What are the most common challenges and pitfalls in RF engineering and antenna projects?

Especially challenging is understanding how, despite designing for a specific frequency, design can affect fields (E and H) and can generate many other (unwanted) frequency components :).

When you are dealing with Phased-Array antenna, for example in mmWave or sub-THz frequency bands, simulation can only get you so far to help you iron your design before hand. But then when the design is manufactured, you need to have a good setup to measure Over-The-Air performance. For this, lots of expertise and experience is necessary. The right instruments and calibrations are also critical.

Software simulation is great these days. The more data points about your model is given, closer you are to a working design, however, the design still needs "tweaking" when built in the real world. Understanding the operation of an VNA and what the S perimeter of the antenna should look like. Knowing how to adjusting, trim, or add material for best return loss when viewing S11. How to get closer to 50 +/- j0 ohms for an 50 ohm system while meeting job requirements for space-size and cost. And then their is the modeled antenna pattern versus measured! Yes, good stuff!

There are always two parts to antenna design: simulation and real-life testing. Modern simulation tools help us reducing the number of prototyping rounds and save time.

Nothing, can beat real-world-environmental-testing. Case-in-point, where I currently live, at the bottom of a river valley, with +300ft high, steep, hills all around me. High Gain Antennas do not work well at all, here, those being +3dBd, and higher. Low Gain Antennas are what I use for local / regional communications to RX / TX out of here, like unity-gain dipoles on HF and Low Band VHF, and a unity gain Discone for VHF High Band and UHF. Why??? Sending my single in all directions, including above and below, takes advantage of the hills via reflection and refraction. That said, multipath interference still plagues this location on VHF / UHF, and a single parasitic reflector does help in that. Know your limit(s), stay within it.

Miniaturizing antennas (Electrically Small Antennas) and maintaining performance (Functionally Small Antennas with impedance matching to 50 Ohms, wide bandwidth, good radiation properties, good antenna radiation efficiency) is another challenge.

There are distinct sets of parameters and issues to test (coverage, power, sensitivity, interference, etc.) that are often mistaken for each other. RF testing needs to be purposeful and robust enough to establish clear root-cause in a reproducable manner. Most real (not static failures) problems are intermittent, and most test equipment and procedures do not adequaltely frame such issues. A useful exception to that is found in "waterfall displays" on quality spectrum analysers, showing amplitude vs. time and frequency. We need more of these sorts of tools to evalute multivariate inputs and make sense in a repeatable manner. Sometimes minor custom coding is best done to suit particlar needs; I prefer python and Anritsu.

After prototyping, a series of tests based on a document called the design verification test (DVT) is conducted. These tests cover various aspects of the RF system, including pure RF testing, digital and DC measurements, and evaluations under different conditions, such as temperature variations, load changes, and extreme operating environments. The biggest challenge I've faced is the mass production of RF systems. This requires designing test fixtures and extensive test automation for RF, digital, and DC measurements. It is a collaborative effort involving hardware, mechanical, and software teams. The test system and measurement protocols must be robust enough to minimize production interruptions.

Properly done simulation can cut costs of prototyping phase. With good initial simulation one can get a good antenna with only one prototype round.

Most issues brought to us deal with TX/RX range or antenna performance. Improper matching values and PCB layout is usually the culprit. Simulations can help provided the correct models are used for the components. The last project I worked on had a PI filter in the antenna path which was a cause for concern. Using the spectrum analyzer, a 20dB drop was measured after the filter I created a Bode plot of the filter, using standard library components, and it showed the -3dB point to be far to the right of the center frequency. I then created new simulation parts using downloaded spice models and re-ran the Bode plot. This yielded completely different results showing the -3dB point to the left of the center frequency and matching my findings.

Dielectric and conductor material properties can often influence fabricated PCB antenna performance vs simulation. All glass reinforced lamiates are anisotropic. Depending on the test method used by laminate suppliers the published values of Dielectric Constant (Dk) will be the result of in-plane or out-of-plane measurements. For planar antenna structures Dk measured out-of-plane is required for modeling and simulation. The roughness of the foil can also affect antenna performance since it increases the effective Dk and and adds additional inductance. The combination of both will increase phase delay and thus the frequency of the antenna when fabricated.