Antennas and the electromagnetic field

Question

I'm an electrical engineering undergrad and have some questions about radio antennas. We've just covered the electronics and maths of radio theory once the signal has reached the receiver, I'm unclear about how the signal actually travels and its effects.

I believe (crudely, no doubt) that as an electron is moved within the transmitting antenna it creates waves in the electric & magnetic fields which, upon interacting with the receiving antenna, creates a voltage which is ultimately demodulated back to the original signal.

Before radio was invented, I imagine the electric/magnetic fields to be calmly doing their thing and to be relatively calm (or in a naturally occurring turbulent state).

• Now that we transmit radio, TV & everything else, does this mean that the electromagnetic field is in constant turbulence, with numerous waves at different frequencies superimposed over the natural state?

• Does this also mean that everything around us (fence posts, cutlery, cars etc) is also having voltages induced in it at the broadcast frequencies? I am aware of antennas having a resonant frequency, is it that all other objects have a sufficient highly impedance as they are not at resonance that this voltage is negligible?

• Are there any tangible side-effects from this increased level of energy in the electromagnetic field since transmission began?

• As these waves in the electromagnetic field are being absorbed (by everything, not just antennas) does this mean the energy of the wave is continually being reduced, and if this is the case how can we broadcast over such distances?

Answer 1

Wow,

Your question has many aspects. First, we are surrounded by electromagnetic fields. We can divide these fields into two parts: 1) Ionizing fields (having wavelengths ~200 nm and below) and 2) Non-ionizing fields (wavelengths above ~300 nm). Ionizing radiation (such as x-rays and UV rays) can affect our body even when their energy density is small because they directly ionize the molecules in our body and break the bonds in tissues. However, in the case of non-ionizing radiation the only health effect is heating, hence if the absorbed power is below a certain level the radiation is termed as safe. The radio waves (from medium wave to microwaves) have their wavelengths in the range of kilometers to meters, hence they are categorized as non-ionizing radiation.

As you may be aware, the strength of the signal is very high near an antenna and it decreases as $1/r^2$, hence the radiation level anywhere, except very close to the antenna, is quite small. There are agencies which define the safe emission levels from our mobile phones, wi-fi connections, TV, and radio towers to keep the radiation within safe levels. The radiation strength away from the devices is about a micro-watt, if you calculate the voltage from power it will be very small.

There are several broadcasting mechanisms, as you know the TV signal comes from a satellite, it is collected by a concave reflector and received by a receiver then it is subsequently amplified and decoded and transmitted in cables. There are several other mechanisms, you may find details on the internet, but there is one common route: 1) Transmitter, 2) Repeaters, the repeaters can be just additional towers which receive signals from one tower, amplify and transmit again, or they can be in the form of wire or optical fibers through which the signal is transmitted, or one can use satellites for this purpose. 3) Receiver, in your device, and 4) Amplifier, which amplifies the weak signal and makes it usable.

No two signals should come at the same frequency in the same area, but if it happens the decoding device gets confused (known as interference), hence if you tune your device to one frequency you will receive only that frequency. The tuning bandwidth is really small hence a large number of signals (channels) can be transmitted in a relatively small band.

I hope that this will clarify some of (if not all) of your queries.