Antenna Basic Concepts
Antenna Basic Concepts
FM DX antenna ears , faint radio waves from the antenna into the receiver through the feeder , to let us hear the voice of the distant station. The quality system of a receiver antenna half. We hope to have a high gain antenna , the weak signal becomes louder , we hope to have some antenna selection capability , the paging station interference and strong local station block on the outside , we want to minimize the loss of antenna system , the microvolt signals are each supplied to the receiver front end .
For most portable radios to listen to FM DX who said they might just whip antenna radio antenna on the antenna so easy though , but for FM DX , it is not enough anyway , although thanks to the ionosphere gifts , such an antenna system is not impossible to receive the DX signals.
I will describe some of the common and easy homemade antennas that can be used in our daily life readily available materials. I will go to produce these antennas will take a photo of the production process , and give as much detail as the size of the data . Although I will spend antenna analyzer in the production process even Tester and other equipment , but I will tell the reader not to use these expensive instruments debugging methods. At least, in full accordance with my material , size can not be wrong .
Before talking about the antenna , can not fail to mention the first wave . Our aim is to make antennas capture radio waves , therefore , to consider the issue before the antenna , it is absolutely necessary to study the problem of radio waves . FM broadcast band frequency from 87.5MHz to 108MHz, the corresponding wavelength is 3.4 meters to 2.7 meters , generally referred to in paragraph 3 meter band is VHF (Very High Frequency) of this section. This band is , 54MHz to 87.5MHz television broadcast band , above , 108MHz to 136MHz band aeronautical communications . VHF band radio wave propagation , there are three main ways :
Direct wave antenna
This means that between the transmitting antenna to the receiving antenna from , without any emission , direct access , like a beam of radio waves as well, 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 be spread not far away . There are two reasons, first, the radio wave emitted from the point whose energy is decreasing power series , and the receiver to be able to satisfactorily demodulated broadcast signal requires a certain strength. So far places, the signal is too weak , insufficient demodulation. If only for this reason , so hard to improve or increase the transmission power gain of the receiving antenna may be able to expand the range of listening . However , there is one important issue is that the Earth is round, waves emitted at any point on Earth , according to the direction of straight forward , will eventually leave the Earth toward the sky. Mainly due to the second reason, generally, a straight line on the ground wave emitted by the transmitter station , can be transmitted only to the receiver at a distance of 70km ground . If the height of both increased , then this distance can be increased, but always limited. Therefore , 70km, to listen to the local maximum is in fact , as mountains, hills , barrier housing , reflector , this distance should be greatly reduced , the estimated distance is generally 35km.
Ionospheric wave transmitter
This refers to the receiver by transmitting a radio wave to reach the ionosphere . There ‘s a lot of tricks . Ionosphere itself has multiple levels , supports shortwave (1.8MHz to 30MHz) reflected the ionosphere is F1 and F2 layers. F1 and F2 are not willing to reflect all of the radio wave reflected by the maximum frequency they can be limited over the radio frequencies are not completely reflected into space but through the ionosphere . Without this feature, it can not exist a communications satellite , Telstar is working outside the ionosphere . The maximum frequency is called MUF (Max Usable Frequency). MUF related to many factors , primarily the extent of sunspots and active , and the seasons . Sunspot activity , MUF is high , the weather is hot , MUF is high. MUF highest energy high to how much? Usually in the summer of sunspot activity , MUF between 20MHz to 40MHz, rarely more than 50MHz. In a time of low or even lower to 10MHz or less. But in the very active sunspot time , MUF there may occasionally reach 100MHz. At this time , it is possible to transmit received by the F layer DX FM . But this is not the main form of FM DX , FM DX mainly through another ionospheric E layer . Originally appeared E F layer layer is destroyed , so we might remember F layer Friend layer , E layer Enemy layers. But the emergence Es layer , but it will form a short-term high -density reflective layer. A reflective layer of high density , meaning better reflection waves. So when opening Es layer , DX radio signals unusually strong. In amateur radio 6 meters and 10 meters amateur band work all know , Es layer open when little power , even 5W, may also make DX contacts . Es opened, is to provide a radio wave propagation path within 800km . Because the signal is very strong, in fact, often do not need good equipment can receive , the need is patience and luck. Apart from these two reflections , FM DX is also possible trail to reach your receiver via tropospheric reflection and meteors.
Ground wave and ducting
Originally , it does not exist in theory, VHF waves . But countless practice shows , VHF there are also some degree of wave propagation . So we can receive radio signals around 200km stable . Amateur radio , Jiangsu and Anhui provinces , the annual National Day when they are carried out of the province VHF mobile communications experiments also proved VHF radio wave propagation can be obtained at a distance of about 200km . Ducting is another possible means of VHF radio wave propagation , but it was not enough research .
Given the existence of these possible , then how do you know what the signal is received by the way , I come from? In general, if the received signal is within 70km from the station , basically can be considered a direct wave ; If it is less than 200km , and signal stability ( not necessarily strong ) , then probably the ground wave ; If it is less than 800km, the signal is very strong, but very unstable , but only appeared occasionally , mostly Es layer propagation ; If the distance is farther, the signal is weak , probably F layer or other forms of ionospheric propagation .
