Antenna Definition and 418 Threads

In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of a radio wave in order to produce an electric current at its terminals, that is applied to a receiver to be amplified. Antennas are essential components of all radio equipment.
An antenna is an array of conductors (elements), electrically connected to the receiver or transmitter. Antennas can be designed to transmit and receive radio waves in all horizontal directions equally (omnidirectional antennas), or preferentially in a particular direction (directional, or high-gain, or “beam” antennas). An antenna may include components not connected to the transmitter, parabolic reflectors, horns, or parasitic elements, which serve to direct the radio waves into a beam or other desired radiation pattern.
The first antennas were built in 1888 by German physicist Heinrich Hertz in his pioneering experiments to prove the existence of waves predicted by the electromagnetic theory of James Clerk Maxwell. Hertz placed dipole antennas at the focal point of parabolic reflectors for both transmitting and receiving. Starting in 1895, Guglielmo Marconi began development of antennas practical for long-distance, wireless telegraphy, for which he received a Nobel Prize.

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  1. Vanadium 50

    Crystal Radios and Followups

    In another message, I discourage a crystal radio as a first project for a kid: Don't get me wrong - I think they are great. But I wouldn't start with one. Anyway, this brought up some issues. (1) The ugly tan earphones that come with the kits. What are they? Piezoelectric? (2) An iron-core...
  2. Flyboy

    Compact highly directional medium wave antenna designs?

    I know that there are highly directional shortwave/HF and VHF antenna designs that can be made fairly compact, but is it possible to do the same for medium wave, i.e. below 3MHz? I can’t seem to find anything helpful online about any theory that would point to a shape, let alone an actual...
  3. Y

    Induced currents on a pair of separated dipole antennas

    Hello, I need to calculate the expression for a current on two dipoles separated as shown in the attached full task . I need to present a solution,lets take the simplest case. Theta_in=0 I need to find the expression for the currents on the dipoles...
  4. Spinnor

    I Plane wave decomposition of the fields of a dipole antenna

    A dipole antenna will have near fields and far fields. Can both the near and far fields can be decomposed into an infinite sum of plane waves? If so, are the plane waves for far fields and near fields of different type or class? Near fields must die off at infinity but far fields do not. Thanks.
  5. G

    Custom UHF Yagi antenna build

    As discussed with Baluncore, I’d like to make a start on building a Yagi customised to my local transmitter. Here are some details: Transmitter: Rosemarkie (https://ukfree.tv/transmitters/tv/Rosemarkie). Horizontal. 33km away. Aerial: inside roof (slate). Landlord won’t allow an external one...
  6. G

    Making a Shortwave listening antenna

    Thanks to @berkeman for the ARRL book recommendation. While trawling through the theory, I’d like to build an antenna to help cement theory with practice. The receiver is an old valve/tube radio, covering the SW band from 6-18 MHz, or 49-16m. For simplicity, I thought a small, tuned loop...
  7. G

    Looking for recommendations for a good Antenna book

    I’ve recently restored a Cossor 501 valve (tube) radio, which can receive LW, MW and SW. It has an external antenna socket (for SW), and I’d like to gain some knowledge on how to design and build something to plug in to it. A single wire about 20m long, draped out of the window, has enabled me...
  8. Y

    Name of the connector used for coax for internet, cable TV, TV antenna

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  9. homeworkhelpls

    I Does the aerial [antenna] not need to be positioned perpendicular?

    Shouldnt the aerial be in perpendicular orientation for all em waves? including radio
  10. X

    Impedance of a half-wave dipole antenna

    73 + j42 means it has an active resistance plus reactance. But this "active" resistance is not a DC resistance, right? We can't just measure 73 ohm with a multimeter since the circuit is open. What closes it then? Let's say the reactive part is zero. How do we measure 73 ohm? By measuring...
  11. E

    I Antenna inductance calculation (RFID applications, inductive coupling)

    Inductance of antenna coils of different shapes can be estimated. Some examples are: A circular coil with single turn. N turn multilayer circular coil. Spiral coil. N turn squae loop coil. etc. For plannar design (spiral, rectangular, etc) I have used this reference (from pag. 9 onwards)...
  12. DC2

    A Calculation of EM fields induced by an antenna in the near field

    The title pretty much covers it. I'm having to calculate the field induced inside the human body by an antenna in the near field (essentially, a phone placed close to a user's head), and I'm drawing a blank on how to relate the field generated by the antenna to the field induced inside the...
  13. amgc63

    Why Is Y Considered 1 in Antenna Power Gain Calculations?

