.4 GHz, a 700 MHz bandwidth will again be a simple task.

It is very typical that the bandwidth potential tapers out at both edges of the frequency spectrum, as the number of frequency points runs out (the closer we get to the start or end of the frequency data, the more narrow the bandwidth potential becomes out of necessity). Also at low frequencies, antennas tend to be short in terms of wavelength, and thus the ability of the antenna to radiate is quite limited, dropping the bandwidth potential further.

With single port antennas, the usage of bandwidth potential computation is very straightforward. Just go to the Assessment menu and choose Bandwidth Potential, set the matching level, and off you go, as in Figure 1.

Figure 1: Bandwidth potential reveals the performance of the bare antenna in terms of bandwidths

The Bandwidth Potential of Multiport Antennas

When the antenna is a multiport antenna, for example an aperture tuned two-port structure, things get a lot more interesting!

Consider the basic building blocks of an aperture tuned antenna with a feeding port 1 and tuning port 2 as shown in Figure 2. Green portion marks the meal strips of the radiator, and orange portions are the conducting ground plane. Tuning port 2 has a switch element with various load impedances (inductors or capacitors), that can be dynamically connected to the second port 2.

Figure 2: Basic building blocks of an aperture tuned antenna

Let’s also assume that the antenna is to operate at three distinct frequency bands, and let’s call them

“low band” at 800 – 900 MHz,
“mid band” at 1700 – 1800 MHz, and
and “high band” at 2500 – 2700 MHz,

so that for each of the bands, the switch is at one of its throw positions.

Now the basic design challenge is: What kind of static matching circuit is the best for port 1, and what the three tuning components should be in port 2?

But before we give this task for Optenni Lab to be solved, we can look into the bandwidth potential of the antenna so that we try different tuning port loads at port 2, and see how things look like from port 1.

In version 6.0 of Optenni Lab, studying tuning port loads together with bandwidth potential is really quite simple. In the past, the user had to export one-port S-parameter file and open it in an “auxiliary project” to generate the bandwidth potential graph. Not anymore! As shown in Figure 3, simply first terminate the port 2 with a load you wish to study (say, a 1 nH inductor).

Figure 3: Aperture tuned antenna subjected to 1 nH tuning port load

Then, choose Additional plots… from the plot selection panel,

Figure 4: Bandwidth potential is now accessible through Additional Plots

Add Bandwidth potential and Radiation efficiency as plot types.

Figure 5: Bandwidth potential and efficiency can be turned on or off

Finally, set the bandwidth potential parameters (matching level and symmetry).

At the same time, it is also beneficial to study the radiation efficiency of the radiator. As the tuner port load changes the currents of the radiator, the tuner port changes also the radiation efficiency. This aspect is easily overlooked when designing tunable antennas.

Figures 6 and 7 show typical results for the resulting bandwidth potential and for radiation efficiency, respectively.

Figure 6: Bandwidth potential with a 1 nH load in the tuning port 2

Figure 7: Radiation efficiency with a 1 nH load in the tuning port 2

As an additional plot, bandwidth potential computation is a game changing feature in Optenni Lab 6.0! It makes the generation of bandwidth potential chart (where several bandwidth potential curves are stacked) almost trivial. Simply tune the load at port 2 to get a selection of bandwidth potentials related to various inductance loads. You may then go also through some capacitor values, and short and open circuit options in port 2. Then, push all these results to a User Plot, and you easily get the following kind of chart:

At the same time, it is very easy to generate the following radiation efficiency chart, almost as a by-product.

Figure 8: Bandwidth potential chart

Figure 9: Radiation efficiency chart

Interpreting Bandwidth Potential and Radiation Efficiency Charts

Now let’s see what these two charts reveal for us.

For high band at 2500 – 2700 MHz, the choice of tuning component is clear: The best bandwidth potential is achieved with a short, and as the radiation efficiency is more or less independent of the tuner loading (as the curves are stacked on top of each other around 2500 – 2700 MHz), we can safely assume that the switch should couple to a short when the antenna is operating in the high band.

In the middle band at 1700 – 1800 MHz, both the bandwidth potential and radiation efficiency are at their highest values when there is a small, 1 – 2 nH inductor in the tuning port. Thus, we can let Optenni Lab optimize the inductor value, but there appears to be no need to consider also a capacitor in the tuning port at the middle band.

