Tiiti Kellomäki, OH3HNY

1.Decibel

2. Transmission lines, SWR 3. Antenna properties 4. Some common antennas

The decibel

Decibel scale

• All circuits either amplify or attenuate power. A = P o /P i = 100000 B = P o /P i = 0.0002

• As the power ratios can be very large or very small, it is handy to use a logarithmic scale, the decibel scale.

A(dB) = 10 log P o /P i = 10 log 100000 = 50 dB B(dB) = 10 log P o /P i = 10 log 0.0002 = –37 dB • Positive decibel readings indicate gain and negative values attenuation.

Decibel calculation

• Multiplication of power ratios corresponds to addition of decibel values.

• If the first block amplifies by 3 dB and the second one attenuates by 10 dB, the whole circuit has an effect of –7 dB, or the output is one fifth of the input.

P o /P i = P o /P a or –7 dB.

∙ P a /P i = 2 ∙ 0.1 = 0.2,

Decibel units of measure

• Decibel readings always mean power related to some known level, e.g. input power, carrier power, or noise level. • A commonly used dB unit is dBm "decibels over one milliwatt". One watt in dBm is 10 log 1 W / 1 mW = 10 log 1000 = 30 dBm.

• One milliwatt is no more than one milliwatt, hence the ratio is one and 1 mW in dBm is 10 log 1 = 0 dBm.

Rules of thumb

• Negative decibels are for attenuation, positive for amplification.

• Adding 0 dB is the same as multiplying by one.

• Adding 10 dB is multiplication by 10.

• Adding 3 dB is multiplication by 2.

• 7 dB = 10 dB – 3 dB, or 10·½ = 5.

• 24 dB = 10 dB + 10 dB + 10 dB – 3 dB – 3 dB, or 10·10·10/2/2 = 250.

Transmission lines

Transmission lines

• Because the wavelength of a RF signal is short (say, 80 m to 23 cm in your normal frequency range, or millimeters), cable lengths are large in terms of wavelengths.

• Signal voltage level varies rapidly in time and space. • Wires must be thought of as transmission lines.

Characteristic impedance

• Characteristic impedance tells the ratio of voltage to current (or electric to magnetic field) on the line.

• On a 50-ohm line, a 1-volt signal will be a 20-milliampere signal.

• This impedance is not related to loss. • 50 ohms is most often used in amateur radio. 75-ohm coaxial cable is used in tv networks.

Mismatch

• If the characteristic impedance of the transmitter, the cable, and the antenna are not the same, power is not delivered properly.

• Power is reflected in every impedance discontinuity.

• At some points, the forward and reflected voltages will add, and at some points cancel each other. Hot and cold spots are formed on the cable.

Standing wave ratio

• Standing wave ratio is the radio of the maximum and minumum voltage on the line.

• SWR = 1 means that there is no standing wave, thus no reflections, and all the power is delivered properly.

• SWR = 2 means that 10 % of the power is reflected at the end.

• SWR = 3 means that 25 % of the power is reflected at the end.

• SWR = ∞ means that no power is delivered to the load.

• To avoid reflections, impedance can be matched so that the transmitter sees a 50-ohm load.

Coaxial cable

• Coaxial cable is formed of two coaxial tubes. • Between the outer and the inner conductor there is some dielectric material.

• All electric and magnetic fields are inside the cable.

• Coaxial cable is not affected by nearby metallic objects or slight bending.

• 50-ohm coax is the most common transmission line used by radio amateurs.

Two-wire lines

• The electric and magnetic fields of a two-wire transmission line spread into the surroundings of the line. • Two-wire lines radiate and cannot be used above HF. • Nearby metallic objects affect the performance of two wire lines. • In twinlead, there is insulating material between the wires. Its characteristic impedance is often 240 or 300 ohms.

• Open wire is essentially air-insulated and of 450 ohms.

Symmetry in transmission lines

• If one of the conductors on a line can be thought of as a ground, the line is unbalanced.

• Coaxial cable is unbalanced.

• If the lines are identical, the line is balanced.

• Two-wire lines are balanced.

• Symmetrical antennas need to be connected to symmetrical lines or else a balun (balanced-to unbalanced transformer).

Antennas

Antennas

• Antenna is the part of an electromagnetic system that either transforms energy from current and voltage into electromagnetic radiation or vice versa.

• An antenna has the same properties regardless of whether it is used as a transmitting or a receiving antenna.

Antenna as a load

• When an antenna is connected to a transmitter, the radio sees it as a load, an impedance.

• The antenna input impedance is strongly dependent on frequency.

• At resonance the impedance is real (resistance).

