Technical Tips

Fundamental forces

There are 4 fundamental forces that have been identified. In our present Universe they have rather different properties.

The strong interaction is very strong, but very short-ranged. It acts only over ranges of order 10～13 centimeters and is responsible for holding the nuclei of atoms together. It is basically attractive, but can be effectively repulsive in some circumstances.

The electromagnetic force causes electric and magnetic effects such as the repulsion between like electrical charges or the interaction of bar magnets. It is long-ranged, but much weaker than the strong force. It can be attractive or repulsive, and acts only between pieces of matter carrying electrical charge.

The weak force is responsible for radioactive decay and neutrino interactions. It has a very short range and, as its name indicates, it is very weak.

The gravitational force is weak, but very long ranged. Furthermore, it is always attractive, and acts between any two pieces of matter in the Universe since mass is its source.

dBi

dB (isotropic) — the forward gain of an antenna compared to a fictitious isotropic antenna, which uniformly distributes energy in all directions.

The decibel (dB) is a logarithmic unit that expresses the magnitude of a physical quantity relative to a specified or implied reference level. Its logarithmic nature allows very large or very small ratios to be represented by a convenient number, in a similar manner to scientific notation. Decibels are useful for a wide variety of measurements in acoustics, physics, electronics and other disciplines.

The idea of decibel is to linearize a physical value which is exponential but perceived as linear (in fact as a logarithm of the original) by human. This concerns a lot of common stuff, such as intensity of light, level of noise, frequency of sound.

A decibel is one tenth of a bel (B). Devised by engineers of the Bell Telephone Laboratory to quantify the reduction in audio level over a 1 mile (approximately 1.6 km) length of standard telephone cable, the bel was originally called the transmission unit or TU, but was renamed in 1923 or 1924 in honor of the Bell System's founder and telecommunications pioneer Alexander Graham Bell. In many situations, however, the bel proved inconveniently large, so the decibel has become more common.