Radio Antennae

In a communication system, the antenna is usually the performance determining part. It will both have to radiate the full transmitter power as well as to pick up weak signals from distant stations to be of use. Antennae are frequency selective, and thus if they are made suitable for operation in a complete frequency band this will always be a compromise between practical use and performance. For "Line-of-Sight" communication equipment such as VHF transceivers, maximum range is obtained through installation of the antenna at the highest possible position on the vessel.

Besides design and installation of the antenna, also the cable connecting transceiver and antenna - the feed line - is an important performance issue.

Antenna Principles

An antenna is a device consisting basically of an electrical wire and designed to produce a dispersing electromagnetic field. The electromagnetic radiation is generated by the accelerating and decelerating motion of oscillating electrons in the electrical antenna wire. This oscillation is driven by introducing an alternating electrical current in the antenna structure. The amount of radiation is proportional to the current fed into the antenna (more current more oscillating electrons) and high current at low impedance can favourably be obtained in a resonant system. But the condition of resonance is not a prerequisite for radiation. Any piece of wire carrying alternating current will radiate and produce a dispersing electromagnetic wave.

Two fundamental types of antennae are the dipole antenna and the loop antenna:

While the dipole looks like a simple open circuit, and the loop a short circuit, these pieces of wire are effective radiators of electromagnetic fields when connected to AC sources of the proper frequency. The two open wires of the dipole act as a sort of capacitor (two conductors separated by a dielectric), with the electric field open to dispersal instead of being concentrated between two closely-spaced plates. The closed wire path of the loop antenna acts like an inductor with a large air core, again providing ample opportunity for the field to disperse away from the antenna instead of being concentrated and contained as in a normal inductor.
As the powered dipole radiates its changing electric field into space, a changing magnetic field is produced at right angles, thus sustaining the electric field further into space, and so on as the wave propagates at the speed of light. As the powered loop antenna radiates its changing magnetic field into space, a changing electric field is produced at right angles, with the same end-result of a continuous electromagnetic wave sent away from the antenna. Either antenna achieves the same basic task: the controlled production of an electromagnetic field.
When attached to a source of high-frequency AC power, an antenna acts as a transmitting device, converting AC voltage and current into electromagnetic wave energy.

Antennas also have the ability to intercept electromagnetic waves and convert their energy into AC voltage and current. In this mode, an antenna acts as a receiving device:

Antennas are frequency selective devices, and for optimal performance the physical dimensions must be matched to the wavelength (λ = c/f) of the electromagnetic waves they are expected to transmit or receive. Optimal electromagnetic radiation is achieved on the resonance frequency of an antenna.
For an ideal dipole, the resonance frequency is related to its length: resonance will occur when the antenna's length is approximately one half of the wavelength it is designed for: l = 1/2 λ. Resonance will also occur at a length equal to an odd multiple of the basic half-wavelength length (l = 3/2 λ, l = 5/2 λ, ...). This means also, that for a fixed length, additional higher frequency resonances will occur at frequencies that are odd numbered harmonics of the half-wavelength resonant frequency.

Radiation Resistance

If power is fed into the antenna, a small fraction will be dissipated in the resistive part of the antenna structure. The bulk of the power will usually be radiated in free space and since power can only be dissipated in a resistance, it is convenient to consider the radiated power to be dissipated in an imaginary resistance which is called radiation resistance of the antenna.
Notice that this radiation resistance is different from the antenna impedance, which is usually a complex impedance and is the electrical load of the feedline.

Polarization

The polarization of a radiated electromagnetic wave is determined by the direction of the lines of force making up the electric field component. If the lines of electric force are parallel to the surface of the Earth, the wave is said to be horizontally polarized. If the lines of electric force are orthogonal to the surface of the Earth the electromagnetic wave is vertically polarized.
If a single-wire antenna is used to receive (absorb) energy from a radio wave it should be oriented along the direction of the electric field component to extract the maximum energy. So principally, the physical orientation of receiver and transmitter antennae should be the same in order to obtain the best possible transmission results.

In the case of long distance propagation through the ionospheric layers, atmospheric reflection and refraction may introduce a degree of cross polarization which results in signals arriving at the receiving antenna with both horizontal and vertical components present. Since the atmospheric conditions are constantly changing, also the polarisation components may vary over short time spans. This is the main reason why long distance stations are received with a noticeable fading effect. If the receiving wave happen to be oriented along the receiver antenna, a maximum signal output is obtained. But after some time the orientation of the receiving wave may have rotated over 90 degrees and a minimum antenna output is obtained.
So in the case of long distance communications, the field of the radio waves rotates and has both horizontal and vertical components and is said to have circular or elliptical polarization.

