Designing More Efficient RF Transmitter

Discussion in 'Wireless & RF Design' started by crazyengineer, Mar 12, 2012.

  1. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    Okay, so a couple weeks ago, I designed and built an RF transmitter. Here's the schematic for the circuit


    I attached a 30 inch telescoping radioshack antenna to the circuit. When operating at 1MHZ and 1vp-p, It pick up the signal but the distance between the two antennas is 2 inches. My goal is to increase the maximum distance to a meter. Can anyone provide any tips for improving my design.
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
    For 1 MHz you would need an antenna 75 meters long. The 30" antenna you have is appropriate for the FM broadcast band ~100 MHz. The most practical thing would be to redesign your circuit for a frequency matching the antenna.
  3. DickCappels


    Aug 21, 2008
    Or use a loading coil.
  4. davebee

    Well-Known Member

    Oct 22, 2008
    At short ranges like a few inches, you may be communicating via the near fields, the electric or magnetic fields, rather than by electromagnetic radiation.

    "it" picks up the signal. What picks it up?

    If you are using a conventional radio receiver, it should be so sensitive that you would almost have a hard time NOT receiving a radio signal from anywhere within a meter or so!

    The other responders are correct in that changes to your antenna could result in greater radiated power, but for short distances like what you're working with, practically any length of wire should be able to radiate enough energy to be received by a sensitive receiver.

    What is R12 doing there? It may be eating up power unnecesarily. I'm guessing it's there to try to establish a 50 Ohm impedance level at the antenna, but with such a short antenna, its impedance will be much more than 50 Ohms, so I'm wondering if that resistor is hurting more than helping.

    Maybe you could think in terms of trying to get a better impedance match between the output impedance of the transistor circuit and the imput impedance of whatever you choose for an antenna, whether 75 meters or 30 inches.