I want to build a simple transceiver (half duplex) for 433.92 MHz ASK modulation. I have found a transmitter chip and a receiver chip that I want to use. I want to use the same antenna for RX and TX but I am unsure of how to approach this (The receiver can handle the full output power of the transmitter). I do not want to buy a ready-made circuit for this, I'm doing this mostly to learn.

The antenna is 50 ohms but the receiver and transmitter both have their own impedances and need some matching network. If I used separate antennas this would be simple (say a PI attenuator).

But I’m unsure of how this would work with having both RX and TX on the same antenna. My very naïve approach to this would be to use a splitter/combiner after the matching networks and accept the -3 dB loss from this. This way every side sees 50 ohms.

But this feels very wrong and any input here would be much appreciated!

  • \$\begingroup\$ If you can make it work satisfactorily, it is not wrong. But if you want to go there, an rf switch is another solution. \$\endgroup\$
    – lakeweb
    Jan 25 '18 at 18:44
  • \$\begingroup\$ Also, if you start over, you could use a transceiver chip. I've used the like of this chip in the past and they work very well. \$\endgroup\$
    – lakeweb
    Jan 25 '18 at 18:52
  • \$\begingroup\$ You can use a circulator. \$\endgroup\$
    – mkeith
    Jan 25 '18 at 19:49

A typical 434 MHz low cost transmission system will have a free-space path loss of: -

Loss (dB) = 32.45 + 20\$log_{10}\$(f) + 20\$log_{10}\$(d) (Friis equation in dB form)

Where f is in MHz and d is in kilometres.

Transmission distance will be about 0.1 km and the pathloss works out at: -

32.5 dB + 52.8 dB - 20 dB = 65 dB

But antennas will provide some gain (about 2 dB each end) so the free space figure is more like 61 dB. However, most RF engineers will add another 30 dB for fade margin and this means the overall path loss is about 90 dB.

Using a splitter at both ends will degrade the power transmitted by 3 dB and degrade the power received by 3 dB; a total of 6 dB.

You then have to ask yourself if that is acceptable or not. For most cases, simplicity overrides performance and it isn't a big deal. However, 6 dB is equivalent to halving the range from 100 m to 50 m. I can't tell you if this is good or bad.


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