Tiny VHF/UHF Transceiver modules from China based upon the RDA1846/7 System On a Chip [SOC] opens the door for inexpensive remote receivers and repeaters. The basic RDA1846 analog chip is available for under $3 each, and the RDA1847 digital/analog version is under $15 each. These chips are second and third generation versions of the original RDA1845 chip that is widely used in many of the $25 FRS radios coming out of China, like the Baofeng. The main improvement of the RDA1846 is the option to control the pre-emphasis/de-emphasis in the transmitter and receiver. Other believed improvements were the squelch circuit and receiver blocking. The specifications are here, and the chips are available on Ebay and at commercial parts houses. The 40 pin chips are very small and would fit on the head of a pencil eraser. They are almost impossible to use by hand, without automated pick & place robots to assemble, they are almost useless.

A number of companies have placed the chips on about 0.75" x 1.42" carrier boards we shall call modules, with all the necessary support components to create a working radio. The chips can work on 3 bands, but all of the current modules operate on a single VHF or UHF band. Most of the modules include micro controllers [MCU], EEPROM memory, crystals or TCXOs, so with the addition of a microphone, audio amp, speaker, PTT button, battery and antenna, you have a working radio that retains the specific programming.  However, the transmitters have no on-board lowpass filters, so have high harmonic output. If you build anything with these modules, you MUST install a lowpass filter.  A comparison of some modules available in April 2018 follows:       

Make Dorji.com NiceRF.com NiceRF.com NiceRF.com sunrisedigit.com sunrisedigit.com NiceRF.com NiceRF.com
Model DRA818U SA818 SA828 SA858 SR_FRS_2WU FRS_DEMO_B DMR818 DMR828
Form Module Module Board Board Module Board Module Board
Main Chip RDA1846 RDA1846 RDA1846 RDA1846 RDA1846 RDA1846 RDA1847 RDA1847
Format Analog  Analog Analog Analog Analog Analog Digital or Analog Digital or Analog
Avg Cost $9.58 $8 $16 $55 $13.50 $21 $50 $55
Notes: Basic Module Basic Module Speaker Amp Speaker Amp & Heatsink Requires External MCU Speaker Amp channel sw Basic Module Speaker Amp
# Channels one one 16 16 one 16 one 16
Tx Power 1 Watt 1 Watt 1 Watt 4 Watts 1 Watt 1 Watt 1.5 Watt 1.5 Watt
Tx Stability unstated 1 ppm 1 ppm 1 ppm unstated unstated 1 ppm