A very breezy day on top of the hill in Enfield for the 2018 summer (yes summer) VHF / UHF field day. All antennas firmly anchored down with transmission lines leading into the main tent. No radios, amateurs or antennas were damaged under these harrowing conditions (the odd Gazebo or two may have been destroyed as a result of constant wind gusts).

Coaxial Cable

Coaxial cable is constructed from a centre conductor (dielectric) that is insulated from the outer conductor which consists of a foil shield (for lower loss) and a braid shield. The outer sheath (black insulation material) protects the outer conductor from the elements. Coaxial cable is unbalanced and as used in radio, is usually 50 ohms impedance (75 ohm coaxial cables are usually used for TV applications).

Coaxial Connectors

The above image shows several types of coaxial connectors. These connectors are adaptors that we use for changing the type or gender of the connector. Socket is the female version and plug is the male version.
1. BNC socket to TNC plug
2. BNC plug to UHF socket
3. UHF socket to UHF socket
4. N plug to UHF socket
5. UHF socket - for mounting an antenna, usually on a vehicle

Parallel / Ladder Line

Parallel or Ladder Line consists of two conductors (wires) that are separated by an insulator at an even distance apart. The insulator may be solid (as per the above diagram) or may have regular gaps - hence the term ladder line because it is formed into a ladder like appearance. Parallel or Ladder line is a balanced transmission line and usually has a 300 ohm impedance (some ladder line may have 600 ohm impedance).

Coaxial Patch Lead - UHF plug to UHF plug

The above coaxial patch lead is typically used to connect the transceiver to an SWR meter or to an antenna tuner. For the practical component of your Foundation assessment, you will be required to demonstrate how to test a similar patch lead to ensure it exhibits the correct characteristics.
1. Test for continuity between the centre pins.
2. Test for continuity between the outside (or shield) of the connectors.
3. Test that there is NO continuity between the centre pin and the outside of the connectors (continuity would indicate a short circuit).

Antenna Purpose

  • The size of the antenna is dependent on the frequency to be transmitted
  • The antenna (aerial) couples power from the transmitter to the space around the antenna
  • The antenna radiates an electromagnetic wave by converting electrical signals to radio waves and vice versa
  • The antenna should be resonant at the frequency to be transmitted / received
  • The antenna that radiates well also receives well
  • Antenna are affected by their height above ground and proximity to buildings, trees etc

Antenna Polarisation

  • The electromagnetic wave radiated from an antenna is made up of electric lines of force and magnetic lines of force
  • Generally the orientation of the antenna with respect to the ground will indicate the polarisation
  • Generally vertical antennas produce vertically polarised signals and horizontal antennas produce horizontally polarised signals
  • Transmitter and receiver antenna should be the same polarisation for best results

Transmission Line

  • The transmission line connects the transmitter to the antenna
  • The transmission line is sometimes called a “feeder” because it feeds the radio frequency to the antenna

Relationship between frequency and wavelength

Recall the relationship between frequency and wavelength is an inverse relationship, therefore there is a relationship between the physical length of an antenna and the frequency of operation (which is also inverse). See Technical Basics

  • The higher the frequency, the shorter the antenna
  • The lower the frequency, the longer the antenna

Identification of common antennas

The Foundation licence candidate will be required to identify common antenna types from a standard set of images. These include a:

  • Vertical antenna with a ground plane (radials)
  • Centre fed dipole antenna
  • Yagi antenna
  • Folded dipole antenna
  • End fed wire antenna

Vertical antenna with ground plane (radials)

The antenna shown in the image above is a typical quarter wave vertical ground plane antenna. The vertical element is one quarter of a wavelength long as are each of the radials. The radials are mounted on a 45 deg angle to provide the antenna with a lower angle of radiation (which makes it better for long distance or DX communications).

Yagi antennas

The antenna shown in the image above is a typical Yagi antenna. The Yagi is constructed from a boom on which all of the elements are mounted. Depending on the design, the elements may be electrically connected to the boom, or isolated from the boom. The smallest Yagi antenna contains only two elements. one is the driven element and the other is the reflector (which is used at the back of the antenna to reflect the signal from the driven element). Director elements can be added to the front of the driven element to provide gain and directivity. The more directors that are added, the higher the gain (increased ERP - Effective Radiated Power) and the antenna will be more directional (i.e., more sensitive to signals in the direction in which it's pointed).