For the low band at 800 – 900 MHz, the radiation efficiency is again quite good with small inductors, but the bandwidth potential is really poor (more or less zero) for these loads. However, there seems to be a compromise based on a large inductor (10 nH) value, which implies a large impedance. Such a large impedance can also be a small capacitance. Thus, it is likely best that this tuning component is left for Optenni Lab to choose, based on the versatile Generic Reactance component.

Figure 10: Tuner optimization setup (RF1 – low band throw port, RF2 – mid band throw port, RF3 – high band throw port)

Benefits of this approach are clear:

For the high band, nothing needs to be synthesized or even optimized as there is simply a short in this port,
For the middle band, we just need to optimize for inductors, not for capacitors at all.

Ability to omit these two synthesis steps saves time considerably.

Conclusion

Taking a critical look into the radiator before any matching is attempted is usually very beneficial, especially for tunable antennas where the synthesis times can be long. With the bandwidth potential and radiation efficiency plots, the secrets of a tunable radiator become a lot more visible even when the tuning port load is varied.

If both bandwidth potential and radiation efficiency are low for any tuning port load and at any one of the frequency bands, redesign of the radiator of the tunable antenna should be considered. On the other hand, if any of the tuning components seems to indicate a good performance in both bandwidth potential and radiation efficiency, we can likely choose that component as a starting point for synthesis, and sometimes even set its value (especially for short or open circuits) without doing any synthesis or optimization. This speeds up the synthesis task considerably and gives the antenna designer a firm handle on the potential performance of the tuned antenna radiator, even before any matching is attempted.

Olli Pekonen
Sales Director
Email: olli.pekonen (at) optenni.com

Modeling the layout of RF switches

Posted by Sergei Kosulnikov | 31 October 2023

Introduction

It is well known that compact wireless devices require small antennas whose size is dictated mostly by mechanical, design and aesthetic aspects, and not so much on the efficiency of radiation. Therefore, such small antennas are often made tunable to cater for multiband requirements. To make an antenna tunable, a switch is often incorporated to the antenna radiator. Such a device is also called an aperture-tuned antenna.

Although a typical RF switch is a small component, its layout can have an effect on the operation of the wireless system, especially at higher frequencies. If layout effects (e.g. connecting traces) are not accounted for, simulation results do not always match well with reality. As layout effects introduce complicated interconnections, a combined study of an antenna, a switch and an element representing the layout has been challenging, even with previous versions of Optenni Lab. But not anymore with Optenni Lab version 6.0, introduced in fall 2023!

Optenni Lab’s New Schematic Entry Feature

With Optenni Lab 6.0 and its Schematic Entry mode we can approach the switch+layout design problem with (at least) three methods, named as follows:

Method 1 “No layout”: Antenna and switch are separate blocks wired together, and the layout aspects are simply not accounted for.
Method 2 “Separated”: Layout aspects are separated into a separate layout representation which is wired to the antenna and switch. In other words, antenna, switch and switch layout model are separate elements.
Method 3 “Combined”: Antenna and switch layout are represented in the same element, wired to the switch.

Method 1 – No layout

In this case antenna is assumed to have Port 1 as input feeding port with a static matching circuit (to be synthesized), and Port 2 (2-2’) as a component port. The switch is placed in the component port and has no layout information. The antenna is represented as a 2-port impedance data block and the corresponding efficiency and radiation pattern data for each port, coming from a 3D EM antenna simulation. This model simply neglects the effect of the layout of the switch. In other words, the model assumes that the switch and its interconnections have a zero or negligible electrical length (size). If this is not true, this model runs into difficulties.

Method 2 – Separated

An example of Method 2 with a separated antenna, switch and a switch layout model is given in the following.

Antenna is connected similarly to the switch as in Method 1. The switch throw ports RF1-RF3 are connected to a layout element simulated in a 3D EM model. The layout element connects also the tuning components to be chosen and optimized by Generic Reactances. In this case, the antenna block is aware of radiation aspects (e.g. radiation pattern, efficiency), whereas the layout element is not – only the six-port impedance data of the layout block is used. Thus, this approach is applicable if it is assumed that the radiation and coupling of the switch layout model to the antenna can be neglected.

Method 3 – Combined

Here both the antenna and the switch layout are combined in a single antenna element in a 3D EM simulation. Thus, the switch throw ports RF1-RF3 are directly connected to the antenna block which is also connected to the optimized tuning components.

In this case, the antenna element captures the “full” electromagnetic behavior of both the antenna radiator and the layout. It takes into account any possible couplings between the antenna the layout, and also accounts for any losses due to radiation from the layout’s interconnections.

