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Receiving/transmitting with a Yagi antenna?

  1. Mar 27, 2017 #1
    Hi, I am learning about different antennas like for radio and internet wifi bandwiths and I came to the Yagi antenna.
    From what I understand, the people use this antenna both for receiving and emitting signal. For example, for pentesting (handshakes and all that) and for radio emitting (also using it in the antennas of routers for places where there is no internet cable connection it increases the download as well as upload speed).

    The strange part I don't understand is why/how emits the signal from the theory behind the antenna. The antenna makes the incoming waves synchronize, adding up the amplitude for the reception. But all this is in a passive way, how people use it for emission? For example in pentest even if you see the AP, you are not able to emit to the same distance with this antenna right?
     
  2. jcsd
  3. Mar 27, 2017 #2
    All antennas have the same gain (relative to a dipole) whether transmitting or receiving.
    One way to look at the Yagi is to consider it as an array of dipoles. Such arrays have a gain which is about equal to the number of dipoles.With a Yagi. the dipoles are phased to encourage end-fire action, so it radiates/receives off the end. Also with a Yagi, there is no feed line driving the elements - they rely on mutual coupling to obtain their energy.
    When the array is transmitting, all the dipoles are supplied with energy, and radiation in unwanted directions is cancelled, causing reinforcement in the wanted direction.
     
  4. Mar 27, 2017 #3

    davenn

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    that was a nice contradiction .... you had better clarify for the OP

    That is describing a log-periodic Yagi
     
  5. Mar 28, 2017 #4
    OK, thank you. Just for clarification, the dipoles are supplied with energy via mutual coupling.
     
  6. Mar 30, 2017 #5
  7. Mar 31, 2017 #6

    jim hardy

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    Where i live we receive only two TV statons.
    Here's a thread where several PF members walked me through building a Yagi antenna optimized for them
    https://www.physicsforums.com/threads/can-i-parallel-yagis.806936/

    Picture over the antenna is for some reason brighter and clearer than over satellite. That should be impossible if i understand anything about digital TV..

    Anyhow we learn best by doing. I second the advice to get ARRL antenna handbook.
     
  8. Mar 31, 2017 #7

    nsaspook

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    It would be impossible if it was exactly the same signal.

    The satellite channel of the same local OTA channel can look worse because of compression to increase the number of TV channels on one satellite (usually a spot beam) transponder. The raw digital speed from a OTA HD channel is about 19.2 Mbps from 6MHz of RF bandwidth. That easily carries a full fidelity digital TV HD MPEG-2 signal with modern compression technology. Most of the satellite HD channels are delivered using MPEG-4 so the original source might see much higher compression and lower speed transmission paths for a typical MPEG-4 channel that could noticeably reduce digital picture fidelity with good viewing equipment.
     
  9. Apr 1, 2017 #8

    jim hardy

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    Thanks ! I had attributed it to my own vanity, pride in the home-made antenna.

    Clearly you are far more conversant than i am on broadcast technology.
    I tried to read some digital TV standards . I was overwhelmed, and astonished at what is possible to do..
    In analog days a lot of information came during retrace interval. I wanted in 1970's to build a device to turn volume and brightness way down during commercials which are surely announced somewhere in all that communication.
    There's laws against building something to turn commercials off, but to just attenuate them ought to be legal.
    Now that so much is digital it's probably something a Raspberry Pi could do.
    Can you imagine what a get rich quick gizmo that would be? I'd buy a kit in an instant.
    Worth a thread?

    old jim
     
  10. Apr 1, 2017 #9

    sophiecentaur

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    Interestingly, Antenna Theory is usually taught, initially using transmitting antennas. It is far easier to start with a model with known amounts of current flowing in the elements than to do the extra step of finding how an incident wave excites those very same currents (the relative proportions). Either way, there is very fuzzy bit about what happens with local fields around the elements and how they relate to the travelling wave that's actually launched. The Yagi is definitely not the antenna design to start with. It's far too complicated because everything affects everything else significantly. An array with each element fed individually (amplitudes and phases controlled in the feeding arrangement. is much easier to understand - even though the interactions between the elements can have a significant effect when you try to be 'too clever' with the design.
    In the old days, there was no timing standard across the networks (and that analogue needed synchronism for clean cuts between sources) To get over the possibility of frame roll or line tearing, they introduced the white star flash before and after the adverts so no one would notice the break in sync before and after the ads. That was detectable by fairly simple circuitry and could be used to attenuate the sound. But it still didn't get over the gap in the programme. Nowadays, many (soon most?) people watch other sources than off-air ands they either pay without adverts or can time slip and step forward over the breaks.
    PS I let my brain do what your circuit would be designed to do and I never remember what the ads are about. Eat yer heart out Raspberry Pie!
     
  11. Apr 1, 2017 #10

    nsaspook

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    There are DIY systems like MYTHTV (I use it) that can detect and auto skip Commercials with close to 100% reliability on recorded or buffered live TV from a DTV source. It can also time compress programs to 1.5x with some loss of A/V quality.

    You could run MYTHTV or similar systems on a Raspberry Pi for a viewer but you will need PC level horsepower for a backend to take the digital HDTV signal, process it and store it for later viewing.
     
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