Feeders and Antennas

A. Outer plastic sheathB. Woven copper shieldC. Inner dielectric insulatorD. Copper core

CoaxCable

Coaxial cable, or coax is a type of cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Coaxial cable differs from other shielded cable used for carrying lower-frequency signals, such as audio signals in that the dimensions of the cable are controlled to give a precise, constant conductor spacing, which is needed for it to function efficiently as a radio frequency transmission line. From Wikipedia

Twin Feed

Twin-lead cable is a two conductor used as a transmission line to carry radio frequency (RF) signals. It is constructed of two multi stranded copper or copper clad steel wires, held a precise distance apart by a plastic ribbon. The uniform spacing of the wires is the key to the cable's function as a parallel transmission line; any abrupt changes in spacing would reflect radio frequency power back toward the source.

Coax is an unbalanced feeder - the inner carries the voltage and the outer is the earth. Coax is used by many stations as the outer acts as a screen

Twinn feeder is considered to be a balanced feeder. It has equal and opposite voltages/currents/fields.

If you wish to connect an unbalanced feed to a balanced antenna (coax feed a dipole) you need an interface. This interface is known as a BALUN (BALanced – UNbalance.

The BALUN prevents RF currents flowing on the braid. If this occurred the screening properties our lost.

Different types of RF connectors:

BNC

PL259

When using a PL259 , BNC plugs and sockets it is essential that the inner conductor and outer braid are connected correctly. Poor connections can lead to equipment mal functioning and can also lead to unwanted expense - having to get equipment repaired.It is essential that the screening is continuous from start to finish.

PL259,s can be used when developing a HF or VHF station. Most amateur stations will use these plugs somewhere. They have a screw thread to secure the connection .

BNC connectors have a bayonet connection and mainly used in circuits that are running low power. There are two types a 50 and 75 Ohm. The difference – different inner pin sizes

Antennas

An antenna (or aerial) is an electrical device which converts electric power into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission a radio transmitter supplies an oscillating radio frequency electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnet waves (radio waves). In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, that is applied to a receiver to be amplified. Antennas are essential components of all equipment that uses radio.

From Wikipedia

Some ant are designed to be directional and therefore give maximum gain in a set direction – beams. Many wire ant will be fixed and it will soon be established which geographical areas the ant favours.Recall that these are polarised according to the orientation of the antenna, e.g. a horizontally oriented antenna will radiate horizontally polarised waves

Review following antenna

DipoleQuarter wave ground planeFive-eights ground planeYagiEnd Fed wire

Dipole:The most common form of dipole is two straight rods or wires oriented end to end on the same axis, with the feed line connected to the two adjacent ends

HF - Would need to be fed via a BALUM

Quarter wave ground plane

Radials represent ground plane and are a quarter wave long

Five-eights ground plane

The 5/8 wave has a slight gain over the 1/4 wave antenna shown above.

Like the 1/4 wave, the 5/8 wave is also used vertically and gives an omni-directional radiation pattern but the thing to note is that the 1/4 wave has no coil, whereas the 5/8 wave requires the coil at the base of the antenna for impedance matching.

Yagi

The Yagi is a directional antenna. It is said to have gain because it focuses the radio waves into a, generally, single direction and is therefore not wasting power radiated in directions where it is not required. The Yagi can be used vertically or horizontally. The diagram shows the antenna in the vertical position. The design of a Yagi antenna is, in its' most basic form, a row of dipoles, held in place by a boom. Spacing between the elements (Reflector, Driven and as many Directors as required) is fairly critical so the whole antenna is mounted on a boom. The Reflector and Directors may be electrically connected or insulated from the Boom but the Driven element must be insulated from the boom. Length of the elements is also a critical factor in design.

End Fed Wire

The end fed wire is simply a random length of wire attached to the centre of a coax feeder or, more usually, linked directly onto the rear of a suitable ATU that can take single wire. This is a poor antenna (a compromise) as it is not tuned to any particular frequency and thus generally preforms badly relative to a dipole.

ERP - Effective radiated Power

Most manufacturers inform you of the gain of their antenna by using the scientific notation dB which stands for decibels. Whilst this may appear more complex you will be meeting it again in the Intermediate and Advanced courses.

Gain in dB Gain multiple factor

3 dB x 2

6 dB x 4

9 dB x 8

10 dB x 10

Power leaves your transceiver and travels up to the antenna. If you are using an antenna which has what is called "GAIN" then effectively you will be getting more out of the antenna than you are putting in. This is only possible because of the antenna construction.

So what is this EFFECTIVE power. As the power is being radiated we called it EFFECTIVE RADIATED POWER (erp) and this is given by this formula :-

ERP = power fed to antenna from the rig x antenna gain

using linear units and no allowance for feeder loss.

So if you have a transceiver which has power output of 10 watts and the antenna has a gain of 10 the ERP = 10 watts (output) x 10 (gain) = 100 Watts EFFECTIVE RADIATED POWER

This means that if the licence conditions state that the maximum ERP is 10 Watts, and the radio in use only gives output 1 watt of RF Power, then an aerial with a 10 times gain will produce the highest legal power for that frequency. Also, if for the same frequency, the radio in use has a maximum RF Power output of 5 watts, and the aerial in use has a gain of 2 times, then the ERP will be 10 Watts.

Polarisation of Antennas

Polarisation of antennas are normally Horizontal or vertical. For the receiving station to get maximum signal strength it needs to have the same polarisation as the transmitting station.

Vertical antenna give vertical polarisation – mobiles with mobile 5/8 mag mounts

Yagis and dipoles may be set up to be either polarisation – depends on how they are erected.

In some complex situations the above rules may not apply.

Antenna Matching and SWR

Antennas preform at their best when they are designed for a particular frequency and used on that frequency. However the challenge come when the antenna is to be used on more than one band. It is essential that the SWR (Standing Wave ratio) is kept as low as possible. This measurement of the antenna system to the normal impedance of the radio. If the SWR is high the output power will be reflected back and could damage the transceiver

Some HF antennas will cover two bands but normal practice is to use ATU (Aerial Tuning Unit). Many ATU’s also have a SWR meter which enable you to monitor any transmission and take appropriate action should the SWR change.

Dummy Loads permit radio tests without radiating a signal

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PL259

BNC

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Twin Feed

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What is the above ??????

Yagi antenna – shown vertically polarised

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Is coax cable a balanced or unbalanced feeder ???

Coax is an unbalanced feeder - the inner carries the voltage and the outer is the earth

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What does this symbol represent

Antenna/Aerial