Feeders and Antennas

Question 1

What are feeders?

Answer 1

the cables connecting the radio to the antenna

Question 2

What is ‘loss’? …and when does loss increase?

Answer 2

When some RF energy is converted to heat. When cables get longer and when frequency increases

Question 3

When would you use a thicker, low loss feeder cable?

Answer 3

When using VHF and UHF

Question 4

What are the two types of feeder?

Answer 4

Coaxial (coax)

twin-feed (or ladder line)

Question 5

Briefly describe the structure of a coax cable

Answer 5

The centre conductor carries the signal, it’s surrounded by an outer screen (braided) to keep the signal within the cable.

Question 6

Briefly describe the structure of a twin-feed cable

Answer 6

This consists of two equal length pieces of wire, separated throughout by plastic spacers.

Question 7

Why is coax an unbalanced feeder?

Answer 7

Because there is only one signal carried.

Question 8

Why is twin-feed a balanced feeder?

Answer 8

Because both wires carry the signal but in opposite directions, balancing each other

Question 9

Why is twin-feed harder to work with than coax?

Answer 9

Because it needs to be kept away from metal to work effectively.

Question 10

What are the 4 main types of connectors?

Answer 10

PL-259 - screw thread - more common on HF
N connector - screw thread - more common on VHF/UHF
BNC connector - bayonet locking
SMA connector - screw thread - common on handhelds

Question 11

In general _____ frequencies need larger antennas

Answer 11

lower

Question 12

Name the 5 common types of antenna

Answer 12

Half-wave dipole
¼ wave ground plane
⅝ wave ground plane
End-fed 
yagi

Question 13

What are the characteristics of a dipole antenna?

Answer 13

Commonly half a wavelength long
It’s a ‘balanced’ antenna
^^ therefore fed by two equal signals from the centre of the antenna. The antenna has two equal halves that send out the signal

Question 14

What are the characteristics of a 1/4 wave ground plane antenna?

Answer 14

Vertical antenna that is quarter of a wavelength long
Omni-directional, that is, signal is sent out in all directions (not upwards into the sky)
It has four horizontal wires, called groundplanes, which form a ‘ground plane’
The ground plane serves as a ‘mirror’ for the radio signals, helping the signals to reflect horizontally

Question 15

What are the characteristics of a 5/8 wave ground plane antenna?

Answer 15

This is a vertical antenna, five-eighths of a wavelength long
It has a noticeable coil at the base, the ‘loading coil’, which is there to provide a good match for the coax feeder
It’s omni-directional
Often used for VHF/UHF mobile operations

Question 16

What are the characteristics of a end-fed antenna?

Answer 16

This is a basic antenna, just a random length of wire
Because it is unlikely to be the correct length for any frequency, it has to be ‘matched’
This type of antenna is more likely than any other to cause interference (EMC)

Question 17

What are the characteristics of a yagi antenna?

Answer 17

This is a directional antenna
It’s basically a dipole with a reflector behind and several directors in front
This antenna is aimed in the direction of the station you want the signal to travel
TV aerials are examples of yagis

Question 18

What is a polar pattern?

Answer 18

An antenna ‘Polar Pattern’ shows how RF signals radiate from the antenna.

Question 19

What is ERP?

Answer 19

Effective Radiated Power (ERP) is the power an antenna radiates.

Question 20

How do you calculate ERP?

Answer 20

Power in watts multiplied by the antenna gain multiplying factor e.g. antenna gain of 6dB has a factor of x4

Question 21

Explain polarisation…

Answer 21

Antennas can be mounted horizontally or vertically
When two stations want to communicate effectively, the polarisation should match so they can capture as much of the signal as possible.

Question 22

When do you require a BALUN?

Answer 22

when an unbalanced feeder is connected to a balanced antenna, you require a BALUN (BALanced to UNbalanced)

Question 23

What is the connection between the feeder and antenna called?

Answer 23

The feed point

Question 24

What is the amount of impedance normally used?

Answer 24

50Ω

Question 25

How do you practically use impedance?

Answer 25

You need to try to match the impedance of the transmitter, feeder and antenna

Question 26

It’s important to match the impedance of the _____, _____ and _____

Answer 26

transmitter, feeder and antenna

Question 27

What are standing waves?

Answer 27

If there is a mismatch [in impedance], the antenna will reflect some of the transmitting power back down the feeder

Question 28

What is an SWR, and what is it used for?

Answer 28

An SWR meter is used to determine whether an antenna presents a good match. These meters typically show the ‘forward’ and ‘reflected’ power.

Question 29

What is an AMU (or ATU)? And what is it used for?

Answer 29

AMU (antenna matching unit) - these are sometimes known as ATU (antenna tuing unit) - are used to resolve mismatches between antenna, transmitter and feeder. AMUs are typically used on HF ‘multi-band’ where it is not possible to have lots of antennas of different lengths and you want to use one antenna.

Question 30

What is a dummy load?

Answer 30

This is used to allow you to use your transmitter without it emitting a signal, to test your station.