However, this method is most challenging in terms of the 3D EM analysis, e.g. involving possible accumulated error due to a very large number of mesh elements and very detailed discretization in details of both antenna’s and layout’s surroundings etc. Therefore, it is not certain that this method yields the most accurate solution with 3D EM simulations.

What is the Effect of the Layout?

We synthesized a matching circuit for 3 bands using Method 1. In the following, let’s compare the results of Method 3 with the results obtained when synthesized component values of Method 1 are inserted into Method 3.

The plots show the comparison between Method 1 (solid lines) vs. the same component values implemented with Method 3 (dashed lines). This comparison brings out the effects of the layout in the synthesis. With the obtained efficiency / S₁₁ results, it is evident that the layout has a major impact: with no layout data, and assuming that the Model 3 represents reality, a realized device based purely on Model 1 data would perform -3.9 dB – (-0.1dB) = -3.8dB worse than predicted. This highlights the value of layout modeling with electrically large elements like switches. As expected, in lower frequencies the effect of layout is not drastic, but at higher frequencies it is!

What is the Effect of Separating Layout from the Antenna?

In this case, we synthesized a matching circuit for 3 bands but using Method 2. Let’s bring the best matching topology from Method 2 as fixed values to the full simulation (Method 3) and compare.

In general, the agreement between Method 2 results (when applied to Model 3) and Model 3 (the full model) is quite good. However, the small shift of the resonance at the highest band indicates that mutual coupling between the layout and antenna physical model may be worth considering. Thus, “to separate or not to separate” the layout from the antenna model needs careful consideration. As usual in engineering, it is ultimately a tradeoff between overall model accuracy and analysis performance.

Summary of the Methods

The teachings of this study can be summarized as follows:

Method 1 (no layout consideration with a simple antenna model):

Has the fastest synthesis.
Not well suited for realistic implementation (at least in frequencies above 1 GHz).
May be used to define an initial matching circuit topology, more accurate methods (2 and/or 3) can continue to optimize the components from this topology.

Method 2 (separate 3D EM simulation of the antenna and the switch layout):

Works as a compromise between high accuracy and good performance. Two simple 3D EM simulations are usually a lot faster than one big 3D EM simulation.
The feasibility depends on the EM coupling of the antenna to the layout structure (pads, interconnecting traces) of the switch. With low coupling, this is a very good method.
The method assumes that the radiation from the switch layout is negligible, and that the layout does not couple electromagnetically to the overall antenna.
Only the S parameter information of the layout is used which speeds up the synthesis task.

Method 3 (simultaneous EM simulation of the antenna and the switch layout):

Most challenging in terms of 3D EM computation.
Takes into account the radiation characteristics of the switch layout and the coupling of the layout to the antenna structure.

Conclusions

Layout effects of switches and tuners may be crucial for accuracy, especially in higher frequencies. The recently introduced Schematic Entry feature of Optenni Lab version 6.0 enables the incorporation of the layout effects accurately and easily with the matching circuit synthesis.

The separation of the EM model into antenna and layout parts speeds up both the 3D EM simulations and the matching circuit synthesis. If the antenna and the layout interact, the accuracy of the method based on such a separation suffers.

As always, modeling is a tradeoff between the desired computation performance and computation accuracy. Optenni Lab version 6.0 offers various ways to easily obtain a good balance between the computation accuracy and time in demanding antenna design and matching tasks.

Sergei Kosulnikov
Field Application Engineer
Email: sergei.kosulnikov (at) optenni.com

Explore – Accelerate – Maximize

Posted by Jussi Rahola | 20 September 2023

With this article I would like to welcome you to the new Optenni blog series. In this blog we will be discussing various matters related to RF circuit synthesis, impedance matching, antenna arrays and our Optenni Lab circuit synthesis software for antenna and RF optimization.

When re-designing the pages, we also re-thought the key messages about our software. The key benefits of Optenni Lab can be condensed in the three verbs: explore, accelerate and maximize.

Using these verbs, let me now explain in more detail what Optenni Lab can offer you.

You may also have noticed a new look and feel for our web pages at www.optenni.com.

Explore theoretical limits:

Optenni Lab has several innovative assessment tools which help to reveal the theoretical upper limits for the wireless performance of your antenna or RF system. If your system requirements are not met with the theoretical upper limits revealed by Optenni Lab, you should likely redesign your antenna radiator structure, and not spend time in vain trying to improve the matching.

