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Syllabus Sections:-

5c Antennas

5c.1 Recall the equation for calculating half-wavelengths and be able to apply 'end factor correction' in calculating the approximate physical lengths of dipole elements.

As a memory jogger let's show you some of the information you have learned in previous courses.

From the Foundation Licence Course came the Frequency to wavelength chart click here to check back.

From the Intermediate Licence Course came the formula v = f relating speed of light v to frequency f and wavelength click here to check back.

v = f

The velocity of light = the frequency x wavelength

and you know that the over all length of a dipole is 1/2 long.

so

so the EQUATION for calculating the length of a half wave dipole is derived from the one above and divided by 2 ( as it is a 1/2 long ):-

Example. What is the length of a dipole for 3.6MHz ?

( 300/3.6 )/2 = 41.66m

'End factor correction'

But that is not the end of the story as this Advanced syllabus introduces 'end factor correction'.

The equation above would be correct for an aerial in free space, however as we are operating from near to the ground the half wavelength aerial will not be exactly equal to the half wavelength depending upon several factors:-

  • the thickness of the wire aerial relative to the wavelength in use.

  • capacitance added to the end due to the use of of insulators.

These items are all taken into account in what is called the "K" Factor which the aerial length calculated above must be multiplied by to give you a "better" approximation as to the length of the aerial.

for wire aerials of wavelengths up to 30MHz the K factor can be taken as 0.95

From above length of aerial is 41.66m

Apply the K factor 41.66 x 0.95 = 39.57m Overall length.

This can then be your starting point to prune the aerial to resonance at the frequency of operation.

So in addition to knowing the formula v = f x and rearranging to the aerial calculation as shown above you must know the "K" factor of 0.95 for wire aerials.

5c.2 Recall that the angle at which the propagated radio wave leaves the antenna is known as the (vertical) angle of radiation and that longer distances require a lower angle of radiation.

Recall the effect of the ground on the angle of radiation.

When a radio signal leaves and antenna, a dipole for instance the radiation takes up a "pattern" as was explained in the Intermediate Course click to check back.

In the Intermediate Course detailed explanation as to height above ground etc was not explained so this is where the Advanced Course develops your understanding.

Ground Effect

The dipole whilst not directional in the same sense as a yagi does not radiate off it ends but only off its sides. The ground has an effect (Ground Effect) of re-enforcing the signal from the aerial which is affected by the closeness or other wise of the radiating element to the ground.

Ground Effect on angle of radiation

If the antenna is low to the ground then the reflection will be at a higher angle than if the antenna was at a higher position above the ground. Although you cannot have a point source the diagram below shows a single radiation point and considers just a single radiated wave.

Low angle of radiation longer distances

You can see that the higher the signal source the lower the "VERTICAL ANGLE OF RADIATION". To be able to achieve long distance communication the angle of vertical radiation wants to be as low as possible and for general use this means AT LEAST half a wavelength at 14MHz but preferable three-quarter to one wavelength ( one wave length @ 14MHz = 21.4 m ) above the ground.

It should be noted that as the ground does not have perfect conductivity that different ground conditions will result in different angles of radiation for the same radiation point above the ground surface.

5c.3 Recall the current and voltage distribution on the dipole and /4 ground plane antennas

When RF energy is passes via the feeder line to a dipole antenna it develop both voltage and current distribution as shown below.

This indicates that there is a high voltage point at the end of the dipole but zero volts at the centre.

The ground plane antenna is derived from the half the dipole and has the same characteristic distribution of voltage and current from one end of the dipole to the centre.

In the ground plane the centre of the dipole is at the bottom so voltage is zero and current at a maximum.

Recall the feedpoint impedances of half-wave dipoles, quarter-wave and loaded 5/8 vertical, folded dipoles, full-wave loops and end fed /4 and /2 antennas.

/2
dipole

/4
vertical

5/8
loaded
vertical

folded dipole

full-wave
loop

/4
end fed

/2
end fed

Feed Impedance of the various antennas

50 to 75 ohms

about 37 - 50 ohms

50 ohms

300 ohms

approx 100 ohms

Low Impedance

High Impedance

Recall the effect of passive antenna elements on feed point impedance and the use of folded dipoles in Yagi antennas.

In the yagi only one element is driven the other elements are called passive elements as they only have a passive effect on the performance rather than the active of the driven element.

The addition of the passive elements, directors and reflector, to the driven element, which is usually a dipole, is to lower the feed impedance. To bring the impedance back up to a level where it can be fed with coax a folded dipole is used as the driven element. If you look to the diagram above you can see that the folded dipole has a higher feed impedance than the dipole.

5c.4 Identify folded and trap dipoles and quad antennas in addition to those in earlier syllabuses.

The folded dipole

The folded dipole is a dipole with an additional /2 element along side the original with the ends joined to make a continuous loop. You can see a diagram representation above.

The trap dipole

The trap dipole is a dipole of /2 length of the lowest frequency of operation and then a trap placed in leg at the ends of the /2 length of the higher frequency that the aerial is to work on.

Quad antenna

The Quad antenna was originally developed by W9LZX in 1942. It is a simple beam antenna consisting to two elements one driven and one reflector or parasitic reflector as it is known. Each side is /4 long. With the feed point at the bottom the antenna exhibits horizontal polarization ( just like a dipole) but if the feed is moved to the side then the antenna exhibits vertical polarization. The quad has a feed impedance of about 75 ohms.

Such antenna can be nested so that higher frequency elements can be positioned their correct distance apart (driven to reflector) and being smaller would sit nicely inside the lowest frequency quad.

5c.5 Recall that an antenna trap is a parallel tuned circuit and understand how it enables a single antenna to be resonant and have an acceptable feed-point impedance on more than one frequency. Recall that this technique may be extended to multi-element antennas such as Yagis.

Trap is a parallel tuned circuit

The trap used in the antenna is a parallel tuned circuit which is resonant at the higher of the two operating frequencies. The trap would not be left exposed but would be covered in a non conducting cover unless that outer covering was the capacitor part of the trap.

So whilst the full length of the antenna is used for the lower frequency and the trap just acts as a small inductance (but otherwise does not have any effect), when the higher frequency is used the trap effectively cut off the antenna at the trap thereby electrically shortening the antenna to the resonance of the higher frequency. By doing this the feed impedance does not change form that of the normal dipole as electrically it is just like having two dipoles and a single feeder is all that connects the antenna to the station system

Multi Band HF Yagi

For HF the same idea is used to make two band and three band YAGI and as such these antennas are called "Dual band" yagi or "Tri band" Yagi. The lowest frequency used in such antennas is usually 14MHz due to the size required to make any lower frequency dipole being so massive to accommodate the driven element reflector and director.

In the same way that traps were used for the dipole traps are also used on the reflector element, driven element, and director element of the beam.

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