Antennas

Question File Number 27



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Wavelength and Frequency

A useful and fundamental measurement in radio antenna work is the half wavelength. We must know how to calculate it. It gives the desired physical length of an antenna for any operating frequency.

Wavelength, frequency, and the speed of light, are related. The length of a radio wave for a given frequency when multiplied by that operating frequency, gives the speed of light.

formula

Knowing that the speed of light is c = 3 x 108 metres per second, and knowing our operating frequency, we can derive the wavelength of a radio wave by transposition as follows:

Wavelength (in metres) = 300 divided by the frequency in MHz.

A simple way to remember this is to remember 10 metres and 30 MHz, (to get the value of the constant, 300).

That gives a wavelength. The half-wavelength of a wave is half of the wavelength figure you obtain.

So a half-wavelength at 10 metres (30 MHz) will be 5 metres. The amateur 10 metre band is 28 to 29.7 MHz so a half-wavelength for that band will be a little longer than 5 metres. Pick a frequency and calculate it.

Dipoles

The fundamental antenna is the dipole. It is an antenna in two parts or poles.

It is usually a one-half wavelength in overall length and is fed at the middle with a balanced feedline. One side of the antenna is connected to one side of the line and the other to the remaining side either directly or through some sort of phasing line.

When making a half-wave dipole for HF frequencies, one usually has to reduce the length by about 2 percent to account for capacitive effects at the ends. This is best done after installation because various factors such as the height above ground and other nearby conducting surfaces can affect it.

The feedpoint impedance of a half-wave dipole, installed about one wavelength or higher above ground (i.e. in free space), is 72 ohm. When the ends are lowered (i.e. into an inverted V), the impedance drops to around 50 ohms.

The ends of the antenna should be insulated as they are high-voltage low-current points. The connections of the feedline to the antenna should be soldered because the centre of the dipole is a high-current low-voltage point.

The radiation pattern of a dipole in free space has a minimum of radiation in the direction off the ends of the dipole and a maximum in directions perpendicular to it. This pattern degrades considerably when the dipole is brought closer to the ground.

Folded-dipole

A modified version of the simple dipole is the folded dipole. It has two half-wave conductors joined at the ends and one conductor is split at the half-way point where the feeder is attached.

If the conductor diameters are the same, the feedpoint impedance of the folded dipole will be four times that of a standard dipole, i.e. 300 ohm.

The Height Above The Ground

The height of an antenna above the ground, and the nature of the ground itself, has a considerable effect on the performance of an antenna and its angle of radiation. See Propagation

The Physical Size of a Dipole

A wire dipole antenna for the lower amateur bands is sometimes too long to fit into a smaller property. The antenna can be physically shortened and it can still act as an electrical half-wave antenna by putting loading coils in each leg as shown in this diagram. With careful design, performance in still acceptable.

Loadcoils

Installing such loading coils lowers the resonant frequency of an antenna.

Multi-band Dipoles

A simple half-wave dipole cut to length for operation on the 40m band (7 MHz) will also operate on the 15m band without any changes being necessary. This is because the physical length of the antenna appears to be one-and-one-half wavelengths long at 15 metres (21 MHz), i.e. three half-wavelengths long.

A dipole antenna can be arranged to operate on several bands using other methods. One way is to install traps in each leg.

Trapdipole

These are parallel-tuned circuits as shown in this diagram (enlarged to show the circuitry). The traps are seen as high impedances by the highest band in use and the distance between the traps is a half-wavelength for that band. At the frequencies of lower bands, the traps are seen as inductive and the antenna appears as a dipole with loading coils in each leg. With clever and careful design, operation becomes possible on a range of amateur bands.

Baluns

Dipoles should be fed with a balanced line. This is discussed at Transmission Lines

Vertical antennas

The simplest vertical is the Marconi which is a quarter-wave radiator above a ground-plane. It has a feedpoint impedance over a perfect ground of 36 ohm. Above real ground it is usually between 50 and 75 ohm. This makes a good match for 50 ohm cable with the shield going to ground. For a given wavelength it 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.

Gndplane

A longer antenna can produce even lower radiation angles although these antennas become a bit large to easily construct. A length often used for VHF mobile operation is the 5/8th wavelength. This length has a higher feed impedance and requires a matching network to match most feeder cables.

Vertical antennas require a good highly conductive ground. If the natural ground conductivity is poor, quarter-wave 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.

Vertical antennas are often used by DX operators because they produce low angle radiation that is best for long distances.

Beams

To improve signal transmission or reception in specific directions, basic elements, either vertical or horizontal, can be combined to form arrays. The most common form is the Yagi-Uda parasitic array commonly referred to as a Yagi array or beam.

Yagi2

It consists of a driven element which is either a simple or folded dipole and a series of parasitic elements arranged in a plane. The elements are called parasitic because they are not directly driven by the transmitter but rather absorb energy from the radiated element and re-radiate it.

Usually a Yagi will have one element behind the driven element (called the reflector), and one or more elements in front (called the directors). The reflector will be slightly longer than the driven element and the directors will be slightly shorter. The energy is then concentrated in a forward direction.

To rotate the beam, the elements are attached to a boom and in turn to a mast through some sort of rotator system.

Other antenna types can be constructed to give directivity. The size and weight, with wind resistance, are important. The cubical quad is a light-weight antenna for home-construction and it can provide good performance. It consists of two or more square wire cage-like elements.

Antenna Measurements

Most antenna performance measurements are given in decibels. Important figures for a beam antenna are the forward gain, front-to-side ratio, and front-to-back ratio.

Forward gain is often given related to a simple dipole. For example, if the forward gain is said to be 10 dB over a dipole, then the radiated energy would be 10 times stronger in its maximum direction than a simple dipole.

Another comparison standard is the isotropic radiator or antenna. Consider it to be a theoretical point-source of radio energy. This is a hypothetical antenna that will radiate equally well in all directions in all planes - unlike a real vertical antenna which radiates equally well only in the horizontal plane. A dipole has a 2.3 dB gain over the isotropic radiator.

A front-to-back ratio of 20 dB means that the energy off the back of the beam is one-hundredth that of the front. Similar figures apply to the front-to-side ratio.

Another antenna measurement is the bandwidth or range of frequencies over which the antenna will satisfactorily operate. High gain antennas usually have a narrower bandwidth than low gain antennas. Some antennas may only cover a narrow part of a band they are used in while others can operate on several bands. Other antennas may be able to operate on several bands but not on frequencies in-between those bands.

Dummy Loads

A dummy load, or dummy antenna, is not really an antenna but is closely related to one. It is a pure resistance which is put in place of an antenna to use when testing a transmitter without radiating a signal.

Commonly referred to as a termination, if correctly matched to the impedance of the line, when placed at the end of a transmission line it will make the transmission line look like an infinite line.

Most transmitters are 50 ohm output impedance so a dummy load is simply a 50 ohm non-inductive resistor load. The resistor can be enclosed in oil to improve its power-handing capacity. The rating for full-power operation may be for only a short time so be aware of the time and power ratings of your dummy load before testing for long periods at full power. The things can get very hot!

Refer to: HF Station



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Compiled Sun Nov 28 2010 at 8:41:44pm


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