Propagation & attenuation of radio waves

Reading time 3 minutes

Updated – May 11, 2023

The topic of WLAN, radiation exposure and shielding comes up again and again. Understanding the propagation and attenuation of radio waves also helps in dealing with radio technology. Therefore, more detailed information is provided below, but first requires the creation of a basic understanding. So, a bit of dry theory to start with.

history

In 1886, Heinrich Hertz, after whom the unit of measurement for frequency (Hz) was named, experimentally demonstrated for the first time the existence of electromagnetic (radio) waves, predicted in theory by James Clerk Maxwell in 1867.

Nikola Tesla received his first patent for wireless energy transmission, radio technology, on March 20, 1900.

Braun and Marconi are other names associated with the development of telegraphy, which, using the electromagnetic telegraph and Morse alphabet invented by Samuel Finley Breese Morse in 1837, bridged messages over distances of up to 3,600 km.

Spread

The propagation of electromagnetic waves depends on the frequency, topological and atmospheric conditions.

The ionospheric layers (average height approx. 1,000 km), which follow the near-Earth troposphere (17...17 km height) and stratosphere (8...50 km height), can reflect electromagnetic waves due to their degree of ionization, which depends on solar activity . In extreme cases, a signal that has just been sent can be received again at the same location by repeated reflection around the globe (circumference approx. 40,000 km), delayed by the transit time (300,000 km / s).

Above about 60 MHz, radio waves propagate in a similar way to light. You can be distracted by obstacles, reflected. In particular, radio waves above 100 MHz are increasingly being disrupted in their propagation by reflections. In the worst case scenario, the wave crest and trough can cancel each other out.

damping

Attenuation is the weakening of a signal. The attenuation is measured in decibels (dB), or decibel milliwatts (dBm) or, for higher powers, decibel watts (dBW) in logarithmic form in order to be able to represent larger levels in a practical manner.

It is important to distinguish between attenuation information in percent and dB, for sound db(A), “A” means adapted to the human hearing curve (taking into account the fact that high and low tones are perceived at different levels at the same volume).

One decibel usually indicates an amplification value, i.e. the value is positive. A negative sign therefore describes attenuation.
100 dB means 100,000 times the sound pressure of 0 dB or a sound pressure of 2 pa (Pascal).

noise	db(A)	Shielding (dB)	%
circular saw	90	10	90
Highway tags	80	20	99
Main street during the day	70	30	99,9
Main street at night	60	40	99,99
Side street during the day	50	50	99,999
Side street at night	40	60	99,999.9
Clock ticking	30	70	99,999.99
Rustling leaves	20	80	99,999.999
Breathing noise	10	90	99,999.999.9
silence	0	100	99,999.999.99

To get a certain idea of how high the damping capacity of a material that can be penetrated by radio waves is, compare a building ceiling <30 cm thick (70%) and a wire mesh that is less than 1 mm thin (100%), or a plasterboard. A partition wall less than 10 cm thick (10%) is provided.

Cable/connector/attenuation losses

If radio waves, for example on the way from the radio to the antenna, pass through cables, plugs, couplings, etc., attenuation losses also occur here. The type and length of the antenna cable used and the plug connections determine the amount of the total attenuation losses.

The rule of thumb is: the sum of the attenuation losses must not exceed the antenna gain. The choice of individual components must be made accordingly.

The type of cable, whether stranded or rigid conductor, shielding, sheath structure, length and frequency to be used are also criteria to be taken into account when choosing an antenna cable. The structural use is also important: halogen cables are heat-resistant and flame-retardant, slotted cables are used for laying in tunnels.

WLAN / radio link

Frequencies of 2.4 GHz, 5 and 6 GHz, and 24 and 60 GHz are intended for WLAN. If you want long ranges, through walls and ceilings, you choose the lowest frequencies, i.e. 2.4 GHz. They have a range of approximately within a single-family home. Please note with semi-detached houses: there is enough for the neighbors too. If you want high data rates over short distances, you should use 5 GHz, because it hardly penetrates the floor or ceiling. The same applies to the 6 GHz band.
24 and 60 GHz, on the other hand, allow distances of 200 to 1,000 m. Line of sight is a requirement here. Rain, haze, snowfall, etc. can lead to losses in throughput, which can partly be compensated for by technical precautions.

Radio field strength

The radio field strength of electrical installations is 1...2 mW, a radio link is around 10 mW, WLAN at 2.4 GHz is 100 mW, at 5 GHz is 200 mW, a microwave is 800 mW and a mobile phone is 2,000 mW.

In general, there are limit values for high-frequency electromagnetic fields, which are known as SAR (specific absorption rate) and describe which power consumption is tolerable by human tissue.

Averaged over the entire human body, 0.08 W/kg is called 2 W/kg based on individual areas, e.g. head (mobile phone).

The cell phone on your ear already puts the highest permissible strain on the organism.

In sensitive people, the effects of electromagnetic radiation exposure can include blood pressure dysregulation, depression, hormonal disorders, immunosuppression, difficulty concentrating, headaches, fatigue, sleep disorders, dizziness, nausea and restlessness.

Protective measures

1. Choose the location of the WLAN router as far away from the workplace as possible.
2. Use the frequency band according to actual requirements.
3. Disable WiFi when not in use.

If you want to protect yourself from external exposure to electromagnetic radiation, you can design your home in the style of a Farady cage. Here, strips of thin metal grids, copper foils, and if necessary aluminum foils are attached under wall coverings/wallpaper or special EMC wallpapers that have a metal-coated back. It is important that each strip has electrical contact with the neighboring strip, that window panes are also metal-coated and that this layer is also connected to the wall shielding.
The entire installation must be connected to the building's equipotential bonding to ensure proper grounding.

In this way, this room is safe from external electromagnetic influences; conversely, the radio traffic of a WLAN router within this room would be protected from external eavesdroppers.

The topic of electrosmog and shielding technology is discussed in this Contribution treated separately.

All work on electrical systems must only be carried out by qualified specialist personnel!