Filter
Top Products
96 Handbook of Intercom Systems Engineering
A brief look at the properties of the electromagnetic spectrum can tell us a lot about the RF
signals that move through it. As you can see, the name electro-magnetic is really a
combination of two words, electron (or electronic) and magnet (or magnetic). The reason
for this is that waves that propagate in the electromagnetic spectrum have two separate and
distinct components, an electrical and a magnetic. As you can see in Figure 7.1, these two
components exist at right angles to each other, as well as, to the direction of propagation.
The electrical component, or field as it is called, is represented by the letter E and the
magnetic field by the letter H. (No, I don’t know why they use H, but they do!)
Figure 7.1
The E an H fields exist in two separate planes, 90° apart from each other.
RF signals at different frequencies have different propagation characteristics and are
affected by external forces in different ways. The reason for this is the ratio of the
magnitudes of the electrical and magnetic components of an RF wave vary dramatically as
frequency changes. Generally speaking, the magnetic component of an RF wave is much
greater than the electrical component at very low frequencies. As the frequency increases,
the electrical component increases and the magnetic component decreases, until, at very
high frequencies, the electrical component is much greater than the magnetic.
This is not just “gee whiz” information. The different makeup of RF waves at different
frequencies is what allows us to use the signals for different and sometimes unusual
applications. For instance, at super low frequencies, such as 5 Hertz, where the magnetic
component is extremely dominant, the US Navy has been able to propagate RF signals
through the Earth’s core to communicate with submarines on the other side of the world.
Try that at 13 GHz! In a more pertinent example, at much higher frequencies the highly
reflective nature of the mostly electrical component wave can cause self-interference,
known as multipath. Multipath can cause an RF signal to be unusable at a very short
distance from the transmitter if not properly handled. We will discuss multipath in more
detail later in this chapter.
Now that we know what RF is, we can discuss what it does, how we can use it and how it
is affected by outside forces. In its most basic form, an RF system puts information on an
RF signal, sends it to a remote location and retrieves the information in exactly the same
form as it originally existed. Let’s take a look at this most basic system and define some
terms so we can talk about this process more easily. Refer to Figure 7.2.
DIRECTION OF
PROPAGATION
Y
Z
E
FIELD
H
FIELD
X
90°