1 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Chelmsford Amateur Radio Society Advanced Course (5) Antennas

2 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Frequency & Wavelength The size of an antenna is determined by the wavelength of operation In free space: Frequency x Wavelength = Speed of Light, ~3x10 8 m/s v = F x = 3 x 10 8 or = 3 x 10 8 / F where F is in Hertz and in metres A wavelength for 14MHz would be 3 x 10 8 / x 10 6 = metres. Therefore a half wavelength = metres and as the feed-point for a DIPOLE is in the centre, each wire should be 5.33 metres long. BUT... The velocity of a RF wave in wire is slight lower than that in free space. This means you can cut your wire approximately 5% shorter

3 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Antenna Environment The environment around an antenna affects its radiation pattern Thus commercial stations have HF/MF antennas in open countryside. Ground effects: The type of ground beneath the antenna modifies the radiation pattern. The environment affects vertical antenna’s more than horizontal ones Only a vertical polarised antenna produces a ground wave. Height above ground and its conductivity affects the radiation pattern. The better the ground conductivity the better (lower) the radiation angle. Salt Water is best, whilst urban concrete not so good, which is one reason to ensure you have a good RF earth mat.

4 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Polar Diagrams & Radiation Angle Polar diagrams give the radiation patterns for an antenna, and graphically describe the direction of radiation. The Radiation Angle is that angle at which the main lobes radiate. This angle can be affected by the height of the antenna above ground. Long distance communication requires a low angle of radiation. Only under certain circumstances will you require a high angle radiation. Vertical at ground levelVertical at 3 metres

5 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Feed Point Impedance Feed Point Impedance is the impedance measured at the point at which the feeder connects to the antenna. It is only resistive at the resonate frequency of the antenna, At all other frequencies it will have some reactive quantity. Feed point impedance for various ideal antenna’s, in the clear:- AntennaFeedImpedance Half Wave DipoleCentre fed70 ohms Quarter Wave VerticalFed at base, radials35 ohms +/- x 5/8 Vertical (¾ with loading coil) Fed at base50 ohms Folded DipolesCentre fed300 ohms Full Wave Loops (Watch These on Tx)100 ohms Quarter wave End fed15 ohms + Reactance Half wave End fedHigh + Reactance

6 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Yagi Antenna Elements Direction of Radiation Beams consist of a Reflector and Directors spaced at a fraction of a wavelength, usually fed by coax. Their presence reduce the Driven Dipole element feed point impedance to 15 to 30 ohms, which can be matched with a BALUN transformer. One matching method is to make the driven element a folded dipole whose 300 ohm impedance will be reduced to ~70/80 ohms which is easier to match to the 50 ohm feeder. Boom Feeder Driven Dipole Directors Rear Reflector -3dB +

7 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Folded Dipoles Balanced Input /2 Folded Dipoles are Dipoles with an extra conductor placed alongside and connected at the ends - equivalent to two in parallel Current is shared(halved) between the two elements for a given power which quadruples the input impedance relative to a standard dipole Folded dipoles therefore have a close match to 300 Ohm feeder The higher impedance enables elimination of Matching Transformers which otherwise would restrict bandwidth. For example:- –A 14MHz Dipole antenna exhibits up to 2:1 VSWR over the band. –A folded dipole would have a flat VSWR over band.

8 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Trap Dipoles One dipole can be turned into two or three by the addition of parallel tuned circuits called TRAPS The impedance of the traps is about 500 ohms. As X L dominates then Z = 2  FL. The trap principle can also be used in beam antennas to construct two or more bands out of one set of rod elements F1: 1 / 2 70 Ohms F2: 2 / 2

9 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas The Quad Antenna Normally used at 14MHz and above and requires a balanced feed Consists of a driven element in the shape of a square loop, placed in close proximity to a square of the same size which acts as a reflector. Can be considered as two closely stacked two element aerials which are end fed. Like a Yagi, it can have extra elements - especially at VHF Has a very low radiation angle is, making it good for HF DX. Reflector Driven Element 75 ohms Radiation /4

10 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Antenna Matching Units Antenna matching units are used to convert the impedance of the antenna system to the output impedance of the transmitter. They are installed as close to the antenna as possible with the VSWR meter as close to the Transmitter as possible. Only at resonance will the antenna system Reactance, X, be zero. The AMU therefore cancels the X part of the impedance and converts the R to a value that equals the Z of the Transmitter. Transmitter Antenna Matching Unit VSWR Meter 50 Ohm Output Feeder

11 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Matching Networks-1 Wide Impedance Matching Range with Tapped Coil can cover a good Frequency Range Good for Matching Lower band Quarter Waves Pi Network “L” Matching

12 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Matching Networks-2 Tank Matching Coax

13 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Baluns Remember BALUN = Balanced Unbalanced Many antennas are balanced devices, such as dipoles etc. Connecting a dipole to an unbalanced coax cable causes currents to flow in the outer sheath. These currents give rise to unwanted radiation which may cause EMC problems. A solution is to match the balanced antenna to the unbalanced line using a BALUN There are three basic types and we will deal with each in turn.

14 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Baluns - Basic Format Below is the basic format of The Transformer BALUN These are normally wound on ferrite cores and are used to match a balanced system such as ladder line or a dipole to an unbalanced line such as a coaxial cable. Unbalanced Coax Balanced Line

15 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Transformer Baluns Unbalanced Balanced 1:4 Transformer BALUN 6 Turns 12 Turns Total Primary turns equals Secondary Turns 1:1 Transformer BALUN Unbalanced Balanced 6 Turns 3 Turns Recall: Zp = Zs. ( Np / Ns ) 2 1:2 Turns Ratio will create a 1:4 Impedance Transformation

16 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Sleeve Balun A /4 long braided or solid extra outer conductor is located around and insulated from the coax screen and connected to the screen at the rear The high impedance of the open circuit present a low impedance to the currents on the coax screen /4 on to the rear The sleeve acts as choke coil to isolate remaining line from antenna. As it is based upon /4 on one band, this is a single band device From Tx /4 Connection to Outer Braid of Coax here

17 Chelmsford Amateur Radio Society Advanced Licence Course Carl Thomson G3PEM Slide Set 14: v1.1, 22-Aug-2006 (5) Antennas Choke Baluns Antenna Transmitter 6 to 10 turns about 25cm diameter tightly wound Current or Choke Baluns prevent current flowing on the screen of the coaxial feeder cable. Such currents may cause unwanted radiation which might lead to EMC problems. Wideband Balun using Ferrite rings From Tx