Transmission Lines

Question File Number 26



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Carrying The Signal

Transmission lines are the link between your station equipment, transmitter, receiver, transceiver, and the antenna. There are many different varieties but two major types of line predominate for frequencies in general use by radio amateurs.

Parallel-conductor line, also known as twin-line, or open-wire line, consists of two parallel conductors held apart at a constant fixed distance by insulators or by insulation. This type of transmission line is balanced. This means that each wire is hot with respect to earth.

Coaxial cable (coax) is the other major type and consists of two concentric conductors. It is a single wire surrounded by insulation and enclosed in an outer conductor, usually a braid. This is an unbalanced line, the outer sheath can be at earth potential, only the inner wire is hot.

The transmitter power radiating from the antenna is less than that generated at the transmitter due to losses in the transmission line. These losses increase with higher SWR values, with higher frequencies and with increasing the length of the line. Most line loss occurs in the supporting insulation so open-wire lines have lower losses than heavily-insulated line.

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Parallel Lines

These come in various types. The flat TV 300-ohm ribbon is an example. Ladder-line, in which two parallel conductors are spaced by insulation spreaders at intervals is another. These lines are relatively cheap. Open-wire lines can be home-constructed using improvised spreaders. These lines have low losses at HF frequencies.

These lines do have the disadvantage that they must be kept away from other conductors and earthed objects. They cannot be buried or strapped directly to a tower.

As the frequency increases, the open-wire line spacing becomes a significant fraction of the wavelength and the line will radiate some energy.

Because it is a balanced line, it can feed a dipole directly without the use of a balun at the antenna. (Baluns are discussed below.) Most transceivers have an unbalanced 50-ohm output impedance and a balun transformer will be required to feed a balanced line.

Parallel lines vary in impedance depending on the diameter and the spacing of the conductors. TV twin lead has an impedance of 300-ohm and ladder-line is usually 450 or 600-ohm.

Coaxial Cable

Coaxial cable consists of two concentric conductors with dielectric insulation in the space between the conductors. The inner conductor carries the signal (i.e. it is hot) the outer conductor is usually at earth potential and acts as a shield. This cable can be buried and run close to metal objects with no harmful effects.

Coax comes in various sizes from very small to large diameters. The small sizes are for low powers and short distances. The larger sizes have higher power-handling capabilities and usually lower losses. Most amateurs use 50-ohm cable while TV coax is usually 75-ohm.

The dielectric insulator is generally the main cause of energy loss. Most coax uses solid polyethylene and some types use a foam version. The foam version is lower loss but the solid version is more rugged. For very low loss purposes, a solid outer is used (hardline), and the inner conductor is supported by a spiral insulator or by beads. This type of coax is hard to work, cannot be bent very sharply and is generally expensive.

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Impedance

An important characteristic of a transmission line is its impedance. This can range from about 30 ohm for high-power coax to 600 to 1000 ohm for open-wire wide-spaced line. The unit of measurement is the ohm, but you cannot simply attach an ohm-meter to coax cable to measure its impedance.

The characteristic impedance of a line is not dependent on its length but on the physical arrangement of the size and spacing of the conductors. (Remember that when simply put, impedance is the ratio of the voltage to the current. A high voltage and low current means a high impedance. A low voltage and high current means low impedance).

Loads attached to the distant end of a line have an effect on the impedance seen at the input to the line.

When a line is terminated at the distant end with a termination impedance that is the same as the characteristic impedance of the line, the input to the line will be seen to be the characteristic impedance of that line. In other words, looking in to the input of this line, you see an infinitely-long line. This is ideal for the optimum transfer of power from the transmitter down the line to the antenna.

Infinite

In this diagram, the termination is the same value as the characteristic impedance of the line. The voltage across the line is shown as E for the various points along the line and the current in the line at those same points is shown as I.

Note that the line is flat - there is no variation in the ratio of voltage to current (i.e. no variation in impedance) at any point along the line.

If there was such a thing as an infinitely long line, cutting a short length off it and terminating that short piece with a load equal to its characteristic impedance, would still make it indistinguishable at its input from an infinitely long line - as shown in the diagram above.

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Line Terminations

There are several classic cases of line termination which must be known and each will be described in turn.

Shorted

For a line with a short-circuit termination, consider this approach:

A signal starts off and travels down the line. It reaches the distant end and finds the line to be short-circuited! What can it do? It turns around and travels back to the source. So there are now TWO waves travelling on the line but in different directions - the forward wave being still sent down the line, and the reflected wave, on its way back.

At any point on the line, the voltage across the line will be the sum of these two component waves, measured using an appropriate voltmeter.

But the voltage across the line at a short-circuit must be zero. So the reflected wave must be phased in such a way that the resultant voltage at the short-circuit is zero. See the red E curve above. Coming back down the line the voltage will increase as shown in the diagram above.

Likewise, at a short-circuit the current will be high. So the current in the line must be high at the termination and will decrease as you measure it back down the line. The current will follow the blue I curve shown above.

quarterwave

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Impedance is the ratio of voltage to current. So at the load (a short-circuit) the impedance will be zero. As you travel back down the line, both E and I vary so the ratio between them is varying. When the line is one-quarter wavelength long, the impedance will be very high - approaching infinity.

A similar thing happens when the line is open-circuited:

Open

In this case, there will be a high voltage at the end of the line - the open-circuit. The current in the line must be zero there. So the impedance will be very high. Travelling back down the line, the impedance (the ratio of voltage to current) will decrease until at a quarter-wavelength point, the impedance will be seen to be zero.

