Ru Bee - Environmental Factors

Environmental Factors

RF is based on physics, and can be reliably modeled with prediction tools and tuned models (see RF Microwaves, and Migraines, Electro Magnetics Explained). RF is not always predictable because the active environment (people, steel shelves, floors, cabinets, doors) are all part of the same tuned circuit, and change with time. For example, a cell phone call to a phone in a building is modified by steel in the building. Reception may be improved by moving the phone near a window, or pointing the antenna in a particular direction. Radio waves are affected by just about everything around us. Many environmental factors influence performance. The more significant ones include steel and water, but people and electrical noise sources are also high on the list.

Magnetic waves can pass through almost anything, even rock. That same rock blocks RF after only a few feet. An RF signal falls off as 1/r, whereas the strength of a magnetic wave falls off far faster at the rate of 1/r³. This means that the magnetic signal will not travel nearly as far as the RF signal.

At first glance this difference in fall-off rate may appear as a negative for the range of a tag using magnetic signaling, but, as explained below, it turns out to be quite a plus in a local visibility network. Secondly, an unexpected advantage is that the noise RuBee sees is also magnetic, so it too falls off 1/r³. Noise and interference sources must be much more local to have significant strength, and tend to be easy to locate and minimize in an IEEE 1902.1 network.

RuBee is 99.99% magnetic waves it therefore is not affected at all by people or animals, mud or water. Steel can alter performance, but steel can actually enhance a magnetic signal. A high frequency (over 1 MHz) RF antenna on or near a steel shelf has three problems: 1. The steel detunes the antenna; 2. RF nulls will appear on the shelf with no signal at all (Swiss cheese field) this is because steel blocks radio waves; and 3. Steel also reflects the radio waves (E in Maxwell's equations) contributing to communication errors and shelf nulls.

In contrast Long Wavelength magnetic transmissions (below 1 MHz) is not blocked or reflected by steel so nulls do not occur. The loop antennas may be detuned by the steel, just like higher frequencies. But, unlike higher frequencies, magnetic loop antennas may be re-tuned with external capacitors, and, in many cases, circuits can be created that dynamically pick the optimal external capacitor for the antenna. Thus the de-tuning issue can vanish in a RuBee network. But the tuning has to be set to the right frequency by adjusting the capacitor to match the tuning curve with steel in place.

Parasitic inductance and capacitance (see Self-resonant frequency) of the antenna wire and the shelf steel limit the range of tuned frequency of any antenna circuit. A simple loop of speaker wire about 100 ft (30 m) in diameter maybe tuned to resonate at 131 kHz with a simple external capacitor. A loop of only a 1 inch (25 mm) may be also tuned to resonate at 131 kHz. At 30 MHz, however, you might be able to tune the 1 inch (25 mm) antenna, but not the 100 ft (30 m) antenna, and not the shelf.At 30 MHz the largest tunable loop is about 1-foot (30 cm). RuBee's frequency is low on purpose so that it can nearly always re-tune to compensate for the parasitic inductance and capacitance despite use in harsh environments like steel shelves (see Roche et al. 2007). Back to the shelf example—the RuBee installation actually tunes the steel in the shelf, and the shelf itself becomes the antenna - the shelf becomes part of the resonate circuit and the H signal gets stronger near the shelf. For frequencies over 1 MHz it's not possible to incorporate most things you find in a warehouse, office building or factory as part of the antenna.

RuBee works well in harsh environments because most steel items resonate well at the RuBee frequency of 131 kHz. As the frequency goes up over 1 MHz fewer steel items resonate. At a frequency of 10 MHz for example, nothing large made of steel can be tuned to resonance.

How big can a RuBee loop antenna be? As the antennas get larger and larger noise becomes the gate keeper. A 100 ft (30 m) diameter loop can detect lighting storms hundreds of miles away. The biggest source of noise is deep space kilometric noise. While it is possible build a second antenna and do differential subtraction, a 10,000 sq ft (1,000 m2) limit of a RuBee network is adequate for most practical visibility applications. RuBee antennas may also use ferrite rods with wire coils and get same range but without large loops.

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