FM DX antenna ears, faint radio waves from the antenna into the receiver through the feed line to let us hear the voice of distant radio stations. The quality of a receiving system, the antenna half. We hope to have a high gain antenna, the signal becomes weak loud, we want the choice of the antenna can have a certain ability to interfere with the paging station and the local strong block on the outside, we want to minimize the loss of feed system, the Each micro-volt signal to the receiver front end.
For most of the use of portable radios to listen to FM DX people that they may be only the antenna whip antenna on the radio, although such a simple antenna, but for FM DX, the case is not enough, though thanks to the ionosphere gift, this antenna system is not impossible to receive the DX signal.
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I will introduce some common and easy homemade antennas can be used in our daily lives readily available materials. I will go through the production of these antennas, the photo of the production process, and as detailed as possible given the size of the data. Although I will use in the production process or even integrated antenna analyzer tester and other equipment, but I will tell the reader not to use the expensive equipment debugging. At least, in full accordance with my material, size, be wrong.
Radio
Before talking about the antenna, can not but mention the airwaves.
Our aim is to produce antenna captures radio waves, therefore, before considering the issue of the antenna, is absolutely necessary to study the problem of waves.
FM radio band frequencies from 87.5MHz to 108MHz, the corresponding wavelength is 3.4 m to 2.7 m, generally referred to as 3 meter band, the VHF (Very High Frequency) section. Following this band, 54MHz to 87.5MHz television broadcast band, above, 108MHz to 136MHz band in aviation communications. VHF-band radio wave propagation, there are three ways:
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Direct wave
This refers to the transmitting antenna to the receiver from the antenna, without any emission, directly, like a beam of light as waves, so some people call it the line of sight transmission. The name also indicates the line of sight propagation of this mode of transmission can spread far away. There are two reasons, first wave starting from the launch point, the energy is decreasing power series, and a good receiver to be able to demodulate the radio, requires a certain signal strength. So too far, the signal is too weak, not enough to demodulation. If only for this reason that so hard to improve or increase the transmit power receiving antenna gain, perhaps you can expand the scope to listen. However, there is an important issue is that the earth is round, in any point on Earth the wave, according to the straight forward, fired the sky will eventually leave the Earth. Mainly due to the second reason, generally speaking, a transmitter on the ground sent straight wave, only 70km distant spread to the reception office on the ground. If both height increase, then this distance can be increased, but is always limited. Therefore, 70km, the limits of the local listening, in fact, as mountains, hills, houses blocked, reflection, this distance should be greatly reduced, the distance is generally estimated to 35km.
Ionosphere wave emission
This refers to the radio emission through the ionosphere to reach the recipient. There's a lot of tricks. Ionosphere itself has multiple levels of support shortwave (1.8MHz to 30MHz) is a reflection of the F1 and F2 ionospheric layers. F1 and F2 are not willing to reflect all of the radio waves, they can reflect the maximum frequency is limited, more than the frequency of the waves are not reflected completely, but fired at through the ionosphere into space. Without this feature, then there can not be a communications satellite, Telstar is working outside in the ionosphere. The maximum frequency is called MUF (Max Usable Frequency). MUF related to many factors, primarily the level of activity and sunspots, and the seasons. Sunspot activity, MUF is high, the weather, MUF was also high. MUF can be high up to much? Usually in the summer period of sunspot activity, MUF between the 20MHz to 40MHz, with little more than 50MHz. In a time of low or even lower to below 10MHz. But in the very active sunspot time, MUF may also occasionally reach 100MHz. At this time, it is possible to transmit through the F layer has received DX FM. But this is not the main form of FM DX, FM DX is another main ionospheric E layer. Would undermine the emergence of E-layer F layer, so we might remember as Friend layer F layer, E layer Enemy layer. But the Es layer appearance, but will form a short-term high-density reflective layer. Reflective layer of high density, which means better reflection waves. So when the opening of Es layer, DX radio signals unusually strong. 6 m and 10 m amateur band amateur radio work know, Es layer opened, little power, even 5W, may also make DX contacts. Es opening, is to provide a radio transmission path within 800km. As the signal is very strong, in fact, often do not need good equipment, you can receive, the need is patience and luck. In addition to these two reflections, FM DX may also reflected through the troposphere and the meteor trail to reach your receiver.
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Ground-wave and atmospheric waveguide
Originally, the theory does not exist to VHF waves. However, practice has shown that numerous, VHF, there are also some degree of ground wave propagation. Therefore, we can stabilize around 200km to receive radio signals. Jiangsu and Anhui provinces, amateur radio, each time for the province's National Day of VHF mobile communications experiments also proved that a VHF radio can be spread a distance of about 200km. Duct is another possible means of communication VHF radio, but it was not enough research.
Since the existence of these possibilities, then how do you know I received a signal on what approach to it? In general, if the received signal is less than 70km from the radio, basically that is the direct wave; if it is less than 200km, and the signal stability (not necessarily strong), then about to wave; if it is less than 800km, the signal very strong, but very unstable, and occasionally appear, mostly Es layer transmission; If the distance is farther, the signal is weak, probably the F layer ionospheric or other forms of dissemination.
Know what use is it? Useful to help us choose the antenna requirements. For example, F layer propagation is more a characteristic distance, about 500km within the radio is not possible to spread through the F layer, and the distance radio signals can only Es layer. As in Taiwan, Hangzhou want to receive FM radio signals, only PNP (Plug and pray), and other Es layer, then the antenna should be considered for Es layer features.
