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How does a resistor to ground from transmit pin effect power output? — Parallax Forums

How does a resistor to ground from transmit pin effect power output?

purplemonkeypurplemonkey Posts: 89
edited 2012-08-17 13:36 in Propeller 1
Just trying to get my head round this... Cant figure out how a resistor instead of antenna is more powerful, Is it that the lower the resistor value the higher the power output? So for instance if I had 25 ohm resistor on transmit pin what would be the watts it would output and what sort of range would it be?

Comments

  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2012-08-17 11:46
    I think you got it about half right.

    Yes, 25 ohm resister causes more current to flow.
    However, the resister is probably not the thing doing the transmitting.
    A resister is physically to small to do it. You need an antenna.

    In your case the antenna is the ancillary wiring to the regulator and the power buses.
    The resistor causes switched current to flow in these things which now act as antennas.

    Duane J
  • purplemonkeypurplemonkey Posts: 89
    edited 2012-08-17 12:12
    So if I had an antenna and also had a resistor going to ground would the resistor actually make any difference?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-08-17 12:13
    BTW, a 25-ohm resistor to ground is not recommended on a Propeller output, due to the current being over the specified limit. Be sure to refer to the Propeller datasheet for maximum ratings.

    -Phil
  • purplemonkeypurplemonkey Posts: 89
    edited 2012-08-17 12:15
    I read on a thread someone was using a 4.6 k resistor instead of antenna and it was broadcasting 80 feet?
  • Mike GreenMike Green Posts: 23,101
    edited 2012-08-17 12:24
    If you use a resistor (any value resistor), most of the energy going into the resistor will be turned into heat and not radio waves. An antenna (all types of antennas) has a characteristic impedance (AC resistance) that's determined by the frequency of the signal and the combination of resistance, inductance, and capacitance that makes up the antenna and its connections (including cabling) to the signal source. Any signal source, including a Propeller I/O pin has its own characteristic impedance where it will produce the most power. The better the match between this and the antenna, the more power will be transferred to the antenna and launched into the "void".

    The ARRL Antenna Book and the ARRL's Amateur Radio Handbook are good references for this sort of stuff.
  • RickInTexasRickInTexas Posts: 124
    edited 2012-08-17 12:36
    Okay, I suggested that you place a resister to ground (e.g. 1k per Heater's comment above) in order to radiate Less power, not more because the propeller puts out a lot of spurious emissions when used in this manner. A simple long piece of wire strung up will put out a signal on the desired frequency and many others for miles possibly resulting in a visit from the FCC or at the very least causing interference far, far beyond that needed for personal experimentation.
  • purplemonkeypurplemonkey Posts: 89
    edited 2012-08-17 12:59
    RickInTexas wrote: »
    Okay, I suggested that you place a resister to ground (e.g. 1k per Heater's comment above) in order to radiate Less power, not more because the propeller puts out a lot of spurious emissions when used in this manner. A simple long piece of wire strung up will put out a signal on the desired frequency and many others for miles possibly resulting in a visit from the FCC or at the very least causing interference far, far beyond that needed for personal experimentation.

    Oh ok i understand thanks, what is the impedence on the prop pins? And what length of wire would match the impedence?

    As when I tested it with a 32cm piece of wire I only was getting about 6 ft before signal dropped off dramatically. What is the unadulterated output of the transmit pin just with this type of antenna?
  • RickInTexasRickInTexas Posts: 124
    edited 2012-08-17 13:36
    purplemonkey wrote: »
    Oh ok i understand thanks, what is the impedence on the prop pins? And what length of wire would match the impedence?

    As when I tested it with a 32cm piece of wire I only was getting about 6 ft before signal dropped off dramatically. What is the unadulterated output of the transmit pin just with this type of antenna?

    On FM choosing a frequency on a relatively quiet channel, (not easy in a big city like the Dallas Fort Worth area I live in), I received a strong signal in a quarter mile radious on my car's OEM FM radio on a random wire w/o any other external components. A very strong signal was received on my Sony stereo receiver On my bench with nothing at all connected to the output pin. What sort of receiver are you using? Something with a PLL design (i.e. has a digital readout) should work a lot better as it will track the TXer's center frequency output drift.

    Antenna design, especially TX can get very complicated. I think it was Chip who suggested an LRC tank circuit that effectvelty increases the output of a TV broadcast power output by at least an order of magnitude.

    Also, a halfway decent receiver will receive the strongest signal on a given channel "ignoring" lower strength signal on that channel.
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