The advantage of vertical polarisation (vertical antenna) is the ability of horizontally omnidirectional communication. This is good for broadcasting but in case of point to point connections, only a fraction of the transmitted power will be beamed in the direction of the receiving station. For all other directions, the transmitter will produce interfering noise. Moreover, due to the omnidirectional characteristic, a vertical receiver antenna will receive noise and interfering signals from all directions.
The advantage of horizontal polarisation is that the transmitted energy is concentrated perpendicular to the antenna (broadside). Horizontal antennae are directional and must be oriented perpendicular to the bearing of the participating station. Communication can be established with less power and less interference of other stations. Horizontal antennae beam a large part of the transmitted energy into space, but taking advantage of reflective characteristics this may be used to enable long-distance transmission over sky waves.

The Dipole Antenna

The basic dipole antenna was developed by Heinrich Rudolph Hertz around 1886. These antennas are the simplest practical antennas from a theoretical point of view. Typically, a Hertz or dipole antenna is formed by two quarter-wavelength conductors or elements placed back to back for a total length of one half of a wavelength. A standing wave on an element of a quarter-wavelength length yields the greatest voltage differential, as one end of the element is at a node (+Vac) while the other is at an anti-node (-Vac) of the wave. The dipole antenna is fed at the (isolated) junction of both quarter-wavelength conductors. At the resonant frequency, the feed-point impedance of an antenna is low. This enables to couple energy into the antenna at a low voltage (at the feeding point) and it can therefore be an efficient radiator.
The picture below shows the voltage and current along the dipole for two instances with half a period time difference.

The current in the dipole elements, is zero at the ends and will swing between the maximum values (+Iac and -Iac) in the center of the dipole (the feeding point). Note that the direction of the current (indicated by the red arrows above the dipole) at a certain instance is the same in both the dipole branches - to the right in both elements or to the left in both elements. Clearly, the antenna does not radiate uniformly along the entire length. At the ends the current (and the acceleration of electrons) is zero and thus the electromagnetic radiation will also be zero at the ends of the dipole.
Contrary to popular belief, the di-pole is so named because it has two electrical poles, not two physical poles. If the length is such that the poles are at ends of the conductor and the zeros are at the center, the antenna will be exactly one half wavelength long.

Radiation is maximal in the plane perpendicular to the dipole and zero in the direction of wires which is the direction of the current. The emission diagram is circular section torus shaped (right image) with zero inner diameter.

A dipole is most commonly fed at the center, where it presents a balanced, 72 Ohm load to the feed line. Notice that as there is a phase shift between current and voltage, the impedance is NOT purely resistive at this point! Although a dipole can be fed anywhere along its length, center fed is the most common, and the easiest because moving the feed point will also change the impedance of the dipole.

The following table shows the impedance of an example dipole antenna for some specific lengths around one half of the wavelength. Besides length, the impedance depends also on the antenna diameter and ground plane characteristics:

For a dipole length of 0.48 times the wavelength, the impedance of the antenna is purely resistive (73 Ohm), whereas for exactly 0.50 times the wavelength, the load has an inductive component and the total impedance is approximately 90 Ohm. Notice however, that only the resistive part of the dipole is dissipative and relevant for power concerns.
In practice, the length of the antenna will be fixed and will be dimensioned for a certain center frequency within a radio band. Depending on the actual transmit frequency, the antenna will then be slightly too short or too long. If the antenna is too short it will behave as a series circuit of a relative low resistance in the range of 60 to 80 Ohms and a low capacitive reactance. Whereas, if the antenna is too long, it will instead have a low inductive series reactance. The reactive part of the antenna will have to be compensated and matched to the impedance of the feeding cable by means of a passive LC (non-dissipative) - matching circuit (often referred to as "L","T" or "π" trans-match circuits).

Grounded Dipole Antenna

Ground is a good conductor for low- and medium range frequencies and acts as a large mirror for the radiated radio waves. This results in the ground reflecting a large amount of energy that is radiated downward from the antenna mounted over it. Using this effect of ground, an antenna only a quarter wavelength long can be made to act as a halve-wavelength antenna.
A halve-wavelength (dipole) antenna is theoretically the shortest antenna that can be used to radiate electromagnetic waves in free space. However, if one end of the dipole is replaced by a perfect ground plane, this antenna will also resonate at the same frequency as the ungrounded halve-wavelength antenna. Such an antenna is referred to as quarter-wavelength or Marconi antenna.

