I have one question regarding the length of the 868MHz antenna. According to the theory, a the wavelength of a 868Mhz signal is 300/868 = 34.6 mm, so a quarter wave antenna should be 8.6mm long. The table above indicates 8.2mm. May someone confirm the correct figure, and also indicate how critical is that difference in terms of range?
I tried 3 types of antennas with RFM69HCW 868mhz.
Just a 8.6 cm wire
- sma antenna
Indoor in my appartement (one floor) it doesn't matter which one I use. All is working fine.
I tried 2 nodes in my metal mailbox 10 meters below which has 2 tiny holes.
One with helical antenna, one with sma antenna.
Gateway (using sma antenna) receives no data.
Testing two repeater nodes (1 helical, 1 sma antenna) near a window I receive data via both repeaters. Sometimes the repeater with sma antenna is not relaying messages for the node with the sma antenna.
In my situation, the helical antenna works better for me. I just buy them on ebay.