In the previous article, I explained the function of an antenna and the criteria which must be considered when choosing the right antenna. Today we are going to construct an imaginary roof antenna for radio reception and I will go into more detail why antennas must have a certain length. I will deliberately leave out electrical engineering details such as wave impedance, base feed and reactance, etc. in order to present the topic as clearly as possible.
Antennas are nowadays installed in a wide variety of places in vehicles: in windscreens, backlights or sidelights, but also on the roof and in the bodywork. The most common antennas in the automotive sector are both dipole and monopole/rod antennas (also known as ground planes).
Today, my example is the radio antenna for the FM radio range. Let's take a step-by-step look at the three criteria from the previous article:
1) Frequency
2) Polarization
3) Area of application
1) Frequency
FM stands Frequency Modulation and is used for ultra-short wave. This covers a frequency range of 30 to 300 MHz. In Germany, the 87.5 to 108 MHz frequency range is reserved for radio broadcasting. For example, in the Ruhrgebiet (area in Germany) the radio station 1LIVE is on 106.7 MHz. The middle of the frequency band is therefore around 98 MHz.
Antennas are usually made for the entire FM range, so that 30 to 300 MHz are assumed as the basis with priority given to the lower frequency range. In simple terms, the average frequency here is 150 MHz.
2) Polarisation
Radio waves are mainly emitted vertically. In an urban environment, reception can be affected by tall buildings between the transmitter and receiver. Reflections can also guide FM waves deep into tunnels, which is beneficial for reception.
3) Area of application
The antenna is to be used for the car radio. It must match the design of the vehicle and at the same time it must provide solid reception. This antenna doesn’t have to transmit. The standing wave (SWR) is therefore secondary, but the dimensions of the antenna must still be suitable.
Summary
Operating frequency: 150 MHz
Polarization: Vertical
Area of application: Vehicle, radio antenna
A suitable antenna shape is the monopole/rod antenna. This is installed in the middle of the car roof and connected to the vehicle body. The monopole points vertically upwards. The counterpart of the monopole is the vehicle mass, which serves as a counterweight. The rule is: the bigger the counterweight, the better.
The length of the monopole is also key. The design of the antenna works on the basis of a λ/4 resonance. Strictly speaking, the monopole must therefore be ¼ of the wavelength of 150 MHz.
Short Calculation
299999 km/s (speed of light) divided by 150 MHz corresponds to a wavelength of about 200 cm. The monopole is ¼ of the wavelength: 200 cm divided by 4 equals 50 cm. This means that the antenna should have a physical length of 50 cm.
Long roof antennas are an absolute no-go these days. Therefore, antennas are shortened electrically and/or physically. The length can be shortened using an extension coil by "winding up" the antenna at the lower end in coil form. Shortening the antenna makes it like a narrow range of frequencies, but this is not a problem for a pure receiving antenna.
Alternatively, electronics can also be installed in the preamplifier to electrically adapt the downstream antenna. In this case, there is no need to use an extension coil. A short flashback in history: Roof antennas were installed ex works with a preamplifier that was remotely fed via the radio using an internal conductor (phantom power supply).
As mentioned above, long roof antennas are no longer popular in modern vehicles due to their design. Instead, antennas are integrated into the glazing of vehicles: Windscreens, backlights and sidelights are prepared for radio reception (DAB + FM), GPS, mobile communications and the Internet thus providing a large area for the use of antennas. In the next blog article, I will go into detail about the antennas installed in automotive glass and will show you how you can easily test a backlight FM antenna.