I’ve bought an emonTX an a RFM69pi that I’ve plugged in a pi zero W. Everything works fine at very close range but signal range is abysmal otherwise: I only get garbage on the pi’s serial port past ~8 meters with clear line of sight and no interference.
I’ve read pretty much all the forum’s posts related to signal range - but what I’m seeing is strange so I thought I’d ask before messing up with antennas and extending wires:
- at close range, the strength of the received signal is ~ -60dB; at 5-6m it’s ~ -64dB (which is pretty good). But just a bit farther, eg. ~8m, with devices still within line of sight, there aren’t any valid packet received.
- I’m in a countryside area with no known UHF devices around so interference/noise should be at a minimum. I’ve done tests at three different locations and still get the same results (in one location close to a long metal hangar the max. range is only 4-5 meters)
- in all 3 locations, wifi from the pi to a laptop works very well at significantly longer distances
- both devices’ antennas aren’t loose, aren’t covered by anything, are perfectly vertical, are in the same plane, etc.
- putting ground planes below the devices or at the base of the antennas doesn’t help at all, with no noticeable improvement in RSSI.
- FWIW the RFM board is a few centimeters away from the pi zero, on a custom made extension board also used for gpios/inputs and a RTC (but there’s nothing I could think of that could create interference).
- tested with two good quality power supplies and with my laptop; same problem.
Like most radio stuff I’d have expected to see a decrease of RSSI and number of valid packets the farther the devices are apart but in this case it’s either “every packet sent by the emonTX is received” (confirmed by looking at the TX led while monitoring the serial’s port output) or “no packet is received at all”. Some forum posts mention RSSIs in the -70dB range (even up to -80dB) while still being able to receive valid packets - even through brick walls - so I’m a bit puzzled. Any clue ?
- in case I transform the RFM’s antenna into a dipole (and/or solder a small length of 50ohm coax cable to go through a wall), what location on the RFM’s pcb would be adequate to solder the ground ? I’m concerned about soldering next to tiny smd components as my soldering skills aren’t very good…
- rather than extend the emonTX’s antenna I’ll probably extend the CT cables by 2 to 3 meters with 2x1mm2 (AWG17) cables; out of curiosity, is there any maximum length (with decent cables) ?