Know what use is it ? Usefulness is to help our choice of antenna requirements. For example , there is a spread of F layer is characterized by the more distance, the radio about 500km within the F layer is unlikely to spread through the radio signals within this distance only to Es layer . As in Hangzhou want to receive FM radio signals to Taiwan , only PNP (Plug and pray), etc. Es layer , then we must consider suitable antenna Es layer characteristics.
Another very important factor is the polarization, it is very easy to overlook a lot of enthusiasts problems. Wave polarization in three ways: horizontal polarization , vertical and circular polarization . No matter how theoretically calculated , a simple way to judge is to see the direction of the transducer , the transducer is placed horizontally horizontal polarization and vertical polarization is vertical , circular polarization is not in FM radio, you can no matter what. Polarization is important, because they require polarization transmitter and the receiver must be consistent in order to have a good reception. Polarization of broadcasting is horizontally polarized , so the level of the receiving antenna should be erected . If polarization is inconsistent , there will be loss of 10dB to 20dB . However , after the wave reflection over the ionosphere has long been reflecting seven dirty eight elements, topsy-turvy , maybe what the polarization . So, receiving DX signals , in fact, vertical polarization is also good, with a benefit , that is, can weaken the impact of the local radio station.
Characteristics of the antenna
Any antennas are resonant at a certain frequency , which we want to receive the signal frequency , it is hoped that in the resonant frequency of the antenna . Antenna resonant antenna is a basic requirement , otherwise , not so much stress , and not just throw out lines also antenna thing.
Resonant antenna issues related to the primary data and a quarter wavelength . Wavelength is very simple formula , 300 / f. Where f is in MHz, and the results obtained in meters . 1 / 4 wavelength is called a basic oscillator, such as a dipole antenna of the basic oscillator, substantially vertical antenna is a transducer .
However, the antenna length of the transducer is not exactly 1 / 4 wavelength , as in the wire traveling wave speed in vacuum , generally be shorter, so there is a shortening factor . This factor depends on the material .
It is also an easily overlooked but important problem . Antenna is a certain bandwidth , which means that although the resonant frequency is a frequency , but within a certain range around this frequency , the performance of these antennas are almost good . This range is bandwidth.
We certainly hope that the bandwidth of an antenna can cover a certain range , preferably the entire FM broadcast band we listen to. Otherwise another station but also for the antenna or adjust the antenna too much trouble .
The bandwidth of the antenna and the antenna of the type , structure, material has a relationship . In general , the oscillator tube , the thicker the line, the more bandwidth ; antenna gain is higher, the narrow bandwidth .
Can be seen as an antenna resonant circuit . A resonant circuit course has its impedance. Our requirement is for impedance matching : circuit and an 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 impedance of the antenna and feeder same. General production feeders , mainly 300 ohm , 75 ohm and 50 ohm impedance three kinds , as well as abroad in the past 450 ohm and 600 ohm impedance of the feeder .
Dipole antenna impedance is substantially 75 ohms , V -type dipole antenna is about 50 ohms , 50 ohms impedance of the antenna is substantially perpendicular . Other antenna impedance is not generally 50 or 75 ohms, then before connecting them with the feeder , do need to have some means of impedance transformation .
Symmetric antenna is balanced, such as dipole antennas, Yagi antennas, and coaxial cable is unbalanced , these two are connected together, we need to solve the problem of a balun .
The antenna is a passive device , but it can have a gain of the antenna . This gain is of course relative gain , as opposed to the basic terms of a dipole antenna . FM DX antenna used , of course, want to gain the higher the better . But do not forget , is often accompanied by high gain narrow bandwidth .
Not all have a directional antenna . Telescopic antenna on a portable radio is no directionality . A weak directional dipole antenna , Yagi directional antenna , etc. can get a better direction. Good directional waves to concentrate the accident to collect the required direction , there is an important capability that can partially reduce the impact of local radio signals.
But under what circumstances directional antenna is not all good . When no target while waiting , directional antenna is possible to make you miss the back of the antenna signal. Therefore, a more reasonable approach is to use a vertical antenna and a pair of directional antennas used in conjunction with a vertical antenna wait , after hearing the signal , and then turn over the alignment of the directional antenna to listen .
Is the elevation angle of the antenna wave elevation , the elevation angle of the antenna element and not on the machinery itself . Which reflects the antenna elevation angle of the wave height of the strongest . For the F layer propagation , we hope the low elevation , can be spread apart, the Es layer , mainly from the high waves , we hope that the high elevation .
The level depends on the type of antenna elevation and erection height. Generally, the vertical antenna has a low elevation , another antenna elevation height variation with erection .
Antenna height erection
Antenna has a set height. This height is actually two heights, we consider it a highly water level height , the height of some use for the local signal for DX actually useful. The second height is often overlooked ground height refers to the electrical ground of the antenna height . For example, set up in the reinforced concrete roof antenna , although the house is 20 meters high , but the height of the antenna from the roof is only one meter , then this is just one meter antennas .
The height of the antenna have different effects on different antennas , generally affect the impedance of the antenna and elevation . Usually we think that the ground antenna height should be 0.4 wavelengths above ground was relatively unaffected .
Finally, this is not the most familiar features of Chinese fans . VSWR reflects the matching of the antenna system . It is used as the transmitting antenna and the antenna out of the reflected energy to measure antenna performance ratio . VSWR antenna system impedance is determined . Impedance of the antenna impedance is consistent with the impedance of the feeder and the receiver is small VSWR . SWR is high and the antenna system , signal loss in the feeder of the great.