use one antenna for two transmitters

Discussion in 'The Projects Forum' started by DarkMavis, Oct 9, 2014.

  1. DarkMavis

    Thread Starter New Member

    Aug 24, 2014
    20
    1
    I use cheap RC transmitters and receivers to control practical effects for video projects. I buy the really cheap ones. You usually get 2 in a pack. One 40mhz and one 27mhz. I'd like to combine the controllers in a single box for convenience and so I can use better quality switches. I have one in there now and it works great. But, rather than have 2 antennas sticking out of the top, can't I just use one and connect both transmitters to it?
    I'd never need to be pushing buttons on both at the same time, so there would be no need to use a diplexer. I'm aware that antennas are supposed to be a certain length in relation to the wavelength of the signal or some such...thing is, while ONE is FAIRLY close to what would be needed for a 16th wave antenna, the other is nowhere near. In fact, I think they are all about the same length. In that case, is it okay to use one antenna? Or would all the other junk from the controller not being used at that moment also being attached to the antenna in use cause a problem? It's no problem to have 2 antennas, I just don't see why if it's not necessary here.

    Why ARE they all the same length when half are 27mhz and half are 40?
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    you need a duplexer., an LC circuit that plays high pass for one transmitter and low pass for the other.
     
  3. DarkMavis

    Thread Starter New Member

    Aug 24, 2014
    20
    1
    As I said, they will never be transmitting at the same time
     
  4. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    The main issue is pumping energy from one transmitter into the other. The one that is not transmitting can act as a short to the one that is transmitting. A simple LC band pass filter on each transmitters output will prevent those issues. Google "bandpass filter." Each filter would require one cap and one inductor. You could even go high pass on the 40 MHz transmitter and a low pass on the 27 MHz transistor.

    Of course you could just connected them directly. You said they were cheap.
     
  5. DarkMavis

    Thread Starter New Member

    Aug 24, 2014
    20
    1
    So, make a filter to filter from 27mhz (or a bit above it) down on the 27mhz transmitter and from just under 40mhz up on the other?
    Oh, since antenna length doesn't seem to be an issue, can't I just use a switch at the antenna to physically isolate the unused transmitter? I could use a dpdt switch and use it to also switch the power over as I'm sure there is some drain through the circuit even when it's not operating
     
  6. DarkMavis

    Thread Starter New Member

    Aug 24, 2014
    20
    1
    Yeah...nevermind. I just used a DPDT switch to switch the antenna and batteries over between the transmitters
     
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