There is also a very important factor is the polarization, it is very easy to be a lot of fans overlook. Wave polarization in three ways: horizontal, vertical polarization and circular polarization. No matter how theoretical calculations, a simple way to judge is to see the direction of the oscillator, the oscillator is placed is the level of horizontal, vertical is the vertical polarization, circular polarization do not have the FM radio, can matter. Polarization is important, because for a launching party and the recipient of the polarization must be consistent in order to have good reception. China's radio polarization is horizontal, so the level of the receiving antenna should be erected. If the polarization is inconsistent, there will be loss of 10dB to 20dB. However, after reflection of the ionosphere over the airwaves, has long been reflected were seven dirty eight elements, topsy-turvy, what might be a polarization. Therefore, the receiving DX signals, in fact, vertical polarization is also good, with a benefit that can weaken the impact of the local radio station.
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Characteristics of the antenna
Resonance
Any antennas are resonant at a certain frequency, which we want to receive the signal frequency, I hope that the antenna resonance frequency in the. Antenna resonance is the most basic requirements of the antenna, otherwise, not so much stress, and not just throw out the root line is the antenna thing.
Resonant antenna problem is related to the main data and a quarter wavelength. The formula for calculating wavelength is very simple, 300 / f. Where f is the unit of MHz, and the result of the unit is meters. 1 / 4 wavelength is called the basic oscillator, such as the dipole antenna is one of the basic oscillator, vertical antenna is a basic oscillator.
However, the oscillator in the antenna length is not exactly 1 / 4 wavelength, because the waves travel at the speed of the wire in a vacuum with a different and generally have shorter, so there is a shortening factor. This factor depends on the material.
Bandwidth
This is an important but easily overlooked. There is a certain bandwidth of the antenna, which means that although the resonant frequency is a frequency point, but in the vicinity of this frequency within a certain range, the performance of these antennas are almost good. This range is the bandwidth.
We certainly hope that the bandwidth of a pair of antennas to cover a certain range, it is best that we listen to the entire FM radio band. Or else for a change in Taiwan but also adjust the antenna or antenna too much trouble.
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Bandwidth of the antenna and the antenna type, structure, materials have a relationship. In general, the oscillator tube, the more thick lines, wider-bandwidth; higher the antenna gain, the more narrow bandwidth.
Impedance
Antenna can be seen as a resonant circuit. Of course there is a resonant circuit impedance. Our requirement is that the impedance matching: the circuit and the antenna connected to the antenna must have the same impedance. And the antenna is connected to the feeder, the feeder impedance is determined, so we hope that the antenna impedance and the same feeder. General production of the feeder, main is 300 ohm, 75 ohm and 50 ohm three kinds of resistance, there is abroad in the past 450 ohm and 600 ohm impedance of the feeder.
Basic dipole antenna impedance is 75 ohms, V-dipole antenna is about 50 ohms, the basic vertical antenna impedance of 50 ohms. Other antenna impedances are not usually 50 or 75 ohms, then connect them with the feeder, you need to have some means to do the impedance transformation.
Balance
Symmetry of the antenna is balanced, such as dipole antennas, Yagi antenna, and coaxial cable is unbalanced, connect these two, we need to solve the problem balun.
Gain
Antennas are passive devices, but the antenna is a gain. This gain is the relative gain of course, is relative to the terms of the basic dipole antenna. FM DX antennas used, of course, want to gain the higher the better. But do not forget, is often accompanied by high gain bandwidth is narrow.
Directional
Not all antennas are directional. Whip antenna on the portable radio is no direction. A weak directional dipole antenna, Yagi directional antenna so you can get a better direction. Accident with a good direction to focus on collecting the required direction of the waves, there is an important part is that we can weaken the ability of local radio signals.
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But the case of directional antennas is not all good. When there is no waiting time targets, directional antennas may make you miss the back of the antenna signal. So a more reasonable approach is to use a vertical antenna and a pair of directional antennas used in conjunction with the vertical antenna waiting to hear the signal, then turn over directional antenna pointed at listening.
Elevation
Antenna elevation angle is the wave elevation, and not their mechanical oscillator antenna elevation. Which reflects the antenna elevation angle to the wave height of the strongest. For the F layer propagation, we hope the low elevation angle, can be spread apart, the Es layer, mainly from the high waves, we hope that the high elevation.
The level of elevation depends on the antenna type and set height. In general, the vertical antenna has a low elevation, the other with the erection of the antenna height of elevation changes.
Erection of a high degree of
Antenna has a set height. The height is actually a two level, we consider it a high water level height, the height of the local signal of some use, for the DX is actually useful. The second is often overlooked is the height above the ground, is the height of the antenna to the electrical ground. Such as the erection of the reinforced concrete roof antenna, although the house high and 20 meters, but the antenna is only 1 meter away from the roof, then pay the height of the antenna which is only 1 meter.
Height of the antenna for different antenna have different effects generally affect the antenna impedance and elevation. Usually we think that the ground antenna height should be 0.4 wavelengths above the ground was relatively free from the impact.
VSWR
Finally the most fans are not familiar with Chinese characteristics.
VSWR antenna system reflects the match situation. It is based on when the transmitter antenna as transmitting antenna and the reflected energy to go out than to measure the antenna performance. VSWR is the antenna system impedance by the decision. The antenna impedance and the impedance of the feeder line and the receiver impedance, VSWR is small. Standing wave is high and the antenna system, the signal loss in the feeder of the great.
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