Hosim S911 2.4 GHz RC car technical review and fix

Related: Increase RC Helicopter range via antenna fix

These $35 - $75 toys have been cropping up in the past year or so, highly capable, repairable, and hackable for the price. I was curious as why I was getting much less than the specified 100 meter range from control to car. I found some surprises and some questions as well as answers.

The control is made by Lansu, a known manufacturer of inexpensive RC servo control systems. There are two chips inside, one had illegible labeling and was connected to handheld control inputs, while the other was connected directly to the antenna with model LT8912SRC, which I could not find on the internet. There is no FCC ID that I could find anywhere on the unit, despite copious approval logos.

Chips inside Hosim S911 controller
Chips inside Hosim S911 controller.

One leg attaches directly to a 4.5 cm wire monopole antenna. On my remote, this wire was tucked down amongst other wires instead of sticking up into the antenna pipe–probably why I was getting 20 m range instead of 100 m range! Also, for 2.4 GHz we would expect about 3cm for a monopole antenna, so we are probably fairly non-resonant, perhaps some high > 3:1 SWR affecting range.

As compared to a helicopter controller spec’d at 3dBm output power, this controller, was about 6dB stronger, so the S911 controller may be just under 10 mW EIRP.

Antenna bent over and tucked away, causing poor range
Antenna bent over and tucked away, causing poor range.

The fix was sticking the wire up from the controller–they tucked the antenna wire inside because the antenna pipe was in the wrong location–some disconnect between mechanical engineering and electrical engineering departments is apparent!

Fixed RC car maximum range by letting antenna wire protrude
Fixed RC car maximum range by letting antenna wire protrude.

Frequencies Used by Hosim RC

Hosim Car Radio

beacons every 49 ms at 2404.3 MHz when in pairing mode, stops transmitting until no sync heard from control (out of range or control turned off). Looks like FSK.

Hosim S911 controller at 2404.3 MHz
Hosim S911 controller at 2404.3 MHz–long time 33ms period.
Hosim S911 controller-short time 100us preamble + 1400us message
Hosim S911 controller-short time 100us preamble + 1400us message.

Hosim Controller Radio 

does not transmit until receiving pairing signal from car, then transmits indefinitely even without control input

Freq (MHz) length (us) cadence (ms) Function
2404.3 1400 48 car sync / pair
2404.3 1500 33 ctrl sync
2422.7  1700  4,60 ctrl
2432.7  1700  20,28 ctrl
2442.7  1700  4,13 ctrl
2452.7  1700  4,20 ctrl
2462.7  1700  4,16 ctrl
2472.7  1700  4,20 ctrl

Secret unpopulated controller switch

All it does is reverse the left-right steering, perhaps to allow for different steering potentiometers. I was hoping it was a “novice/pro” switch like on my RC helicopter, but no joy.