Feeders and Antennas Flashcards
What are feeders?
the cables connecting the radio to the antenna
What is ‘loss’? …and when does loss increase?
When some RF energy is converted to heat. When cables get longer and when frequency increases
When would you use a thicker, low loss feeder cable?
When using VHF and UHF
What are the two types of feeder?
Coaxial (coax)
twin-feed (or ladder line)
Briefly describe the structure of a coax cable
The centre conductor carries the signal, it’s surrounded by an outer screen (braided) to keep the signal within the cable.
Briefly describe the structure of a twin-feed cable
This consists of two equal length pieces of wire, separated throughout by plastic spacers.
Why is coax an unbalanced feeder?
Because there is only one signal carried.
Why is twin-feed a balanced feeder?
Because both wires carry the signal but in opposite directions, balancing each other
Why is twin-feed harder to work with than coax?
Because it needs to be kept away from metal to work effectively.
What are the 4 main types of connectors?
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
In general _____ frequencies need larger antennas
lower
Name the 5 common types of antenna
Half-wave dipole ¼ wave ground plane ⅝ wave ground plane End-fed yagi
What are the characteristics of a dipole antenna?
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
What are the characteristics of a 1/4 wave ground plane antenna?
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
What are the characteristics of a 5/8 wave ground plane antenna?
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
What are the characteristics of a end-fed antenna?
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)
What are the characteristics of a yagi antenna?
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
What is a polar pattern?
An antenna ‘Polar Pattern’ shows how RF signals radiate from the antenna.
What is ERP?
Effective Radiated Power (ERP) is the power an antenna radiates.
How do you calculate ERP?
Power in watts multiplied by the antenna gain multiplying factor e.g. antenna gain of 6dB has a factor of x4
Explain polarisation…
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.
When do you require a BALUN?
when an unbalanced feeder is connected to a balanced antenna, you require a BALUN (BALanced to UNbalanced)
What is the connection between the feeder and antenna called?
The feed point
What is the amount of impedance normally used?
50Ω
How do you practically use impedance?
You need to try to match the impedance of the transmitter, feeder and antenna
It’s important to match the impedance of the _____, _____ and _____
transmitter, feeder and antenna
What are standing waves?
If there is a mismatch [in impedance], the antenna will reflect some of the transmitting power back down the feeder
What is an SWR, and what is it used for?
An SWR meter is used to determine whether an antenna presents a good match. These meters typically show the ‘forward’ and ‘reflected’ power.
What is an AMU (or ATU)? And what is it used for?
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.
What is a dummy load?
This is used to allow you to use your transmitter without it emitting a signal, to test your station.
