Meaning of Standing Wave Ratio (SWR) in HAM Radio
In line with a SWR> 1 the presence of the reflected power does not lead to the loss of transmission capacity, although there are some losses due to the finite attenuation of the line at the feed line is not lossless power losses due to reflection regardless of the value SWR.
All ranges HF cable having low loss, loss mismatched lines are usually minor, but VHF may be material, and a microwave even extremely large. Cable Attenuation depends primarily on the characteristics of the cable and its length. When working at HF cable must be very long or very bad to cable loss became profound.
Reflected power does not flow back to the transmitter and does not damage it. Damage is sometimes attributed to the high SWR operation usually causes the output stage of the transmitter to the mismatched load. The transmitter does not "see" the SWR, it "sees" only the load impedance, which depends on the SWR. This means that the load impedance can be accurately corresponds to the desired (for example, using an antenna tuner), without worrying about the SWR in the feeder.
Effort required to decrease SWR less than 2: 1 in either a coaxial line, generally represented expended in vain - in terms of increase in the radiation efficiency of the antenna, but is suitable in the event that the protection circuit operates the transmitter, e.g., SWR> 1.5.
High SWR does not necessarily indicate that the antenna does not work well - the radiation efficiency of the antenna is defined by its resistance to radiation to the total input resistance.
Low SWR - not necessarily an indication that the antenna system is good. In contrast, a low SWR in wide bandwidth is reason to suspect that, for example, dipole or vertical antenna is large resistance losses due to bad connections and contacts, ineffective grounding system, cable loss, moisture gets into the line, etc. Thus, the dummy load line provides SWR = 1.0, but it generally does not emit a short vertical antenna radiation resistance of 0.1 ohms and 49.9 ohms resistance losses emits only 0.2% of the incoming power, while providing SWR 1.0 in the feeder.
For maximum RF current radiator antenna system does not need to have a resonant length of the feeder and does not require a specific length. A significant mismatch between the power line and the transmitter does not interfere with the absorption of the entire transmitter power actually supplied. By using the appropriate coordination (for example, an antenna tuner) to compensate for the reactivity of non-resonant emitter in place to connect the feed line of random length antenna system is consistent, and virtually all of the power input can be efficiently radiated.
On the SWR in the feed line does not affect the setting of the antenna tuner installed near the transmitter. Low SWR line made by a tuner, is usually an indication that in the course of the tuner was a mismatch between the transmitter and the input of the antenna tuner and transmitter operates at unmatched load.
Contrary to common opinion, with good symmetric (balanced) antenna tuner and an open-wire feed line radiation is fed into the center of the dipole length of 80 m, operating in the range of 3.5 MHz, not much radiation efficiency of the antenna is the same length of 48 meters, work in the same range and with the same transmitter power.
Radiation efficiency of the dipole tuned to resonate at a frequency of, for example, 3750 kHz is substantially the same as at the frequency of 3500 kHz or 4000 using any reasonable length of the feeder; although it can be expected that the SWR at the edges of the range can be 5 and a coaxial cable that will actually operate as a tuned line.
In this case, of course, necessary to use the appropriate matching device (e.g., antenna tuner) between the transmitter and the feeder. If in order to achieve matching any antenna coaxial feeder system requires a certain length, the same input impedance can be obtained with any cable length via appropriate simple matching network of inductors and capacitors.
High SWR coaxial feeder caused a significant mismatch between the characteristic impedance of the line and the input impedance of the antenna itself does not cause the appearance of high-frequency current to the outer surface of the sheath of the cable and the feed line radiation.
Short-wave high SWR in any open line, operating with a high SWR will neither cause leakage antenna current line or lead to the emission line, provided that the line currents are balanced, and the distance between the line conductors is small compared to the working wavelength (this is also true for VHF assuming no sharp bends lines).
The current on the outer surface of the braid feeder and feeder practically no radiation when the antenna is balanced with respect to ground, and a feeder (for example, using the horizontal antenna feeder should be positioned vertically); in such cases need not apply Baluns (baluns) between the antenna and feeder.
SWR meter installed in the area between the antenna and the feeder does not provide a more precise measurement of the SWR. Standing Wave Ratio in the feeder can be adjusted by changing the length of the line. If the reading SWR while moving along the line are significantly different, it may indicate an antenna effect feeder caused by the current flowing through the outside of the braid of the coaxial cable, and / or poor design SWR, but not the fact that SWR varies along line.
Any reactivity added to existing resonant load (having only resistance) to reduce the SWR in the line causes only an increase in reflection. The lowest SWR feeder observed at the resonant frequency of the radiating element, and does not depend on the length of the feeder.
The effectiveness of different types of radiation dipoles (from fine wire loop dipole, "thick" dipole dipole trapovogo or coaxial) is almost the same, provided that each of them has a small ohmic losses and feeds on the same power. However, the "thick" and loop dipoles have a wider operating bandwidth as compared with an antenna of a thin wire.
If the antenna impedance differs from the characteristic impedance of the feeder line, the load impedance of the transmitter can be very much different from the characteristic impedance of the line (if the line electric length is not a multiple of L / 2), and the resistance at the connection to the antenna. In this case, the load impedance of the transmitter depends also on the length of the feeder, which acts as an impedance transformer.
In such cases, if there is no suitable matching circuit between the transmitter and the transmission line, the impedance of the load can be complex (i.e. have active and reactive components), and with it the output circuit of the transmitter can not cope. In this case, the change in the length of the transmission line is sometimes possible to ensure the harmonization of the load with a transmitter - this circumstance rather than any loss related to SWR, has led to many misconceptions about the work of the feeder lines.
Any antenna is fed at the center of any reasonable length with any type of low-loss feeder will provide sufficient effective emission of electromagnetic energy. Thus, as a rule, good antenna tuner is required if the transmitter is designed to operate with a low impedance load (e.g., 50 ohms). This explains the fact that for many years to eat in the center of the dipole is a popular multi-band antenna.