The basic concept antenna
FM DX Antenna is the ears, weak radio waves from the antenna through feeder into the receiver, so that we can hear the distant sound of the radio. A system is only as good reception, the antenna half. We hope that the antenna can have high gain, the weak signal becomes louder, we hope that the antenna can choose to have a certain ability to interfere with paging stations and the Qiang local block outside Taiwan, we hope that the antenna and cable systems to minimize wear and tear, Each micro-volt signals are sent to the receiver's front end.
For most of the use of portable radios to listen to FM DX said they may be the only radio antenna on the rod antenna, although such a simple antenna, but FM DX, in any case it is not enough, even though al ionosphere Gift, so that the antenna system is not likely to receive the signal DX.
I will introduce some common and easily self-made antenna, the antenna can be used in our daily lives easier to get the material. I will go through the production of these antennas will be made into a photo production process, and as much detail as possible given the size of the data. Although I will be in the process of making use of an antenna analyzer or even a comprehensive test equipment and so on, but I will tell the reader not to use the expensive equipment debugging method. At the very least, I am in full accordance with the material, size of the total wrong.
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Radio antenna
In speaking before the antenna can not but mention the first wave. We are making an antenna is designed to capture radio waves, in considering the issue before the antenna, it is absolutely necessary to look at the waves. FM radio band, frequency from 87.5MHz to 108MHz, corresponding to the wavelength is 3.4 m to 2.7 m, generally known as the UHF 3, is a VHF (Very High Frequency) section. Following the band, 54MHz to 87.5MHz is the television band, above, 108MHz to 136MHz band is the aviation communications. VHF band of radio propagation, there are three main ways:
Direct-wave antenna
This is the launch from the antenna to the receiver antenna, is not subject to any launch, directly, as Yi Shuguang airwaves, so some people refer to as the spread of sight. The spread of sight also shows the name of this mode of transmission can be spread not far away. There are two reasons for this, the first wave is launched from the point of departure, its energy is diminishing the power series, and the receiver can be a good way to a radio modem requires a certain degree of signal strength. So far, signals are too weak, not enough to demodulator. If only for this reason, so hard to improve or increase the transmitting power of the receiving antenna gain, perhaps we can listen to the expansion of the scope of the. However, there is another important issue is that the Earth is round, any point on Earth in the wave, according to the way forward in a straight line, will eventually leave the earth shooting at the sky. Mainly due to the second reason, generally, a ground launch pad in a straight line issued by the waves, can only spread to 70km distance on the ground to receive the Department. If the two sides to increase the height, the distance can be increased, but always limited. Therefore, 70km, is to listen to the local limits, in fact, as mountains, hills, the housing block, reflection, but also reduced the distance, the distance can be estimated in general is 35km.
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Ionosphere launch wave
This refers to the radio through the launch of the ionosphere to reach the recipient. There's a lot of names. Ionosphere is a number of levels, to support the short-wave (1.8MHz to 30MHz) is a reflection of the ionosphere F1 and F2 layer. F1 and F2 is not willing reflection of all the radio waves, they Reflectance maximum frequency is limited, more than the frequency of radio waves are not reflection, but through the ionosphere shooting space. If you do not have this characteristic, it can not exist on the satellite, communication satellite in the ionosphere that is outside of work. The highest frequency is called MUF (Max Usable Frequency). MUF with a lot of factors and the sunspots are the main activity as well as the season. Sunspot activity, MUF on a high, the weather, MUF is also high. MUF to the highest high to much? General sunspot activity in the summer period, MUF in the 20MHz to 40MHz, little more than 50MHz. In a time of low or even lower to below 10MHz. But in the very active sunspot time, MUF may occasionally reach 100MHz. At this time, it is possible to receive through the F-launch the DX FM. FM DX, but this is not the main form, FM DX mainly through another layer of the ionosphere E. Originally, E-emergence is the destruction of F layer, so we may wish to remember for the F-Friend-, E-layer for the Enemy. However, Es-story, it would be a short period of very high density of the reflector. Reflector of high-density means better reflection wave. Therefore, the opening of the Es-time, DX radio signal will be exceptionally strong. In the 6 m and 10 m amateur band of the work of amateur radio are all aware, Es-opened, small power, and even 5W, may do DX contact. Es opening is provided within 800km of the radio transmission path. Due to a very strong signal, in fact, often do not need very good equipment will be able to receive, the need is patience and luck. In addition to these two types of reflection, FM DX may also reflective of the troposphere and the Meteor Burst reach your receiver.
To wave in the atmosphere and wave-guide
Originally, the theory does not exist to VHF waves. However, numerous shows, VHF also has to a certain degree of wave propagation. Therefore, we can steady at about 200km to receive radio signals. Jiangsu and Anhui provinces of amateur radio, the annual National Day of time for the province's VHF mobile communication experiments to prove the VHF radio in about 200km distance to be spread. Bird in the atmosphere is another possible means of dissemination of VHF radio, but it was not enough research.
Since the existence of these may be, so I know how the signal received by the way come from? In general, if the signal received from within 70km of the radio and basically can be considered to be a direct wave; if it is within 200km, and signal stability (not necessarily strong), then probably the wave; if it is within 800km, the signal Very strong, but very unstable, but only occasionally, most of the Es layer transmission; if farther away, the signal is very weak, probably the F layer or other forms of dissemination of the ionosphere.
Know what is the use? We chose to help the usefulness of the antenna. For example, F-spread is characterized by a greater distance, about 500km within the station is not possible through the F layer to the spread of this distance, radio signals can only Es layer. As in Hangzhou in Taiwan want to receive FM radio signal, only PNP (Plug and pray), and so on Es layer, then we should consider suitable antenna Es layer characteristics.