For a given wavelength the Marconi antenna is the smallest antenna with reasonable efficiency and so is a popular choice for mobile communication. It can be thought of half of a dipole with the other half appearing as a virtual image in the ground. The current in the reflected image has the same direction and phase as the current in the real antenna. Although the quarter-wavelength conductor and its image together form a half-wave dipole it radiates only in the upper half of space. For the 156 MHz marine VHF band with wavelengths of about 2m, a typical Marconi antenna is only 0.5m long.

The grounded quarter-wavelength antenna has a radiation pattern (directional characteristics) similar to that of a half-wave dipole antenna. It has a feed point impedance over a perfect ground of 36+j*21 ohm (an ungrounded half-wavelength antenna has an impedance of 73+j*43 ohm). Above real ground it is usually between 50 and 75 ohm. This makes a good match for a coaxial (unbalanced) 50-ohm cable with the shield connected to ground.

A longer mono-pole antenna can produce even lower radiation angles than the standard quarter-wavelength antenna, but eventually these antennas become too large to easily construct and handle. Moreover, on rolling and pitching yachts a low radiation angle may have an unfavourable impact on the transmission range. A length often used for VHF mobile operation is the 5/8th wavelength. Theoretically, a gain of 3dB can be obtained compared to the grounded quarter-lambda antenna. This 5/8th wavelength antenna has a higher feed impedance and requires a matching network to match most feeder cables.
Vertical quarter-lambda antennas require a good highly conductive ground. If the natural ground conductivity is poor, quarter-wavelength copper wire radials can be laid out from the base of the vertical to form a virtual ground.

Vertical antennas provide an omni-directional pattern in the horizontal plane so they receive and transmit equally well in all directions. This also makes them susceptible to noise and unwanted signals from all directions.

Effects of a Finite Size Ground Plane on the Monopole Antenna

The simplest monopole antenna consists of a vertical element with an optimal length λ/4 on the ground plane. The radius and the shape of the ground plane will affect the radiation pattern and input impedance of the monopole antenna (usually r >>λ/4).

In practice, monopole antennas are used on finite-sized ground planes. This affects the properties of the monopole antennas, particularly the radiation pattern. The impedance of a monopole antenna is minimally affected by a finite-sized ground plane for ground planes of at least a few wavelengths in size around the monopole. However, the radiation pattern for the monopole antenna is strongly affected by a finite sized ground plane. The resulting radiation pattern radiates in a "skewed" direction, away from the horizontal plane (larger "take-off" angle).

Effects of Diameter Size on the Monopole Antenna

The monopole diameter will affect the antenna bandwidth. A larger diameter will result in a larger bandwidth.

Gain or Directivity

Antenna gain is not gain of power, but rather a refocusing of energy from all directions to a specific direction (directivity of the antenna). Antenna gain can be obtained by changing the physical dimensions of the antenna (longer antennae give higher gain). The gain is expressed in decibel (dBi), with a fictitious antenna with a perfect omni-directional radiation pattern as reference (isotropic antenna).

An antenna that does not radiate equally well in all directions will have a gain compared to the theoretical omni-directional antenna. A halve-wave dipole does not radiate in the longitudinal direction of the antenna structure and has a resulting gain of 2.15 dBi compared to an isotropic antenna. Vertical dipole and Marconi antennas are in fact quite directional in the vertical plane and omni-directional in the horizontal plane. The figure on the left shows the main lobes of the radiation pattern of two antennas of different length: The 1/4-wavelength antenna has the higher angle of radiation (about 26 degrees) however, it also has the lowest gain and thus the lowest horizontal range. The 5/8 wavelength antenna has a lower angle of radiation (about 16 degrees), a higher gain and thus a higher horizontal range.

Ground Plane

The vertical quarter-wavelength antenna works well only when placed over a good ground system. Poor ground causes antennae to operate less efficient. In the radiation pattern the main RF power lobe will be directed towards the ground plane. It is possible, to lose up to 90% of the RF power by heating the space under the main radiation lobe, instead of transmitting the energy as electromagnetic wave.
The overhead to create a good ground plane is proportional to the wavelength of the RF signal used. Especially for MF radio frequencies, used for long range communication and featuring high RF power levels, the requirement for a high-quality ground plane is high but normally it will be hard to realize. However, for marine applications, the ocean and coastal waters form an almost ideal ground plane. With a good antenna, this allows long range communication with considerably less RF power than would be required on land simply because a good ground plane for MF frequencies is hard to realize there.
It should be noticed, that ground losses act as an additional dissipative load and will result in less power being radiated by the antenna. This situation can be frustrating, for the SWR appears to be good. But a low SWR to a 50 Ohm dummy load looks great too. So keep in mind that an SWR reading is NOT a measure for the antenna's efficiency, it only gives a relative indication of the achieved match between transmitter and antenna (including the feed line).

