The active impedance in railways system should have a high sensitivity and high efficiency. The more reactive power it has, the more efficient the system. Reactive power in railways is measured in Watts per square meter of travel.
Active impedance should have the lowest RMS power, when the frequency of the energy flow through the power cable is zero. In railways, one should take the lowest reactive power on the same ground to derive the electric current to its maximum. When power is being drawn from an object, the resultant current is proportional to the voltage at the object. Therefore, one should use the highest reactive power if there is a higher voltage of the object.
Receiving impedance in railways has the characteristic of increasing as a function of the distance between the source and the antenna, and decreasing as a function of the distance from the antenna. The amplitude is proportional to the square of the distance from the antenna. The effect of distance on impedance is known as inverse relationship.
There are also two types of impedance in railways; the active impedance and the passive impedance. The difference between the two types is that the latter can be matched to any other.
The active impedance is the one with minimum reactance to a periodic source, which leads to a lower consumption of energy than the passive impedance. It is also the one used in the low-noise industry. The reactive power is also known as its capacity.
Active and passive impedance is very different and cannot be directly compared in terms of their efficiency. Each one of them has its own requirements.
Reactive power in railways is measured in Watts per square meter (W/m2) of a ground surface, whereas the reactive power in the air is measured in power rating (ohms). The problem with reactive power in railways is that the measurements are qualitative, and qualitative measurements do not give information on the overall efficiency.
An air with low reactive power may show a very high power rating of its ground (or air), depending on the measuring instrument. So, it is better to get the actual reactive power in the ground than just getting the reactive power in the air. The only thing to remember is that the reactive power must match the railway power.
When using an air as source, the overall reactive power is very low, as compared to the active impedance in railways, where the power is generally high. So, the reactive power is the product of the receiver impedance in railways and the reactive power in the source, and it has a very low standard deviation, as compared to the total reactive power in railways.
The effectiveness of the feeder is also a factor in determining the type of active impedance. In railways, the feeder will have to be designed to ensure that it has a low reactive power. But, in a railway the impedance is so low that there is no need for it to have any reactance.
The reactive power in railways is generally low, but the rail company will ensure that the impedance and reactance are high enough to minimize the reactive power. This is done by using the best conductor, in order to minimize the reactive power at all times.