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ETA313-07: Theory Lesson 3: Antennas, Transmission lines, Propagation
Recommended reading: KonCEPT page 191-229 (chapter 7 + 8)
Start with transmission line (TL), the most difficult component.
characterized by: Characteristic impedance, Z_0, a geometry and material parameter. Length and the speed of light in the transmission line. Two metal conductors that guide the fields.
Used for high frequency signals. Lambda = 300/f [MHz], Lambda similar to length in size.
Waves propagate along transmission lines. Reflections along a TL are similar to light in a glass/water. Explain transmission/reflection coefficients.
Reflections cause standing waves and non-optimal power transfer/losses
VSWR = Voltage Standing Wave Ratio = Z1/Z2 OR Z2/Z1 so a x:1 relation is formed.
We want to “match” the antenna, transmission line and radio to minimize losses.
Balanced/unbalanced (differential/single-ended) TL are important and difficult. Explain this in detail.
balanced TL folded out → dipole antenna! Nice SWR achieved. L = lambda/2 = 300/(2*f) ~=(0,96*lambda/2)
Antennas
Antenna = two port that converts energy from propagating in a transmission line to propagation in free-space.
Characterized by:
- Input impedance, SWR, Z_in
- Operation frequency, resonance frequency, f_0
- Directivity D, antenna gain, dBi, dBd
- far field distance = d_f > 2*D^2/lambda, given d_f » D, d_f » lambda
- Radiation efficiency, eta
- radiation pattern, E & H patterns
- Polarization and x-pol suppression and so on…
We can only measure input impedance and operational frequency easily with a VNA/Antena analyzer … so lets do that at ETA.
Different types
- Dipole
- Monopole
- Small loop
- Yagi
- Quad
- Patch
- Bowtie
- PCB/PIFA
- Spiral/Helical
Talk about polarization.
Common mode current on coax = bad!
Propagation
HF/short wave
1) Ground wave (20-40 km typical) 2) Space wave (global range) * Reflections from the ionosphere's D, E, F regions. Sunlight ionize the ionosphere. * Reflections affect polarization chaotically. * D = 60-90 km height. * "Dämpningsskiktet" attenuates signals * Only daytime * Attenuate <10MHz * E = 90-110 km * F = F1 + F2 = 150-350 km - reflection. * Reflect <30-50MHz depending on sun activity * Created during daytime, slow to unionize
3.5-7 MHz
- Attenuated by D
- skyward wave/antenna can reflect back to local contacts (E/F)
14-30 MHz
- Not really affected by D
- skyward wave/antenna cannot reflect back to local contacts (E/F)
- Sporadic-E might enable local connections
DX → 10 degree max antenna gain is best
VHF/UHF/…
- Penetrates the atmosphere, EME possible
- Basically only local line-of-sight connections possible
- Troposphere propagation possible. Heat and humidity gradients guide waves. Ex. hearing Danish FM radio in Göteborg.
- northern lights reflections >25 MHz. Distorts signals, phone sounds creepy.
- Moon/meteor/satellite/airplane scatter
- Reflect signals from big things
Microwaves
- Moon/meteor/satellite/airplane scatter
- Reflect signals from big things
- Also rain scatter
Fading example with cellphone if possible
Radio Antenna Fundamentals Part 1 1947
https://www.youtube.com/watch?v=JHSPRcRgmOw&ab_channel=GerryTrenwith