35/40 MHZ - RC transmitter / diplexer project


Today big aircraft models are big size and price, with weight up to 20 Kg. Loosing such an aircraft due to interference of some careless modeler, who comes to the field and just turns its transmitter on, or due to mailfunction of any component on the receivers side (batery, crystal, cables..) is a big financial and also security risk.

The idea is to controll the aircraft model with two fully independent receiver systems, including power supply. Each receiver will be used for of the model (right or left side). With this setup, in case of mailfunction of one subsystem, the model in most cases could still be brought safely to the ground. There is a problem with throtle servo, which can be solved with 2 servos connected mechanicaly, as shown.

For this task, most suitable is to have one transmitter on two different frequencies. But be carefull,

YOU CANNOT just connect two antenna outputs on the transmit moduler togeather; this causes strong interference signals and also transmit power drops significaly due to intermodulation products!

This could be done with appropriate DIPLEXER circuit, which isolates one transmitter module from the other and matches output circuit to the antena. In our country, we have two allocated bands for RC models, 35 and 40 Mhz, so i made diplexer for this bands, using eliptic HP/LP filter combination:

Response (simulated):

The circuit

The isolation is very good, and all rhe spurs in the tx signal are more than 50db below signals, as shown:

Some measurements:

I used futaba FC-28 transmitter and dual conversion 35 / 40 Mhz PCM receivers. The receivers are programmed for failsafe mode (neutral positions for controls, minimum for throtle). The transmitter modules have about 20dbm output power (i measured 21,3 dbm at 40MHz, 20dbm/35MHz). To use them, the matching coil at the output has to be removed and replaced by the jumper. The matching coil from 35MHz module is used at the diplexer output to match tx antena.

Normaly transmitter operates on 35MHz only, (the 35MHz module is located inside the case, there is just enough space), and when i want to fly big aircraft, i just plug second module into the remote control.

Tx module, diplexer in place:

The system has been tested on 2+ m aircraft for one year, ond it works great.

Throttle servo mechanical backup (in case of failure of one rx, you have abt of throttle freedom):

Important note

It is very important, that this kind of modifications are performed by qualified prsonnell and approved by frequency regulation authority, otherwise there is a big risc of causing unwanted iterference to other users of the frequency band!!!