Effective radiated power (ERP)

  • Antennas such as Yagi’s have gain, and are unidirectional. This means the electromagnetic wave is concentrated into one direction
  • The directivity of antennas has a similar effect as using a higher powered transmitter
  • The Effective Radiated Power (ERP) of the transmitting system is increased
  • Gain of antennas is measured in decibels (dB)
  • Gain is measured relative to a dipole antenna
  • A gain of 3 dB results in twice the Effective Radiated Power

Antenna matching

  • Antenna impedance is measured in ohms.
  • Impedance can be considered AC resistance.
  • Most modern transmitting equipment has an output impedance of 50 or 75 ohms.
  • The output impedance of the transmitter should be matched (have the same impedance) as the transmission line and the antenna.
  • Mismatched transmitter outputs, transmission line and antenna will cause power to be reflected back and forth along the transmission line.
  • The reflected wave is called a standing wave.

Antenna Tuning Unit

  • An Antenna Tuning Unit (ATU) sometimes called a TRANSMATCH can be used to provide the correct operating impedance for the transmitter.
  • The ATU tunes the antenna system allowing operation on multiple bands using the one antenna.
  • Adjusting the ATU is not a requirement for the practical component of the Foundation Licence but knowing its purpose and location between the transmitter output and the antenna is a requirement

BALUN (Balanced to unbalanced)

  • The balun is a type of transformer that can connect a balanced antenna to an unbalanced transmission line.
  • The balun can also be used to match impedances of transmission lines and antennas.

 

 

Centre fed dipole

The antenna shown in the image above is a typical centre fed dipole antenna. The total length of the antenna is half a wavelength long which means that each arm of the dipole is one quarter wavelength long. The centre fed dipole has a typical impedance in the centre of about 75 ohms.

Folded dipole

The folded dipole is typically used as the driven element of a Yagi antenna as shown in the above image. The folded dipole is a half wavelength across and is fed in the centre (same as normal dipole) and has a typical impedance of about 300 ohms.

End fed wire antenna

  • The long wire antenna, more correctly end fed wire antenna consists of a length of wire as high and reasonably long as possible: it is one of the easiest antennas to make and erect.
  • End fed wire antenna details

The on-air performance of an amateur station can be improved significantly by the correct choice of antenna.

  • Vertical – when you need to access a repeater, on a vehicle, limited space etc.
  • Dipole – use on HF bands, horizontal, easy to construct etc.
  • Yagi – when you need gain (low power), determine direction of signal etc.
  • Folded Dipole – often used for VHF / UHF, driven element of a Yagi etc.
  • End Fed Antenna – lower HF bands, access to a tuner etc.

Standing Wave Ratio (SWR)

  • A Standing Wave Ratio (SWR) meter is used to measure how well the antenna system is matched.
  • An SWR meter is connected between the output of the transmitter and the antenna
  • The SWR should be checked each time you change frequency
  • Checking the SWR on a transmission line using an SWR meter is a component of the practical for the Foundation Licence
  • Checking SWR allows for the adjustment of your antenna to bring the antenna closer to resonance by lengthening or shortening the antenna
  • An SWR of more than 2:1 is likely to indicate a fault in the antenna system
  • A good SWR on an antenna system is 1.5:1 or less
  • An SWR greater than 1.5:1 may indicate the antenna needs tuning or a fault condition is developing

Testing Transmitters

  • A dummy load is a resistor typically the same output impedance as the transmitter i.e. 50 ohms
  • The dummy load allows the transmitter to be tested with minimal radiation i.e. not tested “on air” via the antenna.
  • The dummy load also allows the output power of the transmitter to be adjusted.
  • The power for SSB is 10 watts PEP. For CW, FM and AM average power is not to exceed 10 watts.
  • Some power meters can be very inaccurate. Care must be taken in the selection of a power meter to measure output power.