Differences

This shows you the differences between two versions of the page.

--- user:kurser:ham_vt2023_l7 [2023/04/22 19:55] – Added clarification to the Yagi-Uda antenna being built from other antenna types. user
+++ user:kurser:ham_vt2023_l7 [2024/02/13 18:08] (current) – user
@@ Line 1: / Line 1: @@
 =====ETA313-07: Theory Lesson 3: Antennas, Transmission lines, Propagation =====
-[[user:kurser:ham_vt2023|Back to course information]]
+[[user:kurser:ham_vt2024|Back to course information]]
 **Recommended reading: KonCEPT page 191-229 (chapter 7 + 8) **
@@ Line 28: / Line 28: @@
 **Antennas**
+\\ de SA6KRZ
 In this segment, possible exam questions have been marked //**POSSIBLE EXAM QUESTIONS**//.
@@ Line 43: / Line 44: @@
 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
+A selection of different antenna types, roughly in order of most common -> least common
   * Dipole
   * Monopole
@@ Line 82: / Line 83: @@
   * Polarisation and x-pol suppression
-Most of these parameters can be analysed using a vector network analyser or an antenna analyser. Let's do that at ETA.
+Many of these parameters can be analysed using a vector network analyser or an antenna analyser.
 \\
@@ Line 95: / Line 96: @@
 ==Direction==
-The most ideal antenna is a single charge floating in free space, radiating as a sphere in all directions. Such a single charge is known academically as an //isotropic radiator//. Practically, antennas are real objects and thus 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 hypothetic sphere, meaning that most of the EM field is sent/recieved from/to the antenna at that specific direction in space. The illuminated area is known as the //beam area// (SE: ??).
+The most ideal antenna is a single charge floating in free space, radiating as a sphere equally strongly in all directions. Such a single charge is known academically as an //isotropic radiator//. Practically, antennas are real objects and thus 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 hypothetic sphere, meaning that most of the EM field is sent/recieved from/to the antenna at that specific direction in space. The illuminated area is known as the //beam area// (SE: ??).
 How small that illuminated segment of the sphere is as opposed to size the entire sphere, 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 design specification, is its directivity. Different types of antennas have very different directivities.
@@ Line 139: / Line 140: @@
 \\
 ==How wide is a lobe?==
-    * TODO: -3 dB point (SE: halvvärdesbredd)\\
+Stand in front of a transmitting antenna with a power detector. Assume that the strongest power in the main lobe is denoted P. As you move to the side, you see the power going down. The point at which the power has dropped 3 dB, is known as the //-3 dB point// (SE: halvvärdesbredd). By convention, the lobe is said to end at this point. Even though there is some power being transmitted beyond that point, as you move to the side further. Another (less common) datasheet specification, is also the -10 dB point.
-    * TODO: The angle spanned by the circular segment, defines the //beamwidth// of the lobe (SE: öppningsvinkel).\\
+\\
+Now, let's assume that you've found the -3 dB points to the left and to the right of the main lobe. If you draw a triangle between these two points and the antenna, you'll create some angle α at point of the triangle (at the antenna). This angle is known as the //beamwidth// of the lobe (SE: öppningsvinkel).\\
 \\
@@ Line 174: / Line 176: @@
 Answer: Yes, this is allowed, since p.e.p. sets the limit of how much power is fed into the antenna, and does not account for directivity.
+==Wavelength==
+Size-wise, antenna //elements// are usually 0.25 · λ or 0.5 · λ in length/size. An antenna with several elements of different lengths, can usually operate fairly well across a span of frequencies.
+\\
+//**POSSIBLE EXAM QUESTION**// → Show on board, hard to visualise.
+\\
+You have a 40 m dipole antenna. What frequency would create a standing wave pattern on the antenna, that has 5 nodes?
+  * 3.5 MHz
+  * 7 MHz
+  * 14 MHz
+  * 28 MHz
+\\
+\\ Answer: (300 / 40) gives us a frequency of 7.5 MHz; this is the frequency of the current on the antenna that create one full-wave standing wave on the antenna, with one node at each of the two ends of the dipole. If we look at the half-wavelength frequency → 15 MHz, which has three nodes; one at each end of the dipole and one in the very centre. If we look at the quarter-wavelength frequency → 30 MHz, we get yet another two nodes of the standing wave pattern on the antenna. Thus, 28 MHz seems to be the most reasonable answer.
+==Realistic design factors (SE: förkortningsfaktor)==
+Antenna component dimensions have to be scaled down when making real antennas. There are various factors that all make up a 0-1 scaling factor, that effectively makes it so that your antenna is shorter than what it would have been if you only accounted for the wavelength of some frequency. Often, you may see that some factor is given as 0.95, 0.96 or 0.98 etc. For instance, aluminium tubing typically has a factor of 0.96.
+\\
+//**POSSIBLE EXAM QUESTION**//
+\\
+You are building a full-wavelength delta-loop antenna for 7.1 MHz. The wire has a scaling factor of 0.95. How much wire will you use in total?
+  * 20.07 m
+  * 40.14 m
+  * 21.13 m
+  * 42.25 m
+\\
+\\
+Answer: ( 300 / (1.0 · 7.1) ) · 0.95 = 40.14 m
+\\
+\\
+//**POSSIBLE EXAM QUESTION**//
+\\
+Roughly how long is the vertical element of a quarter-wavelength vertical antenna for 145 MHz?
+  * 50 cm
+  * 70 cm
+  * 2 m
+  * 4 m
+\\
+\\
+Answer: (300 / 145) · 0.25 ≈ 50 cm
@@ Line 189: / Line 237: @@
 ==Complicated: circular polarisation==
-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.
+Let's imagine an antenna with two transmitting elements. Thus, there are two electric (E) fields to consider. Let's say that one is vertically polarised, and the other one horisontally polarised. Let's describe the first E field like a cosine with some phase, and the second E field like a sine with some phase.\\
+\\
+If we stand directly in front of where the antenna is pointing, we can 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 other E sine goes left/right, that complex vector will thus 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, due to there being two transmitting E fields.
 Circular polarisation is common in public FM radio broadcast (SE: 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 transmissions.
-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. This effect is achievable using two antenna elements rotated 90 degrees like an "X", and then phase-shifting the fed signal to one of the antenna elements with 90 degrees.
+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 field composants spinning a complex vector around in a circle, with respect to the direction of the propagating EM wave. This effect is achievable using two antenna elements rotated 90 degrees like an "X", and then phase-shifting the fed signal to one of the antenna elements with 90 degrees. Assuming both elements were transmitting with equal magnitude.
-\\
-== Common mode current on coax = bad! ==
-TODO
 \\
 == Antenna impedance matching ==
-TODO
+Different antenna types have different ideal input impedances. And, the the input impedance is typically frequency dependent. For instance, a given antenna may look like a 50 ohm impedance at 5 MHz, but may look more like an open circuit at 10 MHz.
+Example: the ideal dipole antenna has a 73 ohm input impedance. Feeding this antenna with a coaxial cable of 50 ohm characteristic impedance, leads to a 50 ohm -> 73 ohm impedance interconnect. This impedance difference will result in signal reflections, which are unwanted for several reasons.
+Overcoming impedance differences in antennas may for instance be done using matching networks.
+Matching an antenna gets harder as the operational band of the antenna grows larger.