Simple random wire antenna for 1 Mhz

Discussion in 'Wireless & RF Design' started by Synaps3, Aug 18, 2013.

  1. Synaps3

    Thread Starter Member

    Jun 5, 2013
    67
    2
    I am just getting into radio, so bear with me. This is a very simple question about antennas. I have an AM 1 Mhz transmitter that I built. The antenna I am using is just a random wire antenna end fed. The wire is 18 gauge speaker wire that I have cut to what I thought was the right length for 1/4 wave operation (246 feet). Another antenna length site I looked at told me it should be 234 feet, so I am a bit confused. My problem is that it seems like the antenna has way too much reflectance because when I turn on my transmitter anywhere above half power, it starts to have feedback. The amperage draw of the transmitter begins to bounce up and down and then it stops working. I'm afraid it's going to damage it. I need a way to make the antenna less reflective (I think that's the problem). I have read about loading coils, but I thought they are only necessary for tuning an antenna that is not the right length. I always thought if the antenna is the right length it isn't necessary to have a loading coil. How close to the right length does the antenna have to be? Is a few feet ok or do a few inches make a big difference. Any help?

    Thanks.
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    You may be violating RF transmission regulations. Careful you don't end up in trouble with the authorities.
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,145
    1,791
    The wavelength of a 1 MHz. signal is 300 Meters. One forth of that would be 75 Meters, or 246 feet. You have two real problems. The first is that an end fed wire less than 246 feet above the ground may or may not actually radiate a signal. The second is that whatever length you use, the bandwidth will be incredibly narrow like 10-20 kHz.

    Your power level had better be on the order of milliwatts or you will be receiving a visit from some nasty Men in Black.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    246 feet is the length for one-quarter of the wavelength. Then you have to take into account the "end effect" of an antenna. The "end effect" makes the antenna appear to be longer.

    234 feet is the suggested length for a 1/4 wavelength centre-fed dipole, taking "end effect" into consideration.

    For an end-fed antenna, you want the length to be 1/2 the wave length. You need to double your length.
     
Loading...