Antennae for VHF transceivers

Hand-held sets for VHF have an attached flexible rubber-covered antenna of only about 20 cm - not very efficient, but quite convenient. Keep in mind that for optimal transmission performance the orientation of transmitting and receiving antennae must be identical. By construction, most hand-held VHF radios will transmit a vertically polarised radio wave and also fixed VHF antennae are normally mounted in a vertical way. External antennae for VHF radios come in three different sizes. VHF waves are not reflected from the ionosphere, so communications are limited to the visible horizon plus a small percentage of additional range. Range can be increased by "squeezing down" the radiated energy that would normally go upward at an angle closer to the horizon down toward the surface of the water, concentrating and increasing the power level in a useful direction. The extent to which this is done, compared to the ideal dipole is called antenna gain and is expressed in decibels (dBd = Gain over a dipole). Common types are 3 dBd, 6 dBd and 9 dBd, which are roughly 1.5 meters, 2.7 meters and 6.3 meters in length. The 3-db antenna is ideal for small boats that stay close to shore or to other boats, because the wide-angle pattern is unaffected by rolling or pitching in waves. This type of antenna is also the most commonly installed at the top of sail boat masts, since it allows radio transmissions even when the boat is heeled over. The drawback to a 3-db antenna is that the range is reduced. Concentrating the power by about four times, the 6-db is the most popular antenna for mid-sized boats. It allows for some rolling while still giving a good performance in distance. The 9-db antenna produces a much flatter pattern which in theory should be usable only on large stable yachts, or aboard smaller boats that operate on smooth waters at all times. In reality, however, the more powerful signal (better horizontal concentration) of the 9 db antenna is still going to outperform the smaller antennas even in rough seas. Besides the antenna selection also the mounting should be carefully considered. The antenna is a vital part of the communication equipment, which is mounted outdoor where it is permanently affected by wind and salt water conditions. Mounting gear should be of high-quality corrosion-resistant material and well dimensioned for the length and weight of the antenna.

Just as important as the antenna is the cable connecting your VHF radio to the antenna. VHF radios use a special coaxial cable that is really two cables, one inside the other. The central conductor goes to the antenna, while the braided outer conductor (which is insulated from the inner) goes to the ground as well as serving to keep radio energy from escaping between transmitter and antenna. The best type of coax has a stranded copper center conductor, with a high percentage (such as 80 to 90 percent) of copper, while cheaper cables will have a lower amount of copper.

Reduced radio performance can be caused by invisible cracks in the coaxial cable, by corrosion in or around the end fittings, by moisture seeping into the cable, or by using a cable, which is too long.

The common marine installation uses RG-58 cable for short runs of up to 6 meters. For a longer run, such as to the top of a mast, RG-8 cable can be used. RG-8 is a heavier cable and requires special tools to replace the coax fittings at the ends, but it has lower loss per length. For a little extra money RG-213 cable can be installed, which is a military specification cable that has a minimum signal loss like the RG-8 but with an abrasion and ultraviolet proof jacket for long life.

Antennae for HF-SSB transceivers

Antennae for MF-HF sets using single-sideband are physically much larger than those for VHF, but still not as large electrically as they should be. Vertical whip antennae are 8 meters or more in length and still must have an antenna tuner box to make up for a lack of physical length to properly match the transceiver.
For sailboats, it is possible to insert insulators near the top and bottom ends of a backstay and use the electrically isolated portion as the antenna.

A further essential element of a SSB installation is a good "ground plane" connection, which is not necessarily needed for a VHF areal (mainly because the reach is limited by line-of-sight distance rather than the transmission characteristics of the antenna).

Antenna Length

The length of the antenna must be considered in two ways. It has both a physical and an electrical length, and these two are never the same. The reduced velocity of the radio wave on the antenna and a capacitive effect knows as "end effect" make the antenna seem electrically longer than it is physically. The contributing factors for this difference are the ratio of the diameter of the antenna to its length and the capacitive effect of terminal construction (insulators, clamps, ...) used to support the antenna.
A correction factor of 0.95 can be used for frequencies in the HF range. So the physical length of a quarter-wavelength HF antenna for a giveg frequency f is given by:

  length [m] = 1/4 * 300/f * 0.95             with f expressed in MHz

Antenna Performance

For optimal transmission of the transmitter HF power to the antenna, the load impedance of the antenna has to be tuned to the transmitter output impedance. Antennae can normally not be mechanically (length) tuned to the transmission frequency. But they can be electrically tuned with a "matchbox". A matchbox is an electrical device including a bank of switchable impedances. The matchbox is installed directly at the feeding point of the antenna and is usually controlled electrically from the transmitter. The tuning must be performed each time the transmission frequency is changed. The tuning can be initiated manually or automatically by the transmitter.