Openquarterwave

The quarter-wave length of line in effect inverts the impedance at its termination. Quarter-wave lengths of line are very useful for many applications especially at VHF and UHF.

The half-wave length of line can be considered as two quarter-wavelengths in cascade and its performance can be deduced from that approach.

halfwave

The input impedance of a half-wave length of line is a repeat of the termination at the distant end.

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The Voltage Standing Wave Ratio (VSWR)

We have considered the line with a matched load, with a short-circuit termination and with an open-circuit termination. The practical values of load fall somewhere between these limits.

The VSWR (usually shortened to SWR) can be visualised by considering the forward and reflected waves in a line. If the antenna (the termination at the load end of the line) does not exactly match the line (i.e. is not exactly equal to the characteristic impedance of the line), then some energy will be reflected back down the line. So we have a forward wave (high energy) and a reflected wave (smaller than the forward wave) on the line. A pattern of peaks and troughs in the voltage measured between the line conductors will be found as you measure the voltage at points back down the line.

SWR-pattern

The SWR can be measured with a device known variously as a reflectometer or SWR bridge, or plain SWR meter.

The SWR meter is usually placed near to the transmitter. It distinguishes between the forward and reflected waves in the line. It gives an indication of whether the antenna is matched to the line by allowing the standing-wave-ratio to be measured. When inserted in the line between the transmitter and the antenna tuning unit, it also permits the antenna tuning unit to be adjusted.

See: HF Station |  Measurements

Any variations from a correct match at the antenna (or load) end of the line can have a significant effect on the power radiated by the system:

The transmitter requires a correct match (usually 50-ohm) to the line for the best transfer of energy from the transmitter to the line.

The line requires a minimum SWR for least losses, and

the match from the line to the antenna should be correct to minimise the SWR on the line.

Variations from a correct match can also have undesirable effects on a transmitter to the point of causing overheating in the final stage and arcing in tuned circuits.

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The Antenna Tuner

This is usually inserted in the transmission line adjacent to the transmitter with the transmission line to the antenna following and the antenna connected at the distant end of the line. See HF Station

The antenna tuner does not really tune the antenna at all. It does not adjust the length of the antenna elements, alter the height above ground, and so on. What it does do is to transform the impedance at the feedline input to a value that the transmitter can handle - usually 50 ohm. Think of the antenna tuner as an adjustable impedance transformer and you will understand its function.

If the antenna is cut to resonance and is designed to match the impedance of the transmitter and feedline, an antenna tuner is not required. The transmitter is presented with a 50-ohm load (or something close to it) and into which it can deliver its full output power.

The SWR bandwidth is important. The SWR bandwidth of many antenna designs is usually limited to only some 200 or 300 kHz. If a dipole is cut to resonate with a 1:1 SWR at 7 MHz, you may find that the SWR is above 2.5:1 at 7200 kHz. Most modern transceivers will begin to reduce output or may automatically completely shut down at SWR's above 2:1.

With an antenna tuner in the same line, you can transform the impedance seen by the transmitter to 50-ohm, and reduce the SWR in the short piece of line between the transmitter and the antenna tuner to 1:1 again. The transceiver then delivers its full output again. The radiated power will be slightly reduced because of the higher losses on the line between the tuner and the antenna, attenuation due to the higher line currents associated with the higher SWR on that stretch of line.

This attenuation is caused by the fact that the matching function of the tuner has not changed the conditions on the line between the tuner and the antenna.

Velocity factor

A radio wave in free space travels with the speed of light. When a wave travels on a transmission line, it travels slower, travelling through a dielectric/insulation. The speed at which it travels on a line compared to the free-space velocity is known as the velocity factor.

Typical figures are:

Twin line 0.82, Coaxial cable 0.66, (free space 1.0).

So a wave in a coaxial cable travels at about 66% of the speed of light (as an example).

In practice this means that if you have to cut a length of coaxial transmission line to be a half-wavelength long (for, say, some antenna application), the length of line you cut off will have to be 0.66 of the free-space length that you calculated.

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Baluns

A balun is a device to convert a balanced line to an unbalanced line - and vice-versa. It comes in a variety of types.

The transformer type is probably the easiest version to understand. Consider a transformer with two windings, a primary and a secondary. The primary can be fed by a coaxial cable - the UNbalanced input. The secondary could be a centre-tapped winding with the tap connected to the outer of the coaxial input cable. The two ends of the secondary are then the BALanced connections. Impedance transformation can also be made by adjusting the number of turns on the primary and secondary windings.

When a balanced antenna, such as a dipole, is directly fed with coax (and unbalanced line), the antenna currents (which are inherently balanced) will run on the outside of the coax to balance the coaxial cable currents which are inherently unbalanced. This feedline current leads to radiation from the feedline itself as well as by the antenna and can distort the antenna radiation pattern. The RF can travel back down the outside of the coax to the station and cause metal surfaces at the station to become live to RF voltages. RF shocks are unpleasant and burn the flesh. They should be avoided. To correct this, a balun should be used when connecting a balanced line to an unbalanced line and vice-versa.

Baluns are used for connecting TV receivers (75-ohm unbalanced) to 300-ohm ribbon (balanced).

Using a Single Antenna For Transmit and Receive

A lot of trouble and expense goes into erecting a good feeder and antenna system for transmitting. It should also be used for receiving. This is usually the case with a transceiver.

With a station comprising a separate transmitter and receiver, a change-over relay can be fitted to switch the antenna feeder between the two items. It is usual - and desirable - for the unit not being used to be disabled. Extra poles on this same relay can be used to disable the device not being used. HF Station



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Compiled Sun Nov 28 2010 at 8:41:44pm


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