Design for connecting transmitters to antennas often mentions the transmission line, and the standing wave ratio (SWR) for matching. But what if the transmitter is on the antenna?

Background: I've an old 151 MHz narrow band FM 10mW transmit module TX1 from Radiometrix that I'd like to deploy monitoring a solar powered pump on a stock dam.

I plan to attach the RF out and RF ground pins on the module directly to either the base of a quarter wave with groundplane, or at the feed point of a yagi, or across a "slim jim" or J-pole antenna.

An Arduino Pro Mini will be adjacent to the transmit module, and a multi core cable will head down to the other equipment.

So the transmission line will be only a few millimetres long, and measurement of SWR seems impractical.

(This is a licensed use per schedule 1 item 15, 150.7875 to 152.49375, max EIRP 100mW. Source code, which details connections, is here)

  • \$\begingroup\$ I suspect that there isn't any sort of robust lighting protection on the inputs. You should really think about adding some sort of lightning protection to this as well, as I suspect a close by lightning strike would leave this a smoking wreck. \$\endgroup\$
    – rfdave
    Sep 7 '13 at 3:56

Any connection shorter than λ/10 (1/10 of the electrical wavelength, or about 20 cm @ 150 MHz) cannot usefully be treated as a transmission line. In that case, you need to match the transmitter directly to the antennna (or vice-versa). If needed, you can use discrete coils and capacitors to do that.

  • \$\begingroup\$ Thanks. My next question would be: why and how does one match a transmitter directly to an antenna using discrete coils and capacitors? \$\endgroup\$ Sep 6 '13 at 11:31
  • \$\begingroup\$ You should pose it as a question then. Comment fields aren't appropriate for follow-up questions in the StackExchange metaverse. \$\endgroup\$ Sep 6 '13 at 15:39
  • \$\begingroup\$ For l < 0.1 x wavelength, you can still treat the wire as a transmission line. It will be lots of extra calculation work to get close to the same answer as if you ignore transmission-line effects. But you won't get a wrong answer. \$\endgroup\$
    – The Photon
    Sep 6 '13 at 15:44

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