There is also a very important factor is the way of polarization, it is very easy to lose sight of a lot of fans. The wave polarization in three ways: the level of polarization, vertical polarization and circular polarization. No matter how theoretical calculation, a simple method to judge is to see the direction of the oscillator, the oscillator is the level of release is the level of polarization is perpendicular to the vertical polarization, not circular polarization in FM radio, can irrespective. Polarization mode is important because the requirements to launch and the recipient of polarization must be unanimous in order to have a good reception. China's broadcasting of polarization is the level of polarization, so that the receiving antenna should be set level. If the polarization inconsistent manner, there will be 10dB to 20dB of loss. However, after reflection of the ionosphere over the airwaves, has long been a reflection seven eight-meat dish, topsy-turvy, what is perhaps a way of polarization. Therefore, to receive the signal DX, in fact, have a good vertical polarization, with a benefit that can weaken the local radio station.
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The antenna features
Antenna resonance
Any antennas are to a certain resonance frequency, we have to accept what the frequency signal, in the hope that the antenna resonant frequency. Antenna resonant antenna is the most basic requirements, otherwise, not so much about, just throw line root them out and not antenna is.
The antenna resonant issues related to the main data is a quarter of its wavelength. Wavelength calculation formula is simple, 300 / f. F which is a unit of MHz, and the result is a unit of meters. 1 / 4 wavelength is known as the fundamental oscillator, such as the dipole antenna is a basic oscillator, a vertical antenna is a basic vibrator.
But the antenna of the length of the oscillator is not exactly 1 / 4 wavelength, as in the radio wires in the road speed in a vacuum and different, it would usually shorter, so there is a shortening factor. This factor depends on the material.
Bandwidth antenna
This is an important but easily overlooked. Antenna is a certain bandwidth, which means that although the frequency is a resonant frequency point, but at this point in the vicinity of a certain frequency, which pay an antenna performance is almost good. This is the scope of bandwidth.
We certainly hope that a pay antennas bandwidth to cover a certain extent, it is best that we listen to the entire FM radio band. Otherwise Taiwan would also like to change-for-tune the antenna or antennas are too cumbersome.
The antenna bandwidth and the type of antenna, structure, materials have relations. In general, the oscillator used in pipe, the more rough line, the more wide-bandwidth; the higher the antenna gain, the more narrow bandwidth.
Antenna Impedance
Antenna can be seen as a resonant circuit. A resonant circuit, of course, has its impedance. Our resistance is to match the requirements: an antenna and related circuitry and antenna must have the same resistance. And the antenna is connected to the feeder, the feeder is determined by the resistance, so we hope that the antenna impedance and the same feeder. General production of the feeder is 300 ohm, 75 ohm and 50 ohm impedance of the three, was also abroad in the past 450 ohm and 600 ohm impedance of the feeder.
The basic dipole antenna impedance is 75 ohms or so, V-type dipole antenna is about 50 ohms, the basic vertical antenna impedance of 50 ohms. Other antenna impedance in general are not 50 or 75 ohms, then put them in connection with the feeder before a certain resistance as a means of transformation.
Antenna balance
The antenna is symmetrical balance, such as the dipole antenna, Yagi antenna, which is unbalanced coaxial cable, to connect these two together, we need to address the imbalance in the balance conversion.
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Gain Antenna
Antenna is a passive device, but there is an antenna gain. This gain is, of course, the relative gain is relative to the basic terms of the dipole antenna. FM DX used by the antenna gain, of course, hope that the higher the better. Do not forget, however, is often accompanied by high-gain narrow bandwidth.
Directional antenna
Not all have a directional antenna. Portable radio on the bar on non-directional antenna. Dipole antenna of a weak direction, such as Yagi directional antenna can get a better direction. Good direction to focus on the accident to collect the necessary direction of the waves, there is an important part of the capacity that is able to weaken the local radio signals.
But the directional antenna is not a good case. When the goal did not wait, directional antenna so that you may have missed the back of the antenna signal. Therefore, a more reasonable approach is to use a vertical antenna and pay a directional antenna used in conjunction with a vertical antenna to wait to hear signals, directional antenna to use in the past at the hearing.
Antenna elevation
The antenna elevation angle is the angle of elevation of the waves, and the antenna is not a mechanical oscillator itself on the elevation. Elevation which reflects the antenna to receive a high angle to the strongest radio. The F-spread, we hope that the low elevation angle, and can spread far, the Es layer, mainly from the high waves, we hope that the high elevation.
High or low depends on the angle of elevation of the antenna pattern and a high degree of erection. In general, a vertical antenna with a low elevation, the other with the elevation of the antenna to set up a high degree of change.
To set up a high antenna
Antenna has a high degree of erection. This is in fact the two high altitude, we consider it a high degree of a high level, the high degree of local signals by some, the DX is not actually use. The second is often overlooked is the high degree of high ground, is the electric antenna to the ground level. For example, to set up steel and concrete in the roof of the antenna, although the house there are 20 meters high, but the antenna from the roof is only 1 meter, then pay this antenna is only 1 meter high.
The antenna of a high degree of different antenna have different effects on the general will affect the impedance of the antenna and elevation. We believe that the antenna is usually the high ground in more than 0.4 wavelength, compared with only from the ground.
Antenna VSWR
Finally, this was the least the fans are familiar with Chinese characteristics. Standing wave ratio reflects the antenna and cable systems to match the situation. It was launched as an antenna when the antenna reflector and fired out the back of energy than to measure the performance of the antenna. SWR system is fed by days of the decision of the resistance. The antenna impedance and impedance of the feeder and the receiver impedance, VSWR small. The high standing wave antenna and feeding system, the signal is lost in the big feeder.
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