The main assessment tools are as follows:

Bandwidth potential: using the impedance of an unmatched antenna, the bandwidth potential calculation quickly reveals the obtainable bandwidth of the antenna through a matching circuit as a function of frequency. Thus, you can quickly determine if your antenna can provide the requested impedance bandwidth after it has been matched.
Electromagnetic isolation: when working with antenna systems, you may initially think that there is a good isolation between two antenna ports. However, most of the isolation may come from impedance mismatch. The concept of electromagnetic isolation removes the effect of impedance mismatch from the calculation and determines the worst-case isolation for each frequency (when the two antennas are simultaneously matched). In converse, for linear amplifiers, the electromagnetic isolation calculation gives the best possible transducer power gain.
Total scan pattern: when working with antenna arrays and beam steering, the beam can only be steered to a certain amount away from the broadside direction before the gain begins to drop and grating lobes start to appear. Optenni Lab implements the total scan pattern calculation, which reveals the maximum gain of the antenna array to any direction. Optenni Lab can also combine the total scan patterns from multiple arrays to determine if they together can provide a more uniform angular coverage of the beams.

Accelerate your design flow:

Accelerating the RF circuit design flow is at the heart of Optenni Lab. Instead of placing and wiring matching components manually, you can leverage the powerful circuit synthesis capabilities of Optenni Lab. After reading in your impedance and radiation data, just specify the desired operation bands and component types, and within seconds Optenni Lab can provide multiple optimized matching circuit topologies. Optenni Lab can optimize broadband and multiband matching circuits also for tightly coupled antenna systems.

The real power of Optenni Lab’s synthesis and optimization algorithms is revealed when tunable and switchable matching circuits are optimized. With Optenni Lab, frequency bands can be grouped into frequency configurations, where each configuration contains the frequency bands which need to be covered by a single switch or tuner state. At the same time, the input port of the system can have a common matching circuit that is to be identical to all states of the system. The optimization of such circuits is a complex task as the total performance depends on the circuit topology, component values and switch and tuner states.

A key part of the acceleration of the design flow is the capability of Optenni Lab to link with leading electromagnetic simulators and measurement equipment. Together with the EM tool vendors we have developed automated links so that simulated S parameters and radiation patterns can be sent from the EM tool to Optenni Lab with a few clicks of the mouse. And in some cases we can send the circuit back to the circuit simulators embedded in the EM tools for further co-simulation.

We also provide links to leading network analyzer models so that you can quickly get the measured data into Optenni Lab and synthesize matching circuits in real time based on the measurements.

Maximize wireless performance:

In impedance matching, the design goal is not to get the best possible impedance match (which can be obtained with sufficiently lossy components), but to transfer maximum amount of power from the antenna or to a load. Optenni Lab’s optimization setup guides you to the maximization of efficiency or maximization of transferred power.

In the optimization we consider all the loss mechanisms in the circuit: impedance mismatch, component losses, coupling to other ports and finite radiation efficiency. Optenni Lab has a wide range of optimization targets for different purposes: passband and stopband efficiency targets, targets to improve the isolation or to move the impedance to a desired location.

For antenna arrays, Optenni Lab is also able to optimize the beamforming coefficients in conjunction with circuit values. The software takes the radiation patterns of each individual element and the coupling between the elements fully into account based on your original simulated or measured data. It is easy to steer the beam to a given direction, control the side lobe levels, nulls and the active reflection coefficient of the array.

More to come:

Stay tuned for additional blog articles from us. We have a lot to tell, as we are very close to releasing Optenni Lab 6.0, our biggest update yet, containing multiple important improvements in circuit synthesis, optimization speed, ease of use and applicability to new problems in RF design.

CEO Jussi Rahola
Email: jussi.rahola (at) optenni.com

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In a matter of seconds, Optenni Lab produces multiple optimized matching circuits for broadband, multiband or tunable matching problems. Optenni Lab can also synthesize and optimize multiple matching circuits for coupled multiantenna systems.

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Optenni Lab can synthesize matching circuit for challenging coupled multiantenna systems, where the matching of all antenna ports must be optimized simultaneously.

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Optenni Lab can display radiation patterns in various formats: Cartesian and polar cuts, two-dimensional pseudocolor plots and 3D plots.

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For tunable antenna design, Optenni Lab uses multiple frequency configurations. Each frequency configuration has its own optimization targets and corresponds to different states of the switches and tuners of the circuit.

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Optenni Lab array module can quickly show the array performance using canonical solutions, such as progressive phase shifts and closed-form amplitude tapering.

Category Archives: Blog

Exploring Antennas With Optenni Lab