Posted by Jack on 4/14/2003, 11:49:46 an otherwise great site with useful and accurate information Excerpt from the US Marine Corps MCRP 6-22D Field Antenna Handbook. A radio wave consists of electric and magnetic fields arranged per-pendicularly to each other and to the direction the wave travels. The impedance associated with this wave is the ratio of the potential dif-ference (voltage) to the current(amperage) at a given point along a transmission line. The following formula illustrates this. Voltage = Impedance In transmission lines, because of the length-frequency relationship, the characteristic impedance is more often discussed in terms of capacitance and inductance. In conventional circuits that contain inductors and capacitors, the inductance and capacitance are present in definite lumps. In an RF transmission line, however, these quan-tities are distributed throughout the entire line and cannot be sepa-rated from each other. If a transmitter is connected to a transmission line that is terminated in a load whose impedance is different from that of the line, only a portion of the available energy will be accepted by the load antenna, and the remainder will be reflected back down the line in the direc-tion of the transmitter. The energy is actually traveling in both directions along the line. If a transmitter is connected to a transmission line terminated in a load whose impedance exactly equals the impedance of the line, the line will absorb all of the energy except for that lost in the resistive and dielectric losses of the line. Current flowing through the line will be uniformly distributed along its length, and the voltage between the conductors on the line will be equal at all points. When this condition exists, the line is said to be perfectly matched and carries only a forward or incident wave. If the impedance of the transmission line and the load also equal the internal impedance (output impedance) of the transmitter, a maximum transfer of energy (lowest system loss) is achieved (i.e., the transmitter or receiver, transmission line, and antenna are all the same imped-ance), and the best possible transfer of signal energy will occur. When it is necessary to use a transmission line whose impedance is significantly different from that of the load, it is possible to make good use of standing waves and the repetitive impedance variations along the line to match the antenna to the transmitter or the receiver to the antenna by cutting the line to a specific length. An example is when the only available equipment consists of a 300-ohm twin-lead transmission line; a 50-ohm half-wave dipole antenna; and a 50-ohm internal impedance transceiver. (Note: The internal imped-ance of most USMC radios is 50 ohms). Ordinarily, this impedance combination would result in lost energy that could affect the qual-ity of communications. However, if a single frequency is used to communicate, the length between the antenna and the receiver can be matched. This occurs because the impedance of the receiver is repeated at intervals of a half-wavelength along the line. For end-fed, long-wire antennas, a similar impedance match can be made by feeding the long wire with a quarter-wavelength piece of wire that is connected to the transmitter on one end and to the end of the long wire on the other. The quarter-wavelength section doesnt need to be a separate piece of wire. For a 2-wavelength, long-wire antenna, for example, the wire can be cut to 2 1/4 wave-lengths. The entire quarter-wavelength section then becomes the transmission line between the radio and the antenna. 73,
Message modified by board administrator 5/22/2004, 11:46:59
"so much for The Ultimate Guide to 11 Meter CB Antennas"
http://www.signalengineering.com/ultimate/coax_basics.html
with some exceptions.
---------
Current
Optimizing Line Length
228
Message Thread:
« Back to thread