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Considering the Car as a Mobile Antenna “Farm”

I recently walked around a local antique-auto show, for cars which are at least 25 years old. Among the many things which visually struck me as I looked around was that nearly every car had a simple whip antenna, most often mounted on the body panel near the driver’s side.

Fast forward to the present, and that whip antenna on most cars is gone. In its place, we have the beginnings of an antenna “farm”: AM broadcast-band radio (often built into the windshield, between layers), a shark fin for GPS, Wi-Fi, FM broadcast, cellular, and more, Figure 1 .

Figure 1

 It's not unusual to have multiple antennas serving different bands share a location, especially in a fixed-site installation. (Source: Pinterest)

It’s not unusual to have multiple antennas serving different bands share a location, especially in a fixed-site installation. (Source: Pinterest)

Unlike stationary antenna arrays seen at various fixed listening posts (at-home amateur radio, military base stations, and others), this antenna farm is on the move when in use, so protrusions and wind resistance is a factor, Figure 2 . This means that some sort of streamlined, conformal design is mandatory and they can’t just be sticking out, however convenient or RF friendly that may be.

Figure 2

As cars get increased wireless connectivity, they risk looking like a fixed-site installation, except that they are not, with serious consequences as vehicle speeds increase. (Source: WA8LMF site)

As cars get increased wireless connectivity, they risk looking like a fixed-site installation, except that they are not, with serious consequences as vehicle speeds increase. (Source: WA8LMF site)

As a result, auto-antenna design and placement is going to become even more like aircraft design in terms of RF demands and constraints. The situation is going to get more difficult with all the new wireless demands that car designers must accommodate, such as various cellular bands, Wi-Fi, and even 5G with its MIMO requirements, V2V (vehicle to vehicle links), radar (77 GHz, in many cases), to cite a few. Further, as cars get more “autonomous,” their RF connectivity needs will likely increase.

One option is to use a multiband antenna where possible. Of course, this approach has its own tradeoffs. A multiband antenna saves space, cabling, and electronics, but presents wideband front-end issues for preamps, and often reduced performance compared to optimized single-band antennas.

The challenge with cars is made better and worse by the sheet-metal auto body. On one side, that has the potential to be a ground-plane of sorts (not necessarily a very good one, but it’s a start), which thus allows for some whip-like, unbalanced antennas even if they don’t look like those. On the other side, the metal surfaces act as shields, distorting the RF field and blocking signals, and so providing desired directionality (omni or other) can be a real headache.

The good news is that electromagnetic-field modeling and simulation packages, especially those with a significant antenna-modeling toolset, are getting better. (Since I haven’t used any of them recently, I am certainly not going to make recommendations.) Still, while free-space simulation is difficult but doable, providing good models and tools for also accommodating the various surfaces of the vehicle, the engine block, transmission, and more – well, that’s tough.

If there were only a few other constraints on car design in terms of user expectations, appearance, mileage, wind noise, cost, and other factors, there might be easier solutions to the antenna-farm challenge, but those other constraints and demands are not going away. And just think of the repair costs associated with these antennas, RF front ends, metal and radome-like body panels, special connectors, and more – it’s going a real eye-opening and perhaps heart-pounding experience for customers!

I wouldn’t be surprised if, within a few years, cars include more RF-transparent surfaces (mini-radomes) as well as use of non-conductive conformal, streamlined body panels under which the antennas can be located. Will cars in the next decade begin to have antenna-layout diagrams like aircraft, Figure 3 ? We’re getting closer to that situation. It would be nice if standard antenna kits were developed to be used by various manufacturers, to reduce cost and simplify replacements, but I doubt that’s going to happen.

Figure 3

The modern airplane - here, a Boeing 787 - sports a carefully situated, highly optimized, surprisingly diverse set of antennas for the many RF links required. (Source: Boeing Company via MRO Network)

The modern airplane – here, a Boeing 787 – sports a carefully situated, highly optimized, surprisingly diverse set of antennas for the many RF links required. (Source: Boeing Company via MRO Network)

What’s your sense of the evolving antenna situation for cars, as more wireless links, across more bands, and with different bandwidths are added? Will the solutions be highly sub-optimum, due to all the tradeoffs that have to be made? Or will a fairly good automotive antenna farm become realistic?

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