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I have two separate low cost 433 MHz transmitter (datasheet) and receiver modules (datasheet) running at 5V.

I wish to use them both in half-duplex operation, sharing a single antenna.

My questions are:

  • Could I simply connect the antenna to both antenna pins? I am aware the full power transmission could then damage the receiver, but would this be a real risk at the low power I am operating them? I tried it on the breadboard and it worked fine, but I am not sure if there is a long-term risk.
  • If not, which solution would be the simplest way to go? I am looking at this SPDT switch between the antenna and RX/TX. Is there a simpler solution for adequate performance?
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2 Answers 2

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The antenna will work identically for transmitting and receiving, so there is no limitation from the basic physics. The problem is purely one of overdriving a sensitive analog input, which is the receiver input. If the transmitter output is limited to the same power supply that the receiver runs on, this might not matter. It may temporarily make the receiver deaf for a short time after the transmission while its AGC recovers.

Look at the datasheet for the receiver carefully and see what the voltage limits are for the RF input pin. This is actually one of those rare cases where you may be able to use the absolute maximum limits instead of the operating limits if due care is used with all parameters. Your firmware knows when it is transmitting and can know to ignore anything from the receiver during that time and a short time afterward. During this time you only care that the receiver not be damaged.

A better all around solution would be to use a transceiver chip. These have the transmitter and receiver built into the same chip, with a single antenna connection. Whatever protection the receiver might need to not be hurt by transmitter RF levels has already been built in.

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  • \$\begingroup\$ The datasheets in my case are pretty bad... All I can read there is the sensitivity, which is -106dBm, but that probably doesn't say anything about the voltage limit of the RF input pin. Is there a possibility of a creeping damage over time (and RX performance degradation), or would you say that if it works say over a couple of days operation, there is no damage? \$\endgroup\$
    – henning77
    May 29, 2012 at 15:49
  • \$\begingroup\$ I am also interested in what you say to the SPDT switch idea. Is this a feasible solution or overkill? \$\endgroup\$
    – henning77
    May 29, 2012 at 15:57
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    \$\begingroup\$ @henning: I'm not going to guess what might happen due to higher voltage input into the receiver. Probably as long as the voltage is limited to the supply range it's OK, but that is purely a guess. If the manufacturer won't help, get a part from a different manufacturer where this behavior is specified, or talk to a field application engineer. RF relays can work, but will take power. If this is a low power design, then that may be out of reach. \$\endgroup\$
    – Olin Lathrop
    May 29, 2012 at 19:58
  • \$\begingroup\$ What solution did you go for in the end? I have the same 433mhz modules and planning on sharing the antenna for one of my projects. \$\endgroup\$
    – Wayne Shelley
    Feb 14, 2015 at 16:22
  • \$\begingroup\$ @phear: You should ask the OP. I'm not the one with the problem. \$\endgroup\$
    – Olin Lathrop
    Feb 14, 2015 at 16:31
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For low power you can use diode commutator, it is common solution. RF relays are just too slow and expensive. Another trick is directed coupling with circulators.

Another note: 2 antennas next to each other can be as strongly coupled as galvanic connection, meaning that you will still have the same problem even with 2 antennas.

This solutions are from what I remember from decades ago, thinking that not much changed since. Just curious myself what else is possible.

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