HF portable wire antennas Clear Springs Press




Portable Wire Antennas

Antenna System Components

         An antenna system consists of:

         Matching the source and load impedance between the different components is very important because:

         The maximum power transfer theorem states that, to obtain maximum external power from a source with a finite internal impedance, the impedance of the load must be equal to the impedance of the source as viewed from the output terminals. While this yields the maximum power transfer, it is only at a 50% efficiency. In this situation, half the power is dissipated in the source and half in the load.

         In an electrical power system this would be highly inefficient and uneconomical. The efficiency increases as the load impedance increases relative to the source impedance. In a communications system getting the most power to the antenna is more important than doing it efficiently. So, in a communications system, the best result is achieved when the output impedance of the transmitter is equal to the transmission line impedance and the transmission line impedance is equal to the antenna impedance.

         Impedance has a resistance component and a reactance component.

Z=R + jX

         Antennas and transmission lines have inductance and capacitance distributed along their length as well as resistance. The characteristic impedance of a uniform transmission line is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections.

         At resonance, only the resistive components of an antenna are seen by the transmission line.

         Antenna Resistance = Ohmic resistance + radiation resistance

         The radiation resistance represents the loss due to RF energy being radiated off of the antenna into space. Resonant antennas have reflected or standing waves along the antenna length. The radiation is reflected between the antenna ends until all of the energy is dissipated either as radiation or ohmic losses. In a traveling wave antenna, radiation resistance and radiated energy are higher with longer wire lengths. Long means long relative to the wavelength of the signal.

         When not at resonance, a complex impedance is seen. The further from resonance, the higher the impedance becomes. When the transmission frequency doesn't match the resonant frequency of the antenna, the antenna impedance increases and creates an impedance mismatch between the antenna and the transmission line. The impedance mismatch causes reflections and some of the energy delivered by the transmission line is reflected away from the antenna, back toward the source.

         It is the interaction of these reflected waves with the incident waves that causes standing wave patterns in the transmission line. Reflected power has two main implications in radio transmitters. Transmission line losses increase and the transmitter finals can become stressed or damaged by the higher operating temperatures that result from having to dissipate the reflected power.

         The 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 a transmission line. When the standing wave ratio is expressed as a voltage rather than power ratio it is VSWR. For example, the VSWR value 1.2:1 denotes a maximum standing wave amplitude that is 1.2 times greater than the minimum standing wave value.

         Impedance mismatches between the transmission line and load (antenna) causes a reflection of some of the RF energy back toward the source end of the transmission line, preventing all the power from reaching the antenna. An ideal transmission line would have an SWR of 1:1, with all the power reaching the destination load and no reflected power. An infinite SWR represents complete reflection, with all the power reflected back down the cable.

         It is obvious that it is desirable to match the impedance of the transmitter, transmission line and antenna. In real world operations, this becomes difficult because changing frequency changes the impedance of the antenna and affects the transmission line as well. So, what happens when we have significant impedance mismatches between the transmission line and antenna? Standing waves develop as a result of the impedance mismatch. High SWR increases the transmission line losses because the higher effective current that is created in the transmission line increases the line loss from both the resistive and reactive components of the transmission line.

         Another way to visualize this is to consider this analogy. Pause for a moment and consider an analogy of wave propagation in a wave tank (a rectangular tank of water). When a displacement is created at one end a wave is created and it propagates down the tank and is reflected off of the far end. But then it travels back down the tank toward the origin where it is reflected again. This continues until the wave loses all of its energy to the environment. In the transmission line, a similar thing can be imagined with the reflected wave losing some of its energy to line loss each time it travels down the line.

         If the transmission line was lossless, all of the energy would get delivered to the antenna and radiated despite the impedance mismatch. Sadly, transmission lines do have losses. Some of the signal traveling from the transmitter to the antenna is lost. Then a portion of the reflected signal is lost to the transmission line before it gets back to the antenna. When the SWR values are very large and reflected energy is very high, transmission line losses can become a major problem.

         Not all transmission lines are created equal. For example, an open wire line may have a loss of 0.01 dB per 100 ft. while a coaxial line may have a loss of 1.5 dB per 100 ft., depending on the frequency. The losses increase with frequency and higher SWR. The point is that in a high SWR system a coaxial transmission line might eat a significant portion of your transmitter power while open wire line, ladder line or even twin lead will eat only a negligible amount of your transmitter power.

         So, you can use resonant antennas and carefully matched transmission lines which doesn't give you much operating flexibility, or you can use non-resonant antennas with high SWR and low loss transmission lines. In order to get the high SWR scenario to work, particular attention has to be paid to the impedance match between the transmitter and transmission line.

         Before looking at this, there is one more characteristic of transmission lines that one should be aware of. Transmission lines also act as a transformer. A transmission line terminated in its characteristic impedance, and of a length that is a multiple of a half wave length, acts as a 1:1 transformer. It does not change the terminating impedance into something else. However, when any other length of line is not terminated in its characteristic impedance, it does change the impedance at the transmitter end. The transformed impedance is a function of antenna impedance, line loss, frequent, and the length and characteristic impedance of the transmission line.

         Lower loss transmission lines tend to have a high characteristic impedance, typically 300 (twinlead), 450 (ladder Line) or 600 ohms (open line) as compared to the 50 ohm output impedance of the transmitter. Add to this the fact that the impedance of the antenna is changing drastically as you change frequency and band and the transmission line is acting as a transformer that changes its characteristics as a function of frequency. From the transmitter's point of view, this is a disaster. The consequence of a high feed point impedance mismatch is that the transmitter finals have to dissipate a lot of reflected power. Solid state final amplifiers overheat and "blow" under such conditions.

         The older vacuum tube transmitters had no problem with this scenario because they had a built in matching network that could be manually tuned to match the impedance seen at the transmitter output. Modern equipment lacks this feature. Modern transmitters have a fixed 50 ohm output impedance and are not very tolerant of mismatches.

         The solution is the antenna tuner. An antenna tuner consists of a network of inductors and capacitors. The sole purpose of the antenna tuner is to match the output impedance of the transmitter to the input impedance of the transmission line. Another way to say it is that the antenna tuner transforms the impedance of the transmission line - antenna combination into 50 ohms. Automatic antenna tuners are fast and convenient in creating a good match.

         By matching the impedance of the transmitter and transmission line, two important things are accomplished:

         The role and function of the antenna tuner are apparently frequently misunderstood. To be clear:

         Unfortunately antenna tuners do introduce signal loss. High Q capacitors are compact and available. Sadly, high Q coils want to be large while compact design requirements want them to be small. Because actual signal losses in an antenna tuner are affected by multiple variables there is no standard technical specification on this parameter. Generally speaking, the losses should be lower when the SWR is lower.

         Having an antenna system that presents a good impedance match removes the losses of a tuner and reduces the weight and bulk of a portable station. On the other hand, a good tuner makes it possible to use random wire and other non-resonant antennas on all bands.

         There is also some common apparent confusion regarding transmission line characteristics and losses. To be clear:

         If you need a more detailed explanation, I refer you to:

ARRL Antenna Book, 22 nd Edition, Chapters 23, 24, 2011-1013, American Radio Relay League

Practical Antenna HandBook, Fifth Edition, Chapters 4, 2012, McGraw Hill

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