Antenna Basics Antenna Basics Antennas Types Patterns Gain

Antenna Basics

Antenna Basics • Antennas – Types – Patterns – Gain • Feed lines – Coax – Open or Ladder Line • Monitoring station performance – Antenna tuners – SWR

Basic Antenna Types • • Horizontal Vertical Random Directional – Gain • Single band Multi-band

Some General Rules • • • The bigger the better (well most of the time) The higher the better (well most of the time) The lower the SWR the better (well most of the time) Coax is better than twin-lead (well some of the time) Antenna tuners do not change the performance of a bad antenna – Tuners just fool the transmitter into thinking everything is okay • Improve antennas first – then improve power (all of the time) – Improves hearing as well as sending

Some Vocabulary • Antennas are used to send and receive radio waves. – The wave includes both electrical and magnetic fields. • The orientation of the fields when compared to the earth’s surface determines the antenna’s polarization. – Antenna polarization can be horizontal, vertical or circular. – Polarization is based on the orientation of the electric field. • Generally, polarization can be determined by looking at the orientation of the antenna conductors.

Antenna Components 1. Radiator – Radiates and receives the radio wave 2. Feed line – Feeds energy from transmitter to radiator and energy from the antenna to the receiver 3. Matching Network – Matches impedance of the transmitter (i. e. , 50 Ω to 75 Ω) to the radiator and/or feed line.

Simple Dipole Antenna

Half- Wavelength Dipole • Basic building block of most antennas. • Has two poles – di-pole. • Length is a physical ½ wavelength of frequency. – A 20 -meter half-wave dipole would be about 10 meters long. – An 80 -meter half-wave dipole would be about 40 meters long.

Length of a ½ Wave Dipole • Calculate the following dipole dimensions: – – – 14. 200 MHz 3550 k. Hz 146. 52 MHz 10. 05 MHz 7. 150 MHz 440 MHz

Dipole Configurations

Dipole Detail Inverted Vee Trap Dipole

Dipole Kit

Dipole Height Above Ground high ½ wavelength For best performance, a dipole should be mounted at least a half wavelength above the ground.

Dipoles in Free Space Elevation Plot Looking into wire end

Dipoles in Free Space Azimuth Plot

Horizontal Dipoles Over Real Ground

Effect of Height Above Ground 15 feet 45 feet 30 feet 60 feet

Vertical Antennas • Good choice when you do not have room for a dipole or beam. • Vertical polarization. • Used extensively for mobile operations (whip). • Omni-directional pattern. • ¼ wavelength long (some ½ waves available). • Low angle of radiation, good for DX.

Radials for Verticals • For a vertical to work effectively, it needs artificial ground wires since the ground acts as the other half of the antenna. • These wires are called radials. Lots of radials are sometimes needed. • Radials should be placed on the surface of the ground or buried a few inches below the ground.

Ground Radials

Vertical Antennas • If the radials are changed from horizontal to downward sloping, it increases the feed point closer to 50 ohms (to match the transmitter’s impedance). • This type of antenna is called a ground plane.

Length of a Vertical • Calculate the following dipole dimensions: – – – 14. 200 MHz 3550 k. Hz 146. 52 MHz 10. 05 MHz 7. 150 MHz 440 MHz

Vertical Antennas

Dipole vs. Vertical Takeoff Angle = 34 o Takeoff Angle = 25 o

Directional (Beam) Antennas • Take available power and focus the power in desired direction. • When power is focused, it appears that the signal strength has increased. • This apparent increase in strength or power is called gain.

Yagi Antennas • Multi-element (at least two). • Properly called a Yagi-Uda Antenna. • Can be for one band (monobander) or multiple bands (tribander). • Produces gain over a dipole in specific direction.

Yagi Antennas • The driven element is usually about ½ wavelength long – just like a dipole. • The direction of maximum radiated field strength is called a lobe. The “main lobe” is the direction of maximum strength from the antenna. • Adding more directors and increasing the boom length increases the gain of the antenna.

Yagi Gain Antenna

How is Gain Produced? Driven Element

How is Gain Produced? Reflector Element Added

How is Gain Produced? Director Element Added

Other (some specialized) Antennas • • Random wire Multiband Loops Quads Log Periodic Beverage NVIS

Feed Line

Feed Line Types

Feed Line – Flat-Ribbon TV Twin Lead Typical Impedance = 300 ohms • There can be air space or dielectric space between conductors. • The impedance depends on the distance between the conductors and the radius of the conductors.

Feed Line - Coaxial (Coax) Cable • Typical impedance 50 and 75 ohms. • Used in most modern amateur antenna systems.

Feed Line Losses • The higher the frequency, the greater the feed line loss. – This is caused by “skin effect. ” • Feed line losses are measured in d. B/100 ft. • A cable would have a much higher loss on 2 meters (145 MHz) than on 160 meters (1. 8 MHz).

Standing Wave Ratio (SWR) • If the impedances within the antenna system are not matched, the power fed into the antenna is not all radiated into space. • The non-radiated power needs to go someplace. – It is reflected back to the source (the transmitter). – It is dissipated as heat or spurious radiation. • This reflected power is measured by SWR.

SWR Definition • The ratio of maximum voltage to minimum voltage along a feed line. • Also the ratio of antenna impedance to feed line impedance when the antenna is a purely resistive load. • 1: 1 (one to one) is the optimum SWR.

SWR Meters

SWR • SWR is optimum when feed line and antenna impedances are matched. All the power from the transmitter goes to the antenna. None is reflected back. – Up to an SWR or 2: 1, you’re okay. – Above an SWR of 2: 1, modern transceivers start to reduce transmitter power output for protection. – Above an SWR of 3: 1, you need to take corrective action.

SWR Calculation • A dedicated SWR meter will have a direct readout of the SWR. • Some meters will read the actual forward and reflected power (watts), voltage, or impedance. – This data can be used to calculate the SWR.

SWR Calculations • Impedances (ohms): Feed line Load 50 50 50 25 50 300

Antenna Tuners • More accurately: impedance matching transformers. – Transform the input impedance to match the output impedance. – Do not change the actual impedance of the feed line, the impedance of the antenna, nor the impedance of the transmitter. • If the feed line SWR is 5: 1, it is still 5: 1 even with an antenna tuner in line! (But the tuner allows the transmitter to operate under those conditions. )