Field Strength Meter at a dipole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N4LQ, Aug 5, 2017.

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  1. N4LQ

    N4LQ Ham Member QRZ Page

    Assume I have an end fed, half wave, 20 meter wire antenna.
    Then I apply RF at 14 mhz to the end of the antenna via some form of matching system. I.E. an antenna tuner. etc.
    I then take a simple Field Strength meter, one of those simple diode across a meter with a whip antenna, an
    hold the meter's antenna near the antenna wire then move it from one end to the other.
    Where would I see the highest reading on my meter?

    On the fed end?
    In the middle?
    On the far end?
     
  2. KM4DYX

    KM4DYX XML Subscriber QRZ Page

    Current distribution on a half wave antenna is not feed location dependent; max current is in the middle. upload_2017-8-5_17-54-38.png
     
  3. N4LQ

    N4LQ Ham Member QRZ Page

    Sir: That is not what I asked.
    I want to know where along the wire my field strength meter will read the highest. Not the current.
     
  4. KM4DYX

    KM4DYX XML Subscriber QRZ Page

    Maybe I'm in over my head but I would think the greatest strength would come from the greatest current; but I have been wrong before, so I await someone smarter than me to provide us with the answer.

    73,
    Al
     
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I'd say it depends on the FS detector's antenna. "A simple whip" isn't enough of a description.

    If it's a point probe antenna (very short whip, single point connection) I think you'd measure the highest electrostatic field at the feedpoint and again at the tip (far end) of the wire. Professional (calibrated) FS meters never use this, they use loops. With a professional FS instrument and its loop antenna I think you'd measure the highest electromagnetic field at the center of the antenna.
     
    W5DXP likes this.
  6. W5DXP

    W5DXP Ham Member QRZ Page

    Also, the human body can have a large effect on the electric field incident upon a simple whip antenna. I use a ferrite rod loop antenna which is sensitive to the magnetic field on which the human body has little effect.
     
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    It's the reason "commercial" FS measurement systems all use loops; self-contained antennas minus ground influence.

    With a probe antenna (short whip), everything influences the reading, including holding the meter or even standing near the meter.
     
  8. K2WH

    K2WH XML Subscriber Volunteer Moderator QRZ Page

    The strongest RF field would be at the feedpoint ie. highest current.

    Highest voltage would be at the end. Depends if the antenna is resonant.
     
  9. K2WH

    K2WH XML Subscriber Volunteer Moderator QRZ Page

    That would be on a dipole.

    From what he is describing, he is feeding the wire at the end without a counterpoise or, the other half of the antenna.
     
  10. N4LQ

    N4LQ Ham Member QRZ Page

    Here is the FS meter I am using. 20170805_200954.jpg
     
    KC8VWM likes this.

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