One. Long term transmission line
Below high frequency circumstance, when transmission line of electromagnetic wave edge travels, because the wavelengh of electromagnetic wave is very short, can happen on transmission line with the different phenomenon when transmission frequency signal. Must apply, additionally one suits the analytic method of high frequency circumstance.
The wavelengh that uploads sowed electromagnetic wave when edge transmission line is OK when be likened to with the geometrical length of transmission line, right now transmission line is called normally long term.
1. is long term on have voltaic voltage to all do not distributing: ざ of 〕 of sip of Mian of of the Huaihe River holds the post of?0 rice, should connect on the line with high frequency electric current when (long term circumstance) , be like F=150MHZ. Person (wavelengh) =2 rice because wavelengh is shorter, in this paragraph of transmission line electric current has a few periodic change, if pursue,A place is shown. What burn electricity each on same hour line consequently is big little differ somewhat with direction; and fall in low frequency circumstance (namely below short line circumstance) the alternating current that is like F=50HZ, its job wavelengh is 6000 kilometers, differ 3000000 equipment, it is the electric current on the transmission line that grows in 10 meters together big little change can think rarely changeless.
2. is long term it is system of a distributinging parameter: Haw of miscellaneous Lei は this? the differ or is electromagnetic wave of transmission of along the line wavelengh as transmission line length is different, on transmission line itself has distributinging capacitance and distributinging inductance parameter. And these distributinging the impact of parameter is very big, below long term circumstance, differ as a result of what grow along with the line or of working wavelengh different, the impedance that transmission line itself can present a different character (capacitive reactance, inductance or pure resistor) . And only the ability in capacitor has electric field on short line, the ability in coil produces magnetic field, grow to did not concern with the line.
2. The transmission line of terminal open a way
When transmission line receives signal, telegraphic date will distributing by Yu Xian with velocity of light the rule travels to terminal. As a result of terminal open a way, signal cannot continue to travel ahead, meet Xiang Shiduan direction forms total reflection, be equivalent to receiving source of a signal to make telegraphic date travels with velocity of light by terminal Xiang Shiduan again in the terminal of transmission line right now. Say commonly by only then the electric wave that end transmits to terminal calls incident wave; reflex the electric wave that come back to say to reflex by terminal wave. Incident wave and reflection wave are travelling wave. So called travelling wave is the electric wave with voltage and inphase electric current namely here.
Discussion transmission line still introduces the idea of a standing wave, alleged standing wave is voltage and electric current phasic and different and differ 1/4 wavelengh, the amplitude of voltage electric current has not equational cloth. In the transmission line of terminal open a way, existing to reflex wave and incident wave at the same time, the phasic each other that reflexes wave electric current and incident wave electric current is instead, say to there is standing wave on transmission line so, there are travelling wave and standing wave at the same time on transmission line normally, travelling wave, standing wave is formed by the electric current of incident wave and reflection wave. Because the transmission line of terminal open a way forms total reflection, component of its standing wave is very so big, reason is transmitted without energy (get offline in travelling wave condition only go up to just energy is transmitted) suppose bad news of nondestructive of this transmission line. That just stores inside certain period at this moment energy, energy is given off inside another period. To signal source so it is the load of sex of a pure reactance, there is the following characteristic in the transmission line of terminal open a way, transmission line is little pure resistor sex is when the reactance when 1/4 wavelengh is more than for 0; for reactance when at 1/4 wavelengh its reactance is equal to 1/4 wavelengh to allow sexual; is equal to 1/2 wavelengh for inductance sex; .
3. The transmission line of terminal short circuit
The transmission line of the transmission line of terminal short circuit and terminal open a way is contrary, its standing wave character is incident electric current and reflection wave electric current inphase, incident wave voltage and reflection wave voltage are phasic and contrary. Its impedance character is the reactance when the; that allow a gender is being shown to be equal to 1/2 wavelengh when; of pure resistor sex is being shown to be more than 1/4 wavelengh when the transmission line when be less than 1/4 wavelengh shows perceptual; to be equal to 1/4 wavelengh for 0.
In usually the terminal of transmission line is received have load, its load has resistor component to also have reactance component already normally, load of one part shellfish draws the energy that outputs by signal source consequently, another part come back times reflex, so the travelling wave on transmission line and standing wave coexist at the same time, to describe the distributinging relationship of the travelling wave on the line and standing wave further, we introduce a few specific target:
1. reflexes coefficient: P= reflexes wave amplitude / impedance of character of transmission line of = of incident wave amplitude - laden impedance / impedance of transmission line character + laden impedance
Coefficient of 2. travelling wave: K= tension is the least value / voltage maximum = reflexes wave amplitude - incident wave amplitude / reflection wave amplitude + incident wave amplitude
Because be put in incident wave and reflection wave at the same time,be in in transmission line, the voltage that holds the post of He Yi to nod on transmission line so is two amplitude the sum.
3. standing wave is compared: S= voltage maximum / tension is the least value
The place on put together is narrated, when transmission line terminal has load, transmission line inputs impedance to have the following quality:
Terminal of the distance on 1. transmission line of 1/4 wavelengh times odder the square that the equivalent impedance of place is equal to characteristic impedance is divided with terminal load.
The equivalent impedance that 1/2 wavelengh integer is in terminal of the distance on 2. transmission line is equal to laden impedance.
- 4. The general principle of aerial
- Should connect on conductor with high frequency electric current when, in the space can produce electric field and magnetic field all round its. The magnetic field that press report is in distributinging character of the space, can divide for close area, intermediate area, remote region. Set R to was apart from the distance of conductor for the space, in R " λ / the area when 2 π weighs close area, the electric current in the electric magnetic field inside this area and conductor, voltage has close connection.
In R " λ / the area of 2 π calls far area, inside this area electromagnetism field can leave conductor to travel to the space, its metabolic photograph sheds voltage to be about to the report on conductor lag period of time, transmit the electricity that gives the electromagnetic wave that go to already did not go up with lead right now, voltage has direct connection, the electric magnetic field of this area calls radiation field.
Emissive antenna uses this kind of property of radiation field just about, the signal that makes convey distributes land through launching aerial hind to be able to be filled to dimensional radiate, how to make conductor becomes an effective radiator to guide system? We analyse the case on transmission line first here, be transmitted in what there is energy only to make on the transmission line of parallel crewel and do not have radiation, must make sure two routes structure is symmetrical, size of corresponding dot electric current and way are opposite on the line. And the distance of two space " π . Should make electromagnetism field can effective ground radiate goes out, with respect to this kind of symmetry that must destroy transmission line, if use a 2 conductor to become certain angle departure, or be will among them at the same time the method such as take out, can make conductor symmetry is destroyed and produce radiation.
Be like graph TX, open a way of graph lieutenant general is transmitted or be apart from terminal π / the conductor of 4 place parts into straight shape, right now the electric current on terminal conductor already was not to turn over however inphase, make inphase alternate increases this paragraphs of radiation field that conductor chooses in the space thereby. Make an effective radiation system. This is the simplest, the basiccest unit antenna, call half semmetry brace up child aerial, its character impedance is 75 Ω . After electromagnetic wave comes out from emissive antenna radiate, to all sides transmission goes out, one semmetry is put on the direction that is like electromagnetic wave transmission brace up child, fall in the action of electromagnetic wave, aerial brace up child go up to be able to produce inductive electromotive force. Such when aerial and receive equipment to be linked together, be in receive equipment to input end to be able to produce high frequency electricity, such antenna are rising to receive effect and electromagnetic wave translate into high frequency electric current, wear since aerial right now that is to say the action of receving antenna, the stand or fall that receives the result besides electric wave outside losing by force, still depend on of aerial directional with half semmetry brace up child match with what receive facility.
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