• Antenna bandwidth may be defined as the bandwidth where the reflected power due to impedance discontinuities is sufficiently small, or in other words, the SWR is small enough (say, 2 or 3).

Some useful antenna properties

• Radiation pattern is a plot of radiated power in different directions.

• An isotropic antenna radiates equally in all directions. • Directivity tells how efficiently the antenna radiates in the maximum direction, compared to the isotropic antenna (dBi).

• Directivity can also be expressed in dB compared to a half-wave dipole (0 dBd = 2 dBi).

Some useful antenna properties

• Gain is the same thing but multiplied by efficiency. It is the property of a real antenna.

• Gain is also expressed in dBi or dBd.

• Efficiency tells the ratio of radiated power to power delivered to the antenna. If the efficiency is 73 %, 27 % of the delivered power is lost as heat because the antenna is made of lossy materials. • If the directivity is 10 dBd and efficiency 50 %, the gain is 10 dBd – 3 dB = 7 dBd (one half of the original).

Dipoles

• A half-wave dipole is the simplest antenna.

• The dipole is commonly used in HF.

• You need a balun to feed it properly.

• A half-wave dipole has an input impedance of 73 ohms, so you can connect it directly to a 50-ohm radio.

• The bandwidth covers any one HF band.

• The half-wave dipole gain is 2 dBi = 0 dBd (small).

• A dipole can be of any length, e.g. l , 2 l … • Dipoles are balanced.

Monopoles

• A monopole is half a dipole used with a ground plane. • The monopole and its mirror image form a dipole.

• Common sizes are l /4 (gain 0 dBd), 5 l /8 (gain 2 dBd), ½ l and one l .

• The longer the monopole, the higher the gain.

• Monopoles are commonly used in HF, VHF, UHF, and in handheld devices.

• Monopoles are unbalanced.

Images of monopoles and dipoles

5 l /8 radiation pattern l /4 radiation pattern dipole radiation pattern

Loops

• The most common loop size is one wavelength.

• The loop radiates in the direction of the hole.

• Loops are used in HF.

• The exact shape of the loop is not important, just set up some wire in trees.

• The loop has a small gain, 1 dBd.

• Loops are balanced.

radiator boom

Yagis

directors • The Yagi-Uda antenna is a dipole with some additional elements.

reflector • The (half-wave) dipole part radiates, behind it is a long reflector element, and in front are one or more directors.

• The yagi is a directive antenna with a gain of 2 to 20 dBd.

• To achieve more gain, the number of elements must be increased.

Yagis

• Yagis are used in HF above 14 MHz, and up to 1200 MHz.

• Yagis need to be carefully designed and fed.

• Two or more yagis can be stacked in order to achieve gain: – if you place them side by side, the horisontal lobe will narrow, – one upon the other narrows the vertical lobe.

Quads

• A quad is like a yagi but formed of loops instead of dipoles.

• You can place loops of different sizes within each other without problems.

• Quads are used in HF and VHF.

• A quad may have a gain of 4 to 10 dBd.

Helices

• A helix can be used in two ways: – if the diameter is small compared to a wavelength, the helix will radiate like a dipole, – if the diameter is a large fraction of a wavelength, the helix radiates axially and its polarisation is circular.

• The former type is used in handheld radios.

• The latter is used in UHF and up.

Reflector antennas

• To achieve large gains, reflector antennas can be used. • The diameter of the reflector must be several wavelengths.

• Reflector antennas are used in microwave frequencies, SHF.

• A gain of 30 dBi may be achieved.

Exam questions

You are constructing a 432 MHz transmitting antenna. Which one is correct?

• ? 43 cm half-wave dipole • ? 34 cm half-wave dipole • ? 43 cm 5/8-wavelength monopole • ? 34 cm monopole and ground plane

Exam questions

You are constructing a 432 MHz transmitting antenna. Which one is correct?

• – 43 cm half-wave dipole • + 34 cm half-wave dipole • + 43 cm 5/8-wavelength monopole • – 34 cm monopole and ground plane

More exam questions

Coaxial cable has the property • ? not to radiate because the electromagnetic field stays inside the cable • ? its characteristic impedance is affected by the distance between the conductors • ? it can be mounted on a metallic roof • ? it can be bent with a sharp radius (minimum radius 5D)

More exam questions

Coaxial cable has the property • + not to radiate because the electromagnetic field stays inside the cable • + its characteristic impedance is affected by the distance between the conductors • + it can be mounted on a metallic roof • – it can be bent with a sharp radius (minimum radius 5D)

Questions?

• Feel free to ask further questions.

• Check the nice antenna books at the club room: – ARRL Antenna Book – Antennisuunnittelu-kurssin pruju – Simple and Fun Antennas for Hams