Notices
Results 1 to 9 of 9

Thread: Antennas tuning

  1. #1 Antennas tuning 
    Forum Professor
    Join Date
    Apr 2009
    Posts
    1,136
    I don't know lot about antennas and have couple of questions about them:
    Is it always needed to have a smaller size antenna for higher frequency,or there is some
    principal way to make large antenna to accept
    very high frequency,so it will be perfectly in
    resonance and highly efficient?
    I read that they use some tunning capacitors
    in antennas,how it's working?
    How could we influence impedance of antenna?


    Antislavery
    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Sophomore
    Join Date
    Mar 2009
    Posts
    139
    This is a very complex issue.

    I would recommend the AARL Antenna Guide as a good starting location, as well as "RF Circuit Design" by Bowick.

    There are many types of antennas, each brings its own advantages and disadvantages. The size on every antenna is based on the wavelength of the signal being received, so the larger your frequency is, the smaller the antenna should be.

    A tuning capacitor just changes the center frequency of the antenna by adding an inline, or parallel Z element. It can also change the Q factor. Adding it means that your old antenna network is no longer impedance matched, so you should be careful about what you add to an existing network, and rematch the system afterword.

    Impedance is based primarily on frequency, and secondarily on resistance, capacitance, and inductance. If you change the size of the antenna, or the frequency it operates at, or add any circuit elements at the base of the antenna instead of in the circuit, as per my text diagram, you effectively change the antenna impedance, although adding elements does not change the actual impedance of the antenna, it would seem like it did.

    Text diagram:

    V
    *-------------------------------[]

    V-> Antenna
    * -> where you would add the circuit elements
    - -> Wire
    [] -> communication system


    --
    -M

    "Those that would give up essential liberty to obtain a little temporary
    safety deserve neither liberty nor safety."

    -Benjamin Franklin, An Historical Review of Pennsilvanya, 1759
    Reply With Quote  
     

  4. #3  
    Forum Professor
    Join Date
    Apr 2009
    Posts
    1,136
    So, did I understood you right that it is physically and principally impossible to use
    big antenna to receive high frequency waves?
    For example use one meter size antenna to
    receive millimeter waves?And there is absolutely no way to do it (at least with good efficiency?)
    Antislavery
    Reply With Quote  
     

  5. #4  
    Forum Sophomore
    Join Date
    Mar 2009
    Posts
    139
    Theoretically, it should be possible to receive high frequency signals on a large antenna, but there is a physical limit to what you're getting.

    If you have a 1 m antenna, I would expect that you could receive things around 300 MHz. Depending on the bandwidth of the antenna, I'd say to expect roughly 74 MHz to about 1.19 GHz. However, just because you can see it doesn't mean you can recover it. The bandwidth is the critical factor. All antennas have them, and outside of that, it's anyone's guess whether or not the signal is properly recoverable.

    In the 1 m example, your center frequency is 300 MHz. The usable range will probably be + or - 100 MHz from there. After that, you'll need significant processing depending on the q factor and attenuation level. The higher the Q the more attenuation you will get outside of the bandwidth.

    It would benefit you greatly to just read an introductory chapter on antenna basics or communication systems.
    --
    -M

    "Those that would give up essential liberty to obtain a little temporary
    safety deserve neither liberty nor safety."

    -Benjamin Franklin, An Historical Review of Pennsilvanya, 1759
    Reply With Quote  
     

  6. #5  
    Forum Professor
    Join Date
    Apr 2009
    Posts
    1,136
    Actually, the stating point for my interest was the following article:
    https://inlportal.inl.gov/portal/ser...tory=DA_101047
    So,I'm interesting in understanding rather from
    physical then from electronic point of view.
    I thought if there is no way to simplify the concept they are working on and if there is
    real need to print billions and billions of all those minuscule greeds,diodes and capacitors.
    I thought, if the size of antenna's greed should always match the size of wavelength or not necessary.If yes,how then work all those parabolic,satellite antennas, 'cause there is no
    any external greeds visible.
    It's interesting if there is some way to get one
    decimeter size antenna to get in resonance with infrared radiation.So we could use one antenna,one diode and few capacitors instead of billions.
    Antislavery
    Reply With Quote  
     

  7. #6  
    Forum Sophomore
    Join Date
    Mar 2009
    Posts
    139
    I think if there were a way for that to work, we'd already be benefiting from it.

    Ultimately, I think they'll find a large scale version of that object too impractical for real use, and unable to provide enough power for use.

    As far as the satellite dish goes, their idea won't work with one. The reason is simple practicality.

    At best, for a single satellite dish you would get approximately 1.65 eV. 1 J is approximately 6.24150974E18 eV. So to produce 1 J, you need 3.78273318E18 antennas. 1 Watt-hour is 3600 J, so to produce a decent amount of power you need 1.36178394E22 total satellite dishes.

    I am no expert in satellite dishes or antenna theory, so take this with a grain of salt. This is all knowledge from classes I took a few years ago, and may not be 100% up to date.
    --
    -M

    "Those that would give up essential liberty to obtain a little temporary
    safety deserve neither liberty nor safety."

    -Benjamin Franklin, An Historical Review of Pennsilvanya, 1759
    Reply With Quote  
     

  8. #7  
    Forum Freshman
    Join Date
    Dec 2007
    Posts
    66
    It is quite possible to use a long aerial in terms of number wavelengths. The problem is it will have a weird radiation pattern and not be of much use.

    A rhombic is an example of a long aerial which has a useable radiation pattern.

    Generally for high frequency you need a mult-element Yagi, phased array or a dish.
    Reply With Quote  
     

  9. #8  
    New Member
    Join Date
    Oct 2009
    Posts
    1
    Hi, STNLY514

    There are lot of antenna available in the market.Antenna size doesn't important, but work principal & range is much more important .No tunning capacitor's are required for any antenna .
    impedance matching means maximum power transfer to the antenna .
    Reply With Quote  
     

  10. #9 Re: Antennas tuning 
    Forum Professor Wild Cobra's Avatar
    Join Date
    Sep 2009
    Posts
    1,140
    Quote Originally Posted by Stanley514
    I don't know lot about antennas and have couple of questions about them:
    Is it always needed to have a smaller size antenna for higher frequency,or there is some
    principal way to make large antenna to accept
    very high frequency,so it will be perfectly in
    resonance and highly efficient?
    I read that they use some tunning capacitors
    in antennas,how it's working?
    How could we influence impedance of antenna?
    No, at least not to my knowledge. Now there can be antenna arrays that appear longer than the wavelength, but they are multiple smaller antennas tied together.

    You can easily tune an antenna to accept longer waves, but an antenna that is longer than a half wavelength will start canceling the signal out.

    Now parabolic reflectors are an exception if you are looking for a way to cover more signal and concentrate it.

    This may help:

    wiki: Dipole Antenna

    wiki: Log Periodic Antenna

    Here's a satellite view of an RLP (rotatable log periodic) I once worked with:



    Note the shadow and scale.
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •