Antennas and Feeders Flashcards
Where does the impedance in an unbalanced coaxial cable come from (three answers)
Resistance: from the wire shield and centre conductor
Capacitance: centre conductor - insulator material (dialectic) - outer shield, acts like a big capacitor
Inductance: lengths of wire
How to reduce the effect of wire length on impedance in unbalanced coax wire
If the coax is terminated i.e. correct load matching at end as coax (50 Ohm or maybe 75 Ohm)
Summary of ladder line - what happens in each line
What can affect ladder line cable
Current travels in equal and opposite directions
This causes opposite fields around the two conductors
Nearby metal objects may unbalance the feed line.
Unlike unbalanced, balanced relies on RF waves cancelling out to stop it radiating
Pros of using ladder line vs unbalanced coax
What are these pros dependant on
Less loss
Loss is length and frequency dependant
Formula for calculating loss given the loss in dB
Formula given the loss
What to remember
Two common losses
1 / ( 10 ^ (dB/10) )
10 x -log( loss )
3dB = 0.5
10dB = 0.1
How to work out loss given weird length of cables
What order to do it all in
How to work out final power out given loss and gain (in E.R.P)
Add together known dB losses i.e.
13dB loss = 10dB (1/10) + 3dB(1/2)
Divide the power in by 10 first. Then take that value and divide by 2
Work out loss first
Then multiply final value by gain
What is a choke BalUn
What’s the best option
Coiling the coax around a former in order to increase the inductance of the coax
“Chokes” unwanted signals from travelling further down the cable
Using ferrite cores/beads
What is a dummy load
What can you check using this
A resistor/series of resistors which are non-reactive (so doesn’t transmit) and can take the power you are transmitting.
Check coax cable has 1:1 SWR
Check if system is picking up noise
What are the four parts of a Yagi antenna?
Reflector: Long element at the back. Reflects the RF forward
Driven Element (Dipole): Next longest after reflector. Gets fed the RF from coax
Directors: All other elements. Get shorter in direction of reflection
Boom: Piece holding all this together
Yagi design variations:
How can elements be attached to the boom?
Do all antennas have 50 ohm impedance
Single band or multiband
Can be attached by metal or by insulators, depending if you want the boom to be part of the design
Nope, not all. 28 and 12 ohms is common
Can be either
Antenna traps:
How do they work
What can they be used for
A circuit tuned for a given frequency
Will stop frequencies in a certain range but allow all others to pass
Can stick them on a dipole to allow for multiband
Wire length from feed point to trap can be for one frequency.
Wire between feed point and end of insulator can be for another frequency, as the other frequencies can make it through the trap
Can multiple traps (e.g. 10m, 15m, 20m on a 20m antenna)
Main benefit of a Yagi antenna
Where does most RF go out (reference polar pattern)
What’s the ratio called
Its directional
Most out the front, some out the back, little bit out sides
Front to Back ratio
What affect a Yagi’s beam width
The number of elements in the design
The more elements, the narrower the beam.
Also makes antenna longer
What do you need to consider with the angle of radiation?
What are the downsides of a low angle?
What is the recommended height of an long range yagi antenna?
For long distance stations you need a low angle. So it travels further before hitting the ionosphere
The lower it is, the more of an impact the ground will have on it
At least 1/4 wave high
Why is an antennas “ideal” impedance not actually correct in real life
Its impedance is for its resonant frequency but when used for another frequency the impedance will be different.
Also related to factors like current, voltage and proximity to ground
