Transmission Lines
Transmission Lines
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This section attempts to address a number of issues that I have come across during my journey. It is likely that others have also come across these issues:
Reliance on buying misunderstood, poorly built & mislabelled “black boxes” from online vendors
Poor station performance
Inability to repair faults due to lack of knowledge
Inability to source good explanations on how transmission lines work
Throughout this section, math is avoided and instead basic explanations are given along with real world examples.
Radios and Antennas
Radios and Antennas
There are many designs of antennas, with some being very simple and some being quite complicated. Some are cheap to produce and some are expensive. Some are easy to deploy, and some are difficult. Some are picky about what frequencies they will work with (narrow band) and some are less picky (wide band). Some have high gain and some have less gain. It is all a game of compromises, and it is down to the person implementing the radio station what compromises they are willing to accept.
Usually, radios have a 50Ω unbalanced coaxial socket. We would normally plug 50Ω coaxial cable into this socket. The coaxial cable runs up to some antenna, and this is where things get a bit more complicated because an antenna can be of an unbalanced design or a balanced design, depending on the limitations imposed and what is required from the radio station.
A commonly repeated example of a simple antenna is the centre fed half wave dipole, and what is usually depicted is coaxial cable run directly to the antenna elements. The example is often accompanied with the reassurance that “halfwave dipoles are around 70Ω at the centre, which is close enough to 50Ω”. This statement is certainly true, but it is only half the story. The impedance mismatch isn’t worth worrying about because whatever gain can be had by inserting a transformer to match the impedances will likely be cancelled out by the insertion loss of the transformer itself. What IS worth worrying about is the fact that unbalanced coaxial cable is being connected to an antenna which is supposed to be balanced.
If the coaxial cable running from a radio is connected to a balanced antenna, the antenna becomes unbalanced because some of the current will flow back down the outside of the shield leaving less for that leg of the antenna, which in turn will radiate unevenly. The coaxial cable will also radiate, destroying your planned antenna polarization and possibly causing interference to nearby equipment at the radio end of the cable.
Another commonly used antenna example is the end-fed half wave dipole. In this example, unbalanced coaxial cable is being fed to an unbalanced antenna, but this still presents a problem. The 50Ω coaxial cable running from the radio is connected to an unbalanced antenna wich will be in the region of 1800Ω to 5000Ω. This presents an impedance mismatch between the cable and the antenna, and the full power from the radio will not be transferred to the antenna and vice versa.
In both of the above examples, some kind of transformer is required. There are several kinds of transformer and their design depends on what exactly they are expected to do.
The kinds of transformers that will be useful for the above antennas are explored later, but first it would probably be useful to lay out what “balanced” and “unbalanced” really mean.
Balanced Systems
Balanced Systems
A balanced system is one where the two conductors have an equal impedance to ground (Fig1).
An example of balanced feeder is 300Ω or 450Ω twin-lead.
There is an important distinction to draw between balanced line like Twin Lead (Fig1) and differential signalling (Fig2).
Where the signals on both lines are equal in amplitude but opposite in polarity, and the end point derives the signal from the difference between the signals on both lines, this is known as differential signalling. Any noise that finds its way onto one conductor will also find its way fairly equally onto the other. This will result in little to no difference at the end point, making the system resistant to noise.
Differential signalling and balanced lines are usually used together, but they are not technically the same thing. For the sake of simplicity, the word “balanced” is often used to describe a system where balanced line AND differential signalling is used.
A Balanced System in Use
The example given in Fig3 shows a practical example of a balanced system.
Most radios have unbalanced 50Ω coaxial connectors. For this reason, most installations do not have a fully balanced antenna / feeder system, and instead have coaxial cable to carry the signal to a balun and then onward to the rest of the system. Everything AFTER the balun is balanced, and everything before the balun is unbalanced.
Unbalanced Systems
Unbalanced Systems
An unbalanced system is one where the two conductors do not have an equal impedance to ground (Fig4).
Generally, what we’re talking about when we say “unbalanced”, is coaxial cable.
With most commonly found radio systems, the centre conductor of the cable is varied in voltage to carry the signal, whilst the shield is kept at ground potential (Fig5) by the equipment (such as a radio).
The electromagnetic field generated by this arrangement is held between the centre conductor and the shield, and this has the two advantages that coaxial cable can be run alongside cables and other conductive materials (such as cable trays) without interfering with them, and without them interfering with it.
An Unbalanced System in Use
The example given in Fig6 shows a practical example of an unbalanced system.
In this example, there is unbalanced feeder (coaxial cable) and an unbalanced antenna (end fed dipole), so there is no need for a balun. A transformer of some kind will still be required because the coaxial cable expects to see 50Ω, and an end fed antenna will range anywhere from 1800Ω to 5000Ω. For an application such as this, a transformer that provides an impedence transformation is required, and this is called an unun (unbalanced to unbalanced).
Another peculiarity of this design is that a reference plane is required. This can be provided by a low impedance connection to Earth (ground rods), or a counterpoise.
Why 50Ω?
Why 50Ω?
A common number that crops up when dealing with radio communications is 50Ω.
50Ω being used as an impedance is actually a compromise being made in order to balance lowest loss, highest voltage carrying capability and highest power transfer.
Lowest Loss
The lowest loss for air filled cables is around 77Ω.
The lowest loss for dielectric cables is around 50Ω.
Highest Voltage
The highest voltage tolerance occurs at around 60Ω.
Highest Power Transfer
Maximum power transfer happens at around 30Ω.
If you want only one of the above characteristics (such as highest voltage), you could pick some odd impedance (such as 60Ω) to use, but then you would struggle to interface to other equipment with differing impedances. 50Ω is the average between 30Ω and 77Ω, which is why most commercially available radios, coaxial cables and antennas are 50Ω.
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