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A simple DIY antenna - an amplified dipole

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  • #16
    Originally posted by peterhr View Post
    The real attraction of the dipole is the simplicity of making an effective antenna.......
    The simplicity & effectiveness of half-wavelength dipole originates from it's following unique characteristics:

    (1) The impedance of half-wavelength dipole is 75 ohms, which gives full signal power transfer from antenna to receiver**.

    (2) It's length does not depend on insulation's velocity factor. A fixed length of 69mm (center to tip) each limb for 1090 mhz, irrespective whether it is bare conductor or insulated with any insulation.

    (3) It's impedance & gain is relatively insensitive to error in length. Hence if the limbs are not cut precisely to 69 mm, but say 67, or 68 or 70, or 71 mm, there will be no noticeable affect on it's performance.

    It's only drawback is its low gain (2.2 dBi), which cannot handle long cable runs (mine is 50 feet). This problem I solved by adding a low cost satellite in line amplifier 950-2050Mhz, 13-20 dB.

    Realizing these characteristics of half-wavelength dipole, it was the first antenna I decided to make, and after it's successful trial, posted it in FR24 'best antenna' forum.

    My design was indoor type (see peterhr's very first post above, the second photo, one with a hand). This design is not useful for most hobbyists who use outdoor installation. It was very wise and thoughtful of peterhr to develop an outdoor design, which can be used by most hobbyists.

    **When a 75 ohms antenna is connected to 75 ohm coax and 75 ohms usb receiver, the SWR = 1 (100% transfer of signal power from antenna to receiver). Even if 75 ohm antenna is used with 50 ohm coax and 50 ohm receiver, the SWR=1.5 (97% transfer of signal power from antenna to receiver), a very decent value
    Last edited by abcd567; 2014-12-18, 23:58.


    • #17
      Originally posted by Kato View Post
      .......As for an amplifier, I suspect that you would see little to no improvement from using one, due to your short (2-3 metre) feed cable. The main function of an amplifier is to compensate for signal loss in a long cable run, i.e. if the antenna picks up a weak, distant signal, that signal may not reach the receiver if it has to travel down 20 metres of lossy co-ax - in such circumstances, an amp at the mast-head can boost the weak signal so it can reach the receiver. On the other hand, if the signal is just not being picked up due to a poorly constructed or badly sited antenna, then amplification will be no help at all.
      Originally posted by eastons View Post
      ........not sure how much improvement amplification would bring.
      Originally posted by peterhr View Post
      Amplifier adds little to the range (maybe 5%) but does bring in more planes from within the range ... possibly they're putting out less signal in your direction.
      I have tried the half-wavelength dipole + 3 meter coax to receiver, first without amplifier, and then with amplifier. Range without amplifier was 200 km, which jumped to 450 km when I added amplifier.

      This simple experiment proved the same conclusion by theory, as follows:

      (1) Strength of received signal is inversely proportional to the square of the distance between transmitter & receiver. Hence the strength of a signal from a plane when at 400 km as compred to when same plane was at 100 km, will be squre of 100/400 = square of 1/4 = 1/16 !

      (2) All receivers have a cut-off level of input signal below which planes are not shown in the output of receiver. Weak signals from distance planes do get picked up by antenna, travel down to receiver through a very short piece of coax (i.e. negligible loss in coax), but being below the cutoff level of receiver, will not be shown in receiver's output. In this situation, the gain of antenna plays a significant role.

      High gain antennas boost the distant weak signal above receiver's cutoff level, and hence distant planes are also shown, resulting in increase in range.

      If antenna is not high gain, same result is achieved by adding an amplifier.
      Last edited by abcd567; 2014-12-18, 04:06.