One measure of an antennas performance is the standing wave ratio (SWR), which is usually included on most specification sheets so you can compare them. An SWR of 1.5:1 is good, but you're losing energy when you get up into the 2.5:1 or 3:1 areas.

The transmitter must be electrically matched to the antenna so that the antenna will absorb the transmitter power and radiate it into the atmosphere. This is most often accomplished with a microprocessor controlled automatic antenna coupler or "tuner". The coupler varies the "electrical length" of the vertical radiator wire by switching various capacitors or inductors contained in the coupler into the circuit between the radio and the antenna wire. This allows a wide range of operating frequencies to be served by a fixed length antenna. Some couplers provide a readout of the quality of the "match" achieved between the transmitter and the antenna. This is expressed in terms of standing wave ratio, SWR, a ratio of unity, 1.0, is optimum.

Another issue is the matching of the antenna balance to the feed line balance by means of a BalUn. A balanced two-terminal impedance has neither of its terminals connected to ground, whereas an unbalanced impedance has one of its terminals connected to ground. When feeding a balanced antenna such as a half-wavelength dipole, a balanced feed line MUST be used. Conversely, when feeding an unbalanced antenna such as a grounded dipole, an unbalanced feed line MUST be used (e.g. a coax line). When it is necessary to mix balances, a so called BalUn must be used. By definition, a BalUn is a device which transforms balanced impedance to unbalanced and vice versa. In addition, baluns can provide impedance transformation, thus the name BalUn Transformer. The BalUn is basically an HF transformer that is normally incorporated into the design of the matching network since it can also perform the task of matching the antenna impedance to the characteristic feed line impedance. Especially for broadband MF/HF receivers, it is impractical to use full half-lambda or even grounded quarter-lambda dipole antennae. For the 3-MHz band such an antenna still has a length of 25 meters. Shorter antennae can be used without much performance penalty in terms of sensitivity, but shorter antennae have a much higher impedance that the optimally designed quarter-lambda reference antenna. The feeder coax cable, as well as the receiver RF-input have an impedance in the range of 50 Ohm.

So the high impedance of the antenna must be compensated with an integrated amplifier (impedance transformer) directly at the feeding point of the antenna. Because this feeding point is usually outdoor, the antenna amplifier is usually fed through the antenna coax cable.

Antenna Impedance Matching

Impedance matching is important to achieve maximum power transfer between two devices such as a feed line and an antenna. A transformer is a passive device that transforms or converts a given impedance, voltage or current to another desired value. In addition, it can also provide DC isolation, common mode rejection, and conversion of balanced impedance to unbalanced or vice versa.

An HF transformer usually contains two or more insulated copper wires twisted together and wound around or inside a core, magnetic or non-magnetic. Depending on design and performance requirements, the core can be binocular or toroid. Wires are welded or soldered to the metal termination pads or pins on the base. The core and wire ensemble is housed in a plastic, ceramic or metal case.

At low frequencies, an alternating current applied to one winding (primary) creates a time-varying magnetic flux, which induces a voltage in another winding (secondary). At high frequencies, the inter-winding capacitance and magnet wire inductance form a transmission line which helps propagate the electromagnetic wave from the primary to the secondary winding. The combination of magnetic coupling and transmission line propagation helps the transformer to achieve outstanding operating bandwidths (1:10000 or more).

If N1 and N2 are number of turns and V1 and V2 are voltages at the primary and secondary, respectively, the following relations are valid:

  n  = N1/N2
  V2 = n * V1
  I2 = I1 / n
  Z2 = n2 Z1

It shows that the output voltage (V2) is equal to turns ratio (n) times the input voltage (V1). The output current (I2) is input current (I1) divided by the turns ratio and the output impedance (Z2) is input impedance (Z1) multiplied by the square of the turns ratio.
The phase equivalence is marked with a dot on one of the connectors on both primary and secondary windings. If primary current increases on the dotted end of the primary winding, the current out of the dotted end of the secondary winding will also increase. So at the dotted ends the input and output voltages are in phase (at low frequencies, neglecting the small insertion phase).