UHF Telemetry

The 3DR telemetry radios on 433Mhz (and their generic cousins using the HopeRF data link modules) are nice, but on the cheaper versions you never know what kind of antenna you are getting.  Some modules apparently ship with 2.4GHz WiFi antennas instead of 433MHz ones.

3DR Telemetry Radios (433MHz)
3DR Telemetry Radios (433MHz)

On the base station side, it’s not ideal to have the module plugged into the USB slot of the PC as it is very vulnerable to getting broken, and putting it on a USB extension lead means it flops around, and, probably doesn’t have anything like an effective ground plane.  Reportedly the proximity of the FTDI chip to the antenna also creates signal-to-noise issues on the Rx that can limit the range.

All of this caused me to experiment around with an alternative 70cm antenna for the base station side.  I based this on the ‘Slim Jim’ which is a J Impedance Matched design.  A great resource for this is at http://www.m0ukd.com/Calculators/Slim_Jim/

I built my Slim Jim out of 8mm alloy tubing using a mini pipe bender.  There are copper clips that can slide up and down the antenna to find the 50 Ohm feed point.  I also used a typical 1:1 choke balun design, made from about 9 turns of the feeder coax on a 1.5″ plastic tube to prevent the RF signal on the antenna from driving the RG 58 feeder.

433 MHz Slim Jim Antenna
3DR radio with 433 MHz Slim Jim Antenna

The 3DR radios don’t output enough power to use a normal SWR meter for antenna tuning so I used my 433MHz handheld transceiver to tune it while moving the feed point on the antenna.  Once tuned the Slim Jim gave a massive gain to the signal on that handset.  It seems to work really well on the 3DR radios too.  These modules use SMA connectors.  I need an SMA adapter on some radios – the 3DR radios have a pin in the reverse-polarity output socket (same as a WiFi LAN card) while the generic modules have a socket.

As of yet I haven’t flown far enough to have any issues with the telemetry signal.  I’ll update this post in due course.

Ground Station

This is the ground station with UHF telemetry, 2.4GHz RC and ImmersionRC 5.8GHz FPV.