    Question; Given that a dipole antenna has directional gain of 2, calculate the power gain in Dbi? Answer; Power gain = 10 log (x/y), where y =1 Power gain = 10 log (2/1) = 3dbi My question is….how come y = 1?? Thanks in advance
  14. R

    B What parts of an EMW does a ferrite rod antenna respond to?

    Hi. Would it be true to say, that a ferrite rod antenna, operating at fairly low frequencies (say 1Mhz) for all intents and purposes, only responds (in terms of voltage output) to the magnetic field part of a "radio wave"? Thanks.
  15. B

    I Estimation of E-field strength at a distance from dipole antenna

    Hello everyone, I was asking myself about electric field strength estimation at a distance d from - in my case - a half wave dipole antenna. There are pretty much a lot of information about this on internet or in books but still, there are a few things that are confusing to me that I would...
  16. sophiecentaur

    Free simulation software for antenna and feed modelling

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  17. V

    B Why does charge oscillate in an electric dipole antenna?

    Picture: Energy source => LR Oscillator => Transformer => Transmission line => Electric dipole antenna => traveling wave Why would the charge even oscillate in the antenna as opposed to building up in the antenna? The transmission line + antenna is not a closed circuit right?
  18. A

    Engineering Noise Floor/Device Temperature Problem

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  19. T

    Receiver Circuit Question -- What role does this antenna capacitor play?

    Which role plays an additional capacity in a receiver circuit between the antenna and the matching-box part like in this example (found in https://www.frostburg.edu/personal/latta/ee/twinplex/schematic/twinplexschematic.html): Is it necessary for this receiver circuit or just optional? What...
  20. A

    How Does Frequency Affect C/N in Dish Antennas?

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  21. Narayanan KR

    Resonant Frequency for a Coil-Only Antenna

  22. U

    B Physical temperature and antenna temperature

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  23. nomadreid

    TV antenna dish: depth, f/D, or focal distance

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  24. .Scott

    B ARC Centre reports HF Gravitational Wave Antenna

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  25. S

    Does Heinrich Hertz' apparatus include a proper antenna?

    How close to "antenna" was the metal barbell thing which was part of Heinrich Hertz' apparatus? Did he know in theory how to build a radio but just didn't have the right components to do it? I know someone around that time referred to the problem of radio in terms of needing proper sustained...
  26. awen

    Using an inductor choke as a receiving antenna for LW radio

    Hello, I'm interested in using a choke inductor as a receiving antenna for long-wave radio signals (300kHz). What are the "obstacles" in comparison with a standard ferrite rod antenna used in AM radios? So far I'm registering these: 1) low Q at the desired frequency available chokes seem to...
  27. Johan L

    EM re-radiation from a metal object (antennas)

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  28. H

    What makes antenna grounding "good"?

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  29. P

    Calculating Radiation Pattern for a grid of two rows of dipole antennas

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  30. H

    I Telescoping antenna -- why is there a knob on top?

    This is probably a trivial or even a stupid question, the reason I am writing here is I couldn't find something satisfactory online. I am a new amateur radio operator. Currently enjoying listening to SW and MW bands. I have a nice radio receiver with a long telescopic antenna. I have noticed...
  31. G

    I Can an enclosure surrounding a Tx antenna act as a receiving antenna?

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  32. berkeman

    Is this an antenna array on the new Chinese aircraft carrier?

    I saw this photo at CNN.com today, and I'm trying to figure out what the function could be for what looks like a linear antenna array lining both sides of the aircraft ski-jump launching ramp. They look to be an array of about 10m verticals along each side of the bow, but with the base of some...
  33. vutran

    What Happens When Using Long-Wave Frequencies with Starlink Antennas?

    It looks very cool. I have been using phased-array for direction finding & beam-forming, but recently I have come up with an idea that I find difficult to understand. Let's take the star-link phased array as an example, if we used a long-wave (MHz -- much lower than the one it is designed...
  34. U

    Circular antenna array formula

    At page 20 of this pdf the two formulas that characterize the behavior of a linear and a circular antenna array are compared. The reason why I write this post is: I don't understand why the second formula is true. Below I show why I am not convinced. In general, given a configuration of sources...
  35. M

    I How Does Solid Angle Affect Power Calculation in Radiometry?

    I have a doubt regarding the role of the solid angle when calculating the power(W) with the brightness of the source I'm observing with area Asource. I was given the definition: with A the Aantenna. If now I take as an example the picture below to calculate W, we conisder as solid angle the one...
  36. mfb

    How Does Starlink's Phased Array Antenna Track Satellites Without Moving?

