ETAwiki

Back to course information

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)

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!

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