If you need to put the antenna away from the board, outside a housing or replace
it with a directed one you need to attach a shielded cable instead of the
standard wire.

The shield should surround the inner wire as close as possible after the
connection to the antenna port. Everything from the end of the shield works as

The inner wire is soldered to the pin of the (then removed) wire-antenna and the
shield to the third pin, so there is one unconnected pin between antenna and shield.

Pictures follow later…

Antenna Lengths For Bands Covered By The RFM12B Radio

Antenna shape is not crucial for short ranges, so a quarter wavelength monopole
antenna made of a piece of copper wire should be adequate for most uses. The approximate
size for a 1/4 wavelength monopole, for the center channel (1600) of a band, can be
calculated according to the formula:

λ = c / f

  c = speed of light in vacuum, 300000km/s,
  f = frequency in Hz,
  λ = wavelength in meters 

  Band    Monopole Length, λ/4
434 Mhz |  173 mm
868 Mhz |   86 mm
912 Mhz |   82 mm


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