    Starlink has begun its public beta phase, and naturally some people were more interested in the electronics than in fast internet access. Here is a video how the satellite receiver looks inside. The first half is focused on the mechanical side, the second half is looking at the electronics. It...
  37. M

    Engineering Link Budget Antenna: use of effective aperture and the gain in a problem

    Hi, I was just attempting this problem and was confused about the calculation process involved. Context: In earlier parts of the question, we calculate the gain ## G ## and effective aperture ## A ## for the parabolic antennae. My Attempt: We are given the power at the output ## x ## in Watts...
  38. U

    Strange approach to the line-fed slot antenna electromagnetic problem

    There is a beautiful demonstration, available in the text Robert S. Elliot, Antenna theory and Design, Wiley-IEEE Press, page 17 (Stratton-Chu solution), which shows how the electromagnetic field at each point ## \mathbf { r} ## of a volume ## V ##, with boundary ## S_1, ..., S_N ##: can be...
  39. A

    Rotating waveguide antenna (rotating marine radar)

    I've seen these devices on shores as well as on ships , like a horizontal tube rotating slowly around it;'s axis. Now from what I know it's a type of radar, and unlike phased array it rotates it's beam physically by means of using a motor to rotate the antenna itself , what I want to know is...
  40. S

    A hypothetical isotropic antenna

    At 100m: (a) 0.03315 W/m (b)4166 W Since E is inversely proportional to 1/r^2, then E at 150m is 2.22 V/m. (a) 2.22/377= 0.00654 W/m (b) 4*pi*r^2*Wrad= 1665 W Is this reasoning correct?
  41. F

    DIY wideband antenna VSWR - beginner

    Dear forumers, I would like to do some RF measurements by using a spectrum analyzer or an SDR in the ~ 100mhz-6ghz range. I'm new to this and to the concepts in general and I would like to hear a few recommendations. I primarily interested here in how much it is acceptable to use a DIY...
  42. Amartansh

    I Intuition behind fields in Transmission lines

    How can traveling wave exist on transmission line if wires are perfect conductors (how voltages can be different at different positions on one perfect conductor)? I mean electric fiels should be zero on equipotential source. I know if length is too long compared to wavelength, we get phase...
  43. U

    I Stratton-Chu solution, special case

    I will try to ask the question, saving as much calculations as possible, so as not to weigh down those who want to try to help me. Starting from the general electromagnetic problem in empty space, taken as a domain a volume V delimited by a closed surface S, Elliot (1) shows how the field (i.e...
  44. D

    Can a Coiled Antenna Receive Radio Signals at 30 MHz?

    if an antenna 8.2 feet long was coiled up into a coil could it still receive radio signals at 30 megahertz?
  45. mastermechanic

    Cost of a 50 km range RC system & Type of Antenna

    Hello everyone, I'm a mechanical engineering student working on a reconnaissance/surveillance UAV system. To summarize it, 1-) It will either have FLIR camera system (If I can afford it) or GoPro with turret system. In either case it will transmit 4K or lower quality video record. It will also...
  46. D

    How does an antenna work with harmonics?

    if you fed a rectangular waves to a radio antenna, would it still produce radio waves? does it have to be a sine wave?
  47. GuillemVS

    Electromagnetic Waves and different energy manifestations

    When an object is hot its particles are moving faster than when is cold, right? I've searched that particles are electrons and protons, so it means that if we warm a object the electrons will be moving or even accelerating. Every charge accelerated creates Electromagnetic Waves (or light)...
  48. L

    Can a Tiny Antenna Transmit Long Wavelength EM Waves?

    Can you transmit em wave with antenna much smaller than its wavelength? For example. ELF antenna is very long. Can you make one small enough to fit in the pocket by the device constructing the long wavelength part by part?
  49. A

    The simulation of a microwave antenna made of nanofilm Ti3C2 (Mxene)

    Hello! I am a student of the radio engineering faculty and now I am engaged in master's work. The purpose of my work is to find application for Ti3C2 in antenna technology. I found a lot of information from article http://advances.sciencemag.org/content/4/9/eaau0920 But I can't understand how...
  50. L

    Precession in a 45 m high metalic antenna tower

    1- the antenna + motor weights about 400 Kg (the helix weights about 170 Kg and has adiameter of 6.28 m, and the motor of the antenna weights 230Kgs), there is an excentricity between the centre of gravity of the antenna + motor and the centre of rotation of the anetnna which is 42 mm in the yy...
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