Good day to you all. Am new to RF designs. Presently, am working on a device using RF module for transmitting my signal from TX to RX. Am using the common 315-433MHZ RF module bought from ebay.

My questions are:

  1. Can I use 1/4 whip antenna for this considering a distance of 60m?
  2. Can I place the antenna indoor (for both tx and rx), what is the maximum distance it can go if placed indoor?
  3. If I can't place the antennas indoors, what is the best cable to use to extend the antenna outdoor?
  4. the SMA connector of the antenna has the inner pin(antenna) and the screw(body). Do I need to ground this body to my circuit ground?

I have executed this project before using normal portable radio receiver antenna but I had so much interference issue.

  • \$\begingroup\$ What does the data sheet say? When I say "data sheet" I don't mean a link to an ebay page, I mean a properly constructed pdf style data sheet. \$\endgroup\$
    – Andy aka
    Commented Jul 17, 2015 at 11:17

1 Answer 1

  1. That depends on the transmission power and receiver sensitivity. The 433 MHz band has fairly generous power limits for non-continuous (< 0.1%) transmissions.
  2. That should work in general, but will of course attenuate the signal, so you need some power reserve for that.
  3. The best cable is coax, impedance matched to your transceiver circuit. Most use 50 Ohms.
  4. The shielding should be connected to something with a large capacitance. Ground is usually the best choice.

The shielding isn't terribly important for the antenna itself (it wouldn't work if it were shielded), but if you don't connect it, the RF characteristics will be slightly off.

  • \$\begingroup\$ It's not shielding, it's the other half of the connection, the negative terminal. A bit like asking if you need to connect the negative lead of the battery, since the important +12 volts wire is connected. The RF ground needs to be connected to the other side of the transmission line on the board, which is most likely the ground plane. \$\endgroup\$
    – tomnexus
    Commented Jul 17, 2015 at 10:37
  • \$\begingroup\$ No, this is not a return path for any significant current. The antenna attempts to radiate the power away, not sink it into the ground. Not connecting it causes an unwanted impedance mismatch at the connector in addition to the wanted mismatch in the antenna. \$\endgroup\$ Commented Jul 17, 2015 at 10:58
  • \$\begingroup\$ ?? The currents on the centre and outer conductors are equal - Kirchoff current law. Unless you screw a simple monopole to the board, then the outer isn't used I guess. But when driving a length of coax cable, the currents are equal, so the board design needs to support the return path. \$\endgroup\$
    – tomnexus
    Commented Jul 17, 2015 at 15:18
  • \$\begingroup\$ Correct -- we're talking about the connector on the antenna though. My point is that the antenna is unlikely to have a significant length of coax inside, so whether that is connected is not that relevant -- however the shielding on the coax leading from the board to the antenna is important, and should be connected to ground (or something else that behaves like ground -- we don't need to close the loop as we're not sinking DC). \$\endgroup\$ Commented Jul 17, 2015 at 16:14
  • \$\begingroup\$ Ok, true, OP asked about the antenna. I got distracted by the coax extension question. It would still be unusual not to ground the outer, at UHF and up, the antenna might have a loading coil etc. \$\endgroup\$
    – tomnexus
    Commented Jul 17, 2015 at 19:18

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