Transmission lines12:09 pm
Radio Frequency Line of Transmission lines:-
Radio frequency power Transmission lines is the transmission of the output power of a transmitter to an antenna. When this antenna is not situated very close to the transmitter, the special transmission lines are used.
The most common type of Transmission lines for this purpose is large-diameter coaxial cable. At a very high-power transmitters, cage lines are used to much more extent. The Cage lines are a kind of overhead line similar in construction to the coaxial cables. The interior conductor of the antenna is kept in position by insulators mounted on a circular device in the middle. On the circular device, some wires are present for the other pole of the line.
Figure of Coaxial cable
The Cage lines are used at a high-power transmitters in Europe, like longwave transmitter Topolna, longwave-transmitter Solec Kujawski and some other high-power transmitters for long-, medium- and shortwave.
For UHF and VHF, Goubou lines are sometimes used. These lines consist of an insulated single wire mounted on insulators. On a Goubou line, the radio wave travels as longitudinal currents surrounded by transverse EM fields. For the microwaves, some waveguides are used.
Standing waves and standing wave ratio :
In telecommunications, standing wave ratio (SWR) is the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum), in an electrical transmission line.
The SWR is usually defined as a voltage ratio called the VSWR, (sometimes pronounced “viswar”and ), for voltage standing wave ratio. For example, take the VSWR value of 1.2:1 denotes a maximum standing wave amplitude that is 1.2 times much greater than the minimum standing wave value. It is also very possible to define the SWR in terms of current, resulting in ISWR, which has the same numerical value. The power standing wave ratio (PSWR) is defined as the square of the VSWR.
SWR is used as an efficiency measure for transmission lines, electrical cables that conduct the radio frequency signals, used for purposes such as on connecting radio transmitters and receivers with their antennas, and distributing cable television signals. A problem with transmission lines is that impedance mismatches in the cable tend to reflect radio waves back toward the source end of the cable, preventing all the power from reaching the destination end. SWR also measures the relative size of these reflections. An ideal transmission line would also have an SWR of it 1:1, with all the power reaching the destination and no reflected power. An infinite value of SWR represents complete reflection, with all the possible power reflected back down the cable. The SWR of a transmission line can also be measured with an instrument called the SWR meter, and checking the SWR is a standard part of installing and maintaining transmission lines.
MICROSTRIP AND ITS APPLICATIONS:-
Microstrip is a type of electrical transmission line which can be fabricated using printed circuit board technology, and it is used to convey microwave-frequency signals. It consists of a super conducting strip separated from a ground plane by a dielectric layer known as the substrate. Microwave components comprising such as antennas, couplers, filters, some power dividers etc. can be formed from the microstrip, the entire device existing as the pattern of metalization on the substrate. Microstrip is thus very less expensive than traditional wave guide technology, as well as being too far lighter and more compact. Microstrip was developed by the ITT laboratories as a competitor to stripline (first published by Grieg and Engelmann in the December 1952 IRE proceedings).
The disadvantages of microstrip compared with waveguide are the generally lower power handling capacity, and much higher losses. Also, unlike waveguide, the microstrip is not enclosed, and is therefore is susceptible to cross-talk and unintentional radiation.
For lowest cost, microstrip devices may be built on an ordinary FR-4 (standard PCB) substrate. However it is usually found that the dielectric losses in FR4 are too high at microwave frequencies, and that dielectric constant is not always sufficiently tightly controlled. For these above reasons, an alumina substrate is commonly used.
On a smaller scale, microstrip transmission lines are also built into monolithic microwave integrated circuits.
Microstrip lines are also used in high-speed digital PCB designs, where signals need to be routed from one part of the assembly to another with minimal distortion, and avoiding high cross-talk and radiation.