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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

What is an antenna? An antenna is a device that converts EM waves between a bound medium and a free medium, for instance between a cable and free space.

Alternative: Antenna = two port that converts energy from propagating in a transmission line to propagation in free-space.

Antenna types.

There are many different types of antennas, since different antenna designs are optimised for solving different problems. There is no antenna that is perfect for every single operational situation. However, the opposite is true; there are indeed antennas that are quite bad at everything.

A seletion of different antenna types, roughly in order of most common → least common

• Dipole
• Monopole
• Loop antennas
• Yagi (Yagi-Uda)
• Patch
• Quad
• PCB/PIFA
• Waveguide slot
• Bowtie
• Spiral/Helical
• Vivaldi
• J-pole

Antennas may often be combined together for different effects. These group of antennas acting together as one, are denoted antenna arrays (gruppantenner). Some antenna arrays are more common than others, for example:

• Log-periodic dipole array (LDPA)

Very often, specific antennas are are combination of other antenna types. For example, the very common Yagi-Uda antenna, is a combination of three (or more) dipole antennas, and one magnetic loop antenna.

All antennas can be characterised using at least the following parameters:

• Input impedance, Z_in
• Standing wave ratio, SWR
• Operational frequency band
• Resonance frequency, f_0
• Directivity, D, dBi, dBd
• Antenna gain
• Far-field distance = d_f > 2*D^2/lambda, given d_f » D, d_f » lambda
• Radiation efficiency, eta
• Radiation pattern, E & H patterns
• Polarisation and x-pol suppression

Of these parameters, only input impedance and operational frequency are arguably easy to analyse. This analysis is done with either a vector network analyser, or an antenna analyser. Let's do that at ETA.

The actual metal sticks that point at different directions on an antenna, are commonly known as elements (antennelement, spröt). These elements are often mounted to a frame of sorts, like a large metal bar that supports all elements. This bar is known as the boom (bomm).

Often, antennas are shielded from the evil world of rain, snow, pidgeons etc. by placing them in a plastic cover, box etc. This plastic cover is known as a radome (SE: radom).

The most ideal antenna is a single charge floating in free space, radiating in a sphere in all directions. Such a single charge is known academically as an isotropic radiator. Practically, antennas are not single charges, meaning that the “radiation sphere” (made-up word) is very much not a sphere. Most antennas only illuminate (actual term) a smaller part of that sphere, meaning that most of the EM field is sent/recieved from/to the antenna at that direction in space.

How small that illuminated segment of the sphere is, is known as the antenna directivity. The directivity is often given in dB with respect to that ideal isotropic radiator. The unit is thus *dBi*. A very large part of an antenna's specification, is its directivity. Different antennas have different directivity.

A higher directivity means that more of the emitted/recieved field is sent through a smaller portion of that sphere around the antenna. The antenna is in a way “pointing” more to one location.

A high directivity, means that the antenna is very good as emitting/receiving to that specific direction. And as a drawback, the antenna become worse at emitting/receiving in the other directions.

*dBd*

Practically, it is not uncommon to describe the directivity of some antenna with respect to something more real. Such as, a dipole antenna's directivity. How much more “directive” (made-up word) is my antenna, in comparison to a dipole antenna?

A dipole antenna is -2.15 dB less “directive” than an isotropic radiator. Thus, the unit *dBd* = dBi - 2.15. dBd = directivity in dB with respect to an ideal dipole antenna.

Important: almost always, antennas do not emit/receive in a single direction. Most antennas emit/receive in one “very good” direction, and a small set of “less good but OK” directions. How well an antenna is emitting to a particular direction, is known as a lobe.

The direction with the highest directivity of the antenna, is known as the main lobe (SE: huvudloben). Thus, the smaller lobes are known as sidelobes (SE: sidolober). The main lobe in practice defines which way the antenna is pointing.

Simple antennas, like a monopole antenna, only have a single lobe. Very complex antennas, can have lobes that are shaped practically in any way you want it to be shaped. Example: an antenna in a satellite orbiting above a nation, might have an antenna with a lobe pattern that is shaped according to the borders of that nation. The antenna is thus good at transmitting/receiving to/from that nation, and worse at transmitting/receiving to/from locations outside of that nation's borders.

In practice, it is very hard to design an antenna with such a complicated lobe pattern. Companies approach this problem for instance using evolutional AI algorithms that brute-force designs until the lobe pattern is achieved. Another very common approach, is to make a very large array of antennas, and control the phase of the signal reaching each antenna, in order to achieve a more complicated lobe pattern.

All antennas emit/receive EM waves. Meaning, that the antenna emits electric and magnetic fields. By convention, the orientation of the electric field is said to define the polarisation of the antenna. Example: let's say that the antenna is vertically polarised, then that would mean that the E-field is sent from the antenna like a sine wave moving up (and down) along the Z-axis, i.e. vertically with respect to the ground. Vice versa, horisontal polarisation means that the E-field is propagating like a sine wave that is laying down flat with respect to the ground.

Very often, it's fully possible to just look an an antenna, and figure out its polarisation. “Which way are the antenna elements pointing?”

Why polarisation is important: typically, the best transmission efficiency between two antennas, is achieved when their polarisations are matching.

SE words: vertikal polarisation, horisontell polarisation, vertikalpolariserad, etc.

Some antennas transmit mainly using the electric field, some transmit mainly using the magnetic field, and some use both. Let's imagine an antenna with an electric (E) field that is vertically polarised, and a magnetic (H) field that is horisontally polarised. Let's describe the E field like a cosine with some phase, and the H field like a sine with some phase. If we stand directly in front of where the antenna is pointing, we could see both the cosine and sine waves like composants in a complex vector. As time progresses, and the E cosine goes up/down while the H sine goes left/right, that complex vector would be spinning around in a circle as time progresses. This type of polarisation is known as a circular polarisation; the emitted/received EM field is in a way “spinning around” in a circle.

Circular polarisation is common in public FM radio broadcast (rundradio). The advantage is that the receiver can be rotated at almost any direction with respect to the ground, and still be somewhat optimal to the transmitter. Another example is in satellite-to-Earth orientation.

Common misconception: even though many circularly-polarised antennas are shaped like round objects, it is not necessary to have a round antenna to create circular polarisation. The circular polarisation itself stems from the E and H field composants spinning a complex vector around in a circle, with respect to the direction of the propagating EM wave.

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