In my last post we discussed comparing RF power between 2 transmitters. In that comparison we used a logarithmic function called the decibel (dB) to compare two absolute power values. When we compare two absolute values one is considered the source of interest and one is called the reference or the source of comparison. This works simply enough if we just want to compare transmitter A to transmitter B, but what if we need to see the bigger picture? When working with wireless networks we always have a transmitter and a receiver, as well as the path in between. To ensure that the receiver has enough signal strength we could just compare the absolute transmit power to the absolute receive power to find the total loss. However if we need to make a change to this signal path we may want to change transmit power, or we may want to make a change to the path in between. To really see the big picture we need to know what the change is at each point on the signal path.
To find the change at each point along the signal path it would be easiest to compare them all against a common reference so that the values could be added or subtracted along the path. In wireless networks the reference power level is usually 1mW which is where we get dBm (dB-milliwatt). Instead of comparing the absolute power of each end of the path, we compare each end to a common value so that we can see the change which is occurring.
To expound on this further, we’ve always discussed transmitter and receiver as though they were a single unit. However in reality we know that there exists the transmitter, the antenna, and usually the antenna cable. When we connect an antenna to a transmitter it provides a certain amount of gain, or amplification. However when we remove the antenna it does not generate any power by itself. That is to say that the antenna itself does not generate any amount of absolute power, and therefore we cannot measure its gain in milliwatts or dBm.
Since we can’t compare millitwatts for an antenna what can we compare it to? We could compare it to another antenna, and to keep everything equal we should always use the same antenna as a comparison. This is where the isotropic antenna was created, or at least the idea of it. The isotropic antenna does not actually exist because it is the perfect antenna. It is a tiny point which sends RF power out equally in all directions. It is basically a concept which can be worked out mathematically. So when we compare our actual antenna to this ideal isotropic antenna we get a value in decibel-isotropic (dBi).
An important concept to note here is Effective Isotropic Radiated Power (EIRP). EIRP is the combination of the transmitter absolute power value, the loss from the antenna cable, and the gain from the antenna. This is the first point where the dBm makes itself useful. When measuring EIRP we are not comparing two transmitters, we want to add up the gain and loss for the components of a single transmitter so we have to rely on a common reference point. We measure the transmitter power in dBm, the antenna gain in dBi, and the antenna cable manufactures give us the cable loss in dB per foot of cable. We can then start with the transmitter’s dBm value, subtract the dB loss of the cable, and add the dBi value of the antenna gain to give us the EIRP. The EIRP value is important because it is regulated by the government bodies such as the FCC.
We can’t leave well enough alone, can we? The dBi was a comparison of our actual antenna to the conceptualized isotropic antenna. However sometimes antenna gain is measured against a real antenna. This value is called the decibel-dipole (dBd) since the comparison is between our actual antenna and a simple dipole antenna. The dipole antenna has a gain itself of 2.14 dBi. So when we describe our antenna gain in dBd we are comparing it to the dipole antenna. However we can’t add up the EIRP values using dBd because we have to take into consideration the gain of the dipole antenna it was originally compared against.
In other words, when calculating EIRP or the total signal path, we always have to convert dBd to dBi (by adding 2.14 dBi).
All of these measurements can be confusing, but if you take the time to slow down and consider what you are comparing then it will begin to make sense. And remember when we are adding things up we need to make sure that we are adding values that are all compared to the same reference value. In other words, we can only add/subtract with dB, dBm, and dBi. If we introduce dBd we have to take into account it’s own gain.