Module 4 Flashcards
What are feeders
What can happen when an RF signal is put through them
How can you prevent this happening
Connect the transmitter/receiver to the antenna
Exhibit’s loss as some energy converted to heat-longer the cable the greater the loss
Loss increases with frequency, so thicker cables needed for VHF/UHF
Two types of feeder cables and their characteristics
Coaxial cable (coax):
Used by amateurs
Unbalanced
Inner conductor carries the signal, outer braid keeps signal within cable
Twin feeder (ladder-line):
Balanced
Equal and opposite signals in two wires
Constant separation between them
What are all the connectors listed used for
Four types of connectors
Used for co-ax
PL259 (shit game for the PL):
Long telescope looking body
Screw-thread locking
Used for HF
N (narrow):
Short telescope body
Screw-thread locking
Used for VHF/UHF
BNC (big nasty cone):
Cone shape at top
Bayonet locking
Use 50Ω not 75Ω
SMA (screw my arse):
Looks like a hex bolt
Screw-thread locking
Found on handheld radios
Key facts about antennas
Convert electrical signals into radio waves, and vice-versa
Designed for specific frequencies
Different sizes for different bands
Longer wavelengths need larger antennas
Dipole:
What type of antenna is it
How long is it
What is a polar pattern
What does its polar pattern look like
Balanced
Half a wavelength long
Shows how RF signals radiate from the antenna
Two circles on top and below the antenna
Looks like a donut when you view it in 3D
5/8 Wave Ground Plane
Type of antenna
What size is it
What features does it have (more than 1/4)
What’s it used for
Vertical antenna
5/8 of a wavelength long
Four radials
Better signals towards horizon
Coil at the base for coax matching
Used for VHF/UHF mobile
1/4 Wave Ground Plane
Type of antenna
What size is it
What features does it have (clues in name)
Vertical antenna
Quarter of a wavelength long
Four horizontal wires, called radials form a ground plane which act as a mirror for the radio signals
End-fed antenna:
Type of antenna
What size is it
What’s worse about it than other antennas
Long-wire antenna
Unlikely to be correct length so needs to be matched
More likely to cause EMC interference than other types
Yagi antenna:
Type of antenna
Features
What parts is it made up of
What does the polar pattern look like
Directional
Focuses signal mainly in one direction
Dipole with a reflector and several directors
Looks like a blimp: ellipse (main lobe) towards correct direction and small side lobes behind and to the sides of the dipole
Antenna gain:
What is it
What’s it measured in and relative to
With an antenna that focuses the signal, it will have a gain
Measured in decibels relative to a half-wave dipole
What is ERP
Equation for calculating it
What do you need to keep in mind regarding the units
Effective Radiated Power
Directional power of an antenna
ERP (watts) = transmitted power (watts) * gain
Need to convert dB first before multiplying
What is polarisation
Particularly to do with Yagi’s
Difference between VHF/UHF and HF polarisation
Orientation of the antenna
Yagi’s can be horizontal or vertical
Most VHF/UHF is vertical
Receive and transmit antennas should have same polarisation
Less of an issue at HF as polarisation can be changed by ionosphere
What is a BALUN
BALanced to UNbalanced
Used when connecting an unbalanced feeder to a balanced antenna
Feed point:
What is it
What do you have to worry about at this point
How does this relate to the antenna
Connection point of the feeder to the antenna
Antennas are designed for specific frequencies-feed point impedance should match both feeder and transmitter
Impedance related to the dimensions of antenna and wavelength of signal
What happens if the impedance matching is wrong?
How can you test for this?
How to read it
Standing waves will be reflected back down the feeder, the worse the match, the more energy is reflected
SWR( Standing Wave Ratio) meter tests for the match
Ratio of 1:1 is ideal
2:1 means 10% is reflected back
High WER (greater than 2:1) indicates mismatch
