A "rubber duck" low-profile antenna In my job, I travel a lot. And in those travels, I rent a lot of cars, which gives me a good bit of exposure to the various entertainment system offerings of domestic and foreign automakers. Most rental cars have the “standard equipment” package, meaning no premium sound, but from time to time I get lucky and find an upgrade installed in my rental.
It’s always interesting to listen to local radio as I drive around the various markets in rent cars. In most of the markets I am well acquainted with the signals, ours and our competitors’. For the most part, I know what the coverage areas should be, and I have at least some awareness of signal problem areas.
One trend that I have seen in the last couple of years is the low-profile antenna. This often takes the form of a “blade,” a “rubber duck” or “dimple” on top of the vehicle. Some of these antennas, particularly the blade style, apparently incorporate AM/FM and satellite radio in the one package.
There are, I’m sure, all kinds of reasons for the move to low-profile antennas, including aesthetics and aerodynamics, but whatever the reasons, these antennas do not — cannot — perform as well as a “regulation” antenna or even in-glass wires such as were popular in vehicles ten or more years ago. The laws of physics dictate that received field strength is directly related to antenna aperture. Reduce the aperture and you reduce the signal capturing ability of the antenna and the signal level delivered to the input of the receiver.
Because of its long wavelength, AM, more than FM, suffers from the reduction in antenna size. I would find it very interesting to pull the antenna lead out of the radio in a vehicle with a low-profile antenna and connect it to the external input of a FIM. My guess is that the signal level delivered is many dB below the incident field strength. Microvolts are likely delivered to the radio rather than millivolts, reducing the reception range of any station being tuned, likely to the detriment of the radio station.
To add insult to injury, automobile manufacturers are increasingly making use of microprocessors and other noise generators in their products. As a result, a goodly portion of the noise with which signals, particularly AM signals, must compete now comes from within the vehicle itself.
It’s the perfect storm: Reduce the signal level of the desired signal by drastically shrinking the antenna aperture, and at the same time increase the noise level in the overall environment with CFLs, microprocessors and plasma televisions as well as the local noise level within the vehicle.
A "dimple" or "abbreviated blade" low-profile antenna Back in January, I rented a 2013 Ford Explorer, a very nice vehicle that had a “glass cockpit” with touch screen controls for just about everything, Ford’s “Sync” system and premium sound. I thought I’d hit the jackpot as I was pulling out of the rental garage. But it didn’t take long for me to figure out there was a problem. As I drove around L.A., even powerhouse stations KFI and KNX weren’t noise-free, and our own 50 kW signal seemed marginal in some locations. On several occasions, in locations where signals were noisy and weak sounding, I stopped the car and got out with the FIM … and measured 5 mV/m plus signal levels, which should have been absolutely solid. At one such location, I shut the engine off and noted probably a 10 dB improvement in the signal-to-noise ratio of the demodulated signal. The car itself, presumably the engine computer, fuel injector signals and even the fuel pump motor were definitely big parts of the problem.
Just last month, I rented a Dodge Journey with the standard sound system package. It performed very well. I noted that the antenna was more in the “rubber duck” category than “blade” or “dimple,” so there was presumably more aperture and thus more signal to work with.
And then in late March, our chief engineer in L.A. was driving a Scion rental with a Pioneer premium sound system installed. That vehicle had a “dimple” antenna. Its AM performance was abysmal, with the receiver unable to maintain an HD lock on even the strongest signals, and the FM performance wasn’t much better with the HD lock in and out (and multicast signals out more than in). I’m sure the Sirius/XM reception worked just fine.
Like it or not, this is the mobile reception environment we must live in these days. As radio engineers, we are inevitably going to field complaints from managers, programmers, salespeople and listeners about “signal problems” that did not previously exist. My bet is that many of these will be directly attributable to low-profile automobile antennas and electrical noise generated within vehicles.
Short of drilling holes and mounting real antennas on these vehicles, I’m not sure what the answer is.
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Cris Alexander, CPBE, AMD, DRB, is director of engineering for Crawford Broadcasting. This article originally was written for the SBE Chapter 48 newsletter.