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VHF Experiment

  1. Jan 19, 2009 #1
    Hi. I am designing an experiment focused on radio waves for my experiment and this is the basis of it:

    Three antenna's will be tested for point to point transmission (VHF): [ 144 MHz ]

    >>3/4 Wave antenna

    >>1/4 Wave antenna

    >>5/8 Antenna (Best Angle of Radiation Usually)

    These antennas will be connected to a VHF radio transceiver, a voice signal will be transmitted and this will be measured with a field strength meter. The purpose of the experiment is to test whether vertical polarization vs horizontal polarization will make a large difference in field strength. (basically testing antenna up straight and then again when horizontally polarized and looking for differences in field strength).

    My aim is to produce data which can be graphed illustrating the differences in field strength based on the different antennas as well as the polarization differences. I am hoping that this can lead on to an explanation of how angles of radiation affect directivity and gain.

    Also I am wondering whether to build the antenna's professionally (I have the equipment to do so), or to demonstrate the simplicity and to simply use wire. (The reason I have chosen these antennas is because they are the types that will match my transciever impedance)

    Any suggestions/help/advice are NEEDED!, I would highly appreciate it.
  2. jcsd
  3. Jan 20, 2009 #2
    I would like to know how far the transsmission will be measured over i.g miles
    the power used watts

    As a ameture radio operator if you have some one on a vertical and some one on a horizontal there can be a diiference and vice versa and sum time they can be very distorted.

    What type of antenna are u using?

  4. Jan 20, 2009 #3
    Hi. :)

    As for antenna's I am not sure whether to simply use wire cut to the appropriate wavelength or to build them professionally myself.

    I will use:

    3/4 Wave l.

    1/4 Wave l.

    5/8 Wave l.

    regardless of whether it is wire or a professional antenna. What do you think would be best in this regards: Wire vs Proffesionally built ( I can make it at home). Because maybe to illustrate the simplicity of it a wire may suffice. Any advice?

    Power used: 25 Watts

    As for distance..... I have no idea what would be an effective distance to for point to point to get good readings. Any advice?

    The Rx reciever will remain constant with a vertical antenna at all times. Only the transmitter Tx antenna will change in polarity.
  5. Jan 20, 2009 #4
    The best antenna you could use would be a dipole they are very efficient and you will get the standing wave ratio down to about 1.2.1. bought antennas are a gimmick they place capacitors in to burn off unwanted power to get there swr down so there is aloss of power.

    I would try the setup hozizontal to horizontal and vertical to vertical and vertical to horizontal.

    Throught my experience of radio I listen to most of the broadcast on a hozizontaly long wire end feed this also picks up a lot of interference ect if i get sum one transmiting on a vertical there is distortion and the signal is not as strong as me switching to my 35 foot whip the signal is stronger.

    Hope this helps

  6. Jan 20, 2009 #5
    Thankyou very much. :)

    What do you think would be a good distance between Tx and Rx to take measurements with a homemade field strength meter?
  7. Jan 20, 2009 #6
    VHF does not travell far as the wave is small and anythink in its way will effect it strength. what distance where you thinking? couple of miles i was thinking because the power will just bend the needle rite over.

    Is it possible to have a copy of the schematics of the homemede strenght meter.

  8. Jan 20, 2009 #7
  9. Jan 20, 2009 #8
    I'm confused, if you have a receiver why do you need a field strength meter?
  10. Jan 20, 2009 #9
    The Reciever is just to make sure that a signal is being transmitted (just incase what is picked up is interference or another station). The field strength meter will measure the differences in the signals strength based on vertical and horizontal polarization of the antenna.
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