user:kurser:ham_vt2023_l7
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user:kurser:ham_vt2023_l7 [2023/04/22 12:12] – user | user:kurser:ham_vt2023_l7 [2023/04/22 13:51] – Added segments to the Antenna section. user | ||
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balanced TL folded out -> dipole antenna! Nice SWR achieved. L = lambda/2 = 300/(2*f) ~=(0, | balanced TL folded out -> dipole antenna! Nice SWR achieved. L = lambda/2 = 300/(2*f) ~=(0, | ||
- | ==Antennas== | + | **Antennas** |
//Antenna = two port that converts energy from propagating in a transmission line to propagation in free-space.// | //Antenna = two port that converts energy from propagating in a transmission line to propagation in free-space.// | ||
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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. | 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. | ||
//Antenna types.// | //Antenna types.// | ||
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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. | 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. | ||
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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. | 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. | ||
- | Talk about antenna polarisation. | + | ==Introductory important terminology== |
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+ | The actual metal sticks that point at different directions on an antenna, are commonly known as // | ||
+ | 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). | ||
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+ | 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). | ||
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+ | ==Direction== | ||
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+ | 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, | ||
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+ | 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' | ||
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+ | A higher directivity means that more of the emitted/ | ||
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+ | A high directivity, | ||
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+ | == Lobes (SE: Lober) == | ||
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+ | Important: almost always, antennas do not emit/ | ||
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+ | ==Polarisation== | ||
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+ | All antennas emit/ | ||
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+ | Very often, it's fully possible to just look an an antenna, and figure out its polarisation. "Which way are the antenna elements pointing?" | ||
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+ | Why polarisation is important: typically, the best transmission efficiency between two antennas, is achieved when their polarisations are matching. | ||
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+ | SE words: vertikal polarisation, | ||
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+ | 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; | ||
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+ | 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. | ||
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+ | Common misconception: | ||
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Common mode current on coax = bad! | Common mode current on coax = bad! | ||
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== Propagation == | == Propagation == |
user/kurser/ham_vt2023_l7.txt · Last modified: 2024/02/13 18:08 by user