Complete Antenna Handbook

The antenna is the ear of FM DX. The weak radio waves from the antenna enter the receiver through the feeder, so that we can hear the sound of the distant radio station. The antenna accounts for half of the quality of a receiving system. We hope that the antenna can have high gain to make weak signals louder. We hope that the antenna can have a certain selectivity to block the interference of paging stations and local strong stations. We hope that the antenna feed system can minimize the loss and send every microvolt signal to the front end of the receiver.
For most people who use portable radios to listen to FM DX, their antenna may be just a telescopic antenna on the radio. Although such an antenna is simple and convenient, it is not enough for FM DX. Although thanks to the gift of the ionosphere, such an antenna system is not impossible to receive DX signals.
I will introduce some common and easy-to-make antennas. These antennas can be made with materials that are easily available in our daily lives. I will make these antennas one by one, take photos of the production process, and give as detailed size data as possible. Although I will use equipment such as antenna analyzers and even comprehensive testers in the production process, I will tell readers how to debug without using these expensive instruments. At least, it's always right to follow my materials and sizes.

Radio waves
Before talking about antennas, we must first mention radio waves.
The purpose of making antennas is to capture radio waves. Therefore, before considering the problem of antennas, it is absolutely necessary to study the problem of radio waves first.

The FM broadcast band has a frequency of 87.5MHz to 108MHz, and a corresponding wavelength of 3.4m to 2.7m, generally known as the 3m band, which is a section of VHF (Very High Frequency). Below this band, 54MHz to 87.5MHz is the TV broadcast band, and above this band, 108MHz to 136MHz is the aviation communication band. There are three main ways for VHF band radio waves to propagate:

Direct wave
This means that the radio wave reaches the receiving antenna directly without any transmission. The radio wave is like a beam of light, so some people call it line of sight propagation. The name of line of sight propagation also indicates that the distance that this propagation method can propagate is not far. There are two reasons for this. First, the energy of the radio wave decreases exponentially from the transmitting point, and the receiver needs a certain signal strength to demodulate the broadcast well. Therefore, the signal is too weak to be demodulated at a distance. If this is the only reason, then desperately increasing the transmission power or increasing the gain of the receiving antenna may expand the listening range. However, there is another important issue. The earth is round. The radio wave emitted from any point on the earth will eventually leave the earth and shoot into the sky in a straight line. It is mainly due to the second reason. Generally speaking, a straight wave emitted by a transmitter on the ground can only propagate to a receiving point on the ground 70km away. If the height of both parties increases, then this distance can be increased, but it is always limited. Therefore, 70km is the limit of local listening. In fact, due to the obstruction and reflection of mountains, hills and houses, this distance is greatly reduced. The generally estimated distance is 35km.

Ionospheric emission waves
This refers to the radio waves transmitted through the ionosphere to reach the receiver. There are many names for this. The ionosphere itself has multiple layers. The ionosphere that supports shortwave (1.8MHz to 30MHz) reflection is the F1 and F2 layers. F1 and F2 do not reflect all radio waves willingly. The highest frequency they can reflect is limited. Radio waves exceeding this frequency are not reflected at all, but pass through the ionosphere into space. Without this feature, communication satellites would not exist. Communication satellites work outside the ionosphere. This highest frequency is called MUF (Max Usable Frequency). MUF is related to many factors, mainly the activity of sunspots and the season. When sunspots are active, MUF is high, and when the weather is hot, MUF is also high. How high can MUF be? Generally, in the summer when sunspots are active, MUF is between 20MHz and 40MHz, and rarely exceeds 50MHz. When it is low, it can even be as low as 10MHz. However, when the sunspots are extremely active, the MUF may occasionally reach 100MHz. At this time, it is possible to receive DX FM through the F layer. However, this is not the main form of FM DX. FM DX mainly passes through another ionosphere, the E layer. Originally, the appearance of the E layer destroyed the F layer, so we might as well remember the F layer as the Friend layer and the E layer as the Enemy layer. However, the appearance of the Es layer will form a reflection layer with extremely high density in a short period of time. The high density of the reflection layer means that it can better reflect radio waves. Therefore, when the Es layer is opened, the signal of the DX station will be abnormally strong. Amateur stations working in the 6-meter and 10-meter amateur bands know that when the Es layer is opened, it is possible to make DX contacts with very small power, even 5W. The opening of the Es layer mainly provides a propagation path for radio waves within 800km. Because the signal is very strong, in fact, you don’t need very good equipment to receive it in many cases. What you need is patience and luck. In addition to these two reflections, FM DX may also reach your receiver through tropospheric reflections and meteor trails.

Ground waves and atmospheric waveguides
Originally, theoretically, there are no ground waves in VHF. However, countless practices have shown that VHF also has a certain degree of ground wave propagation. So we can stably receive signals from radio stations about 200km away. Amateur radio stations in Jiangsu and Anhui provinces conduct VHF mobile communication experiments throughout the province every National Day, which also proves that VHF waves can be propagated at a distance of about 200km. Atmospheric waveguides are another possible means of propagating VHF waves, but people have not studied them enough.
Since there are these possibilities, how do I know how the signal I received came from? Generally speaking, if the received signal comes from a radio station within 70km, it can basically be considered a direct wave; if it is within 200km, and the signal is stable (not necessarily strong), then it is probably a ground wave; if it is within 800km, the signal is very strong, but extremely unstable, and only appears occasionally, it is mostly Es layer propagation; if the distance is farther, the signal is very weak, probably F layer or other forms of ionosphere propagation. What is the
use of knowing these? The use is to help us choose the requirements for the antenna. For example, one characteristic of F-layer propagation is that it can reach a certain distance. Radio stations within about 500km cannot reach through the F-layer. Radio signals within this distance can only reach through the Es-layer. Just like if you want to receive FM radio signals from Taiwan in Hangzhou, you can only PNP (Plug and Pray) and wait for the Es-layer. Therefore, the antenna must be suitable for the characteristics of the Es-layer.

Another very important factor is the polarization mode, which is easily overlooked by many enthusiasts. There are three polarization modes of radio waves: horizontal polarization, vertical polarization and circular polarization. No matter how it is calculated in theory, the simple judgment method is to look at the direction of the vibrator. If the vibrator is placed horizontally, it is horizontal polarization, and if it is vertical, it is vertical polarization. Circular polarization is not used in FM broadcasting and can be ignored. The reason why the polarization mode is important is that the polarization modes of the transmitter and the receiver must be consistent in order to have a good reception effect. The polarization mode of broadcasting in my country is horizontal polarization, so the receiving antenna should also be set up horizontally. If the polarization mode is inconsistent, there will be a loss of 10dB to 20dB. However, the radio waves reflected by the ionosphere have long been reflected in a mess and upside down, and it may be a matter of polarization mode. Therefore, vertical polarization is actually not bad for receiving DX signals. An additional benefit is that it can weaken the influence of local radio stations.

Antenna characteristics

Resonance
Any antenna resonates at a certain frequency. We want the antenna to resonate at the frequency we want to receive the signal. Antenna resonance is the most basic requirement for an antenna. Otherwise, there is no need to pay attention to it. Just throwing a wire out is also an antenna.
The main data involved in the resonance problem of the antenna is the wavelength and one quarter of it. The formula for calculating the wavelength is very simple, 300/f. The unit of f is MHz, and the unit of the result is meter. 1/4 wavelength is called a basic oscillator, such as a dipole antenna is a pair of basic oscillators, and a vertical antenna is a basic oscillator.
However, the length of the oscillator in the antenna is not exactly 1/4 wavelength, because the speed of radio waves traveling in a wire is different from that in a vacuum, and is generally shorter, so there is a shortening factor. This factor depends on the material.

Bandwidth
This is also an important but easily overlooked issue. The antenna has a certain bandwidth, which means that although the resonant frequency is a frequency point, within a certain range around this frequency point, the performance of this antenna is almost the same. This range is the bandwidth.
Of course, we hope that the bandwidth of an antenna can cover a certain range, preferably the entire FM radio band we listen to. Otherwise, it would be too troublesome to change or adjust the antenna when changing a channel.
The bandwidth of the antenna is related to the type, structure, and material of the antenna. Generally speaking, the thicker the tube and wire used in the vibrator, the wider the bandwidth; the higher the antenna gain, the narrower the bandwidth.

Impedance
An antenna can be regarded as a resonant circuit. A resonant circuit certainly has its impedance. Our requirement for impedance is matching: the circuit connected to the antenna must have the same impedance as the antenna. The feeder is connected to the antenna, and the impedance of the feeder is fixed, so we hope that the impedance of the antenna is the same as that of the feeder. The feeders generally produced are mainly 300 ohms, 75 ohms and 50 ohms. In the past, there were also feeders with 450 ohms and 600 ohms in foreign countries.
The impedance of the basic dipole antenna is about 75 ohms, the V-dipole antenna is about 50 ohms, and the basic vertical antenna impedance is 50 ohms. Other antennas generally have impedances that are not 50 or 75 ohms, so before connecting them to the feeder, some means are needed to perform impedance transformation.

Balance
Symmetrical antennas are balanced, such as dipole antennas and Yagi antennas, while coaxial cables are unbalanced. To connect the two, you need to solve the problem of balanced-unbalanced conversion.

Gain
Antennas are passive devices, but they can have gain. This gain is of course relative to the basic dipole antenna. The antenna used for FM DX, of course, hopes that the gain is as high as possible. But don't forget that high gain is often accompanied by narrow bandwidth.
Directivity
Not all antennas are directional. The telescopic antenna on a portable radio has no directionality. Dipole antennas have weak directionality, and directional antennas such as Yagi can achieve better directionality. Good directivity means that the radio waves in the required direction can be collected in a concentrated manner. Another important ability is that it can partially reduce the influence of local radio signals.
However, directional antennas are not good in all situations. When there is no target and you are waiting, the directional antenna may cause you to miss the signal on the back of the antenna. Therefore, a more reasonable way is to use a vertical antenna and a directional antenna together, wait with the vertical antenna, and then turn around and aim the directional antenna to listen after hearing the signal.

Elevation Angle
The elevation angle of an antenna refers to the elevation angle of the radio wave, not the mechanical elevation angle of the antenna vibrator itself. The elevation angle reflects the altitude angle at which the antenna receives the strongest radio waves. For F layer propagation, we hope that the elevation angle is low so that it can propagate far. For Es layer, radio waves mainly come from high places, so we hope that the elevation angle is high.
The height of the elevation angle depends on the antenna type and the installation height. Generally speaking, vertical antennas have low elevation angles, and the elevation angles of other antennas vary with the installation height.

Mounting height
The antenna has a mounting height. This height is actually two heights. One height is the horizontal plane height, which is somewhat useful for local signals, but not very useful for DX. The second height that is often overlooked is the ground height, which refers to the height from the antenna to the electrical ground. For example, an antenna mounted on a reinforced concrete roof, although the house is 20 meters high, but the antenna is only 1 meter away from the roof, then the height of this antenna is only 1 meter.
The height of the antenna has different effects on different antennas, and generally affects the impedance and elevation angle of the antenna. It is usually believed that the ground height of the antenna should be above 0.4 wavelengths to be less affected by the ground.

Standing wave ratio
Finally, let's introduce this feature that is least familiar to Chinese enthusiasts.
The standing wave ratio reflects the matching of the antenna feed system. It measures the performance of the antenna by the ratio of the energy emitted and reflected when the antenna is used as a transmitting antenna. The standing wave ratio is determined by the impedance of the antenna feed system. If the impedance of the antenna is consistent with the impedance of the feed line and the impedance of the receiver, the standing wave ratio is small. In an antenna feed system with a high standing wave ratio, the signal loss in the feed line is large.