Chelmsford Amateur Radio Society
Advanced Course
(5) Antennas
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
1
Frequency & Wavelength
• The size of an antenna is determined by the wavelength of operation
• In free space:
Frequency x Wavelength = Speed of Light, ~3x108m/s
v = F x  = 3 x 108
or
 = 3 x 108 / F
where F is in Hertz and  in metres
• A wavelength for 14MHz would be 3 x 108 / 14.05 x 106 = 21.35 metres
• Therefore a half wavelength = 10.67 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 slightly lower than that in free space
• This means you can cut your wire approximately 5% shorter
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
2
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 is not so good, which is one
reason to ensure you have a good RF earth mat
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
3
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 level
Chelmsford Amateur Radio Society
Advanced Licence Course
Vertical at 3 metres
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
4
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 resonant 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:Antenna
Feed
Impedance
Half Wave Dipole
Quarter Wave Vertical
5/8 Vertical (¾ with loading coil)
Folded Dipoles
Full Wave Loops (Watch These on Tx)
Quarter wave
Half wave
Centre fed
Fed at base, radials
Fed at base
Centre fed
70 ohms
35 ohms +/- x
50 ohms
300 ohms
100 ohms
15 ohms + Reactance
High + Reactance
Chelmsford Amateur Radio Society
Advanced Licence Course
End fed
End fed
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
5
Yagi Antenna Elements
Rear
Reflector
Driven
Dipole
Directors
Direction of
Radiation
Feeder
+
-3dB
Boom
• 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
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
6
Folded Dipoles
/2
Balanced Input
• 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.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
7
Trap Dipoles
F1:  1 / 2
F2:  2 / 2
70 Ohms
• 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 XL 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
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
8
The Quad Antenna
/4
/4
Reflector
Radiation
75 ohms
Driven
Element
• 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, making it good for HF DX
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
9
Antenna Matching Units
Transmitter
50 Ohm
Output
VSWR
Meter
Antenna
Matching
Unit
Feeder
• 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
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
10
Matching Networks-1
“L” Matching
Pi Network
• Wide Impedance Matching Range
with Tapped Coil can cover a
good Frequency Range
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
• Good for Matching
Lower band Quarter Waves
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
11
Matching Networks-2
Tank Matching
Coax
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
12
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…
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
13
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.
Balanced Line
Unbalanced Coax
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
14
Transformer Baluns
1:1 Transformer BALUN
3 Turns
Primary turns equals
Secondary Turns
Unbalanced
6 Turns
Balanced
3 Turns
1:4 Transformer BALUN
Unbalanced
6 Turns
Balanced
12 Turns
Total
Chelmsford Amateur Radio Society
Advanced Licence Course
• Recall: Zp = Zs . ( Np / Ns ) 2
• 1:2 Turns Ratio will create a
1:4 Impedance Transformation
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
15
Sleeve Balun
 /4
From Tx
Connection to Outer
Braid of Coax here
• 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
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
16
Choke Baluns
Antenna
Transmitter
From Tx
6 to 10 turns
about 25cm diameter
tightly wound
Wideband Balun
using Ferrite rings
• 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.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 14: v1.2, 20-Dec-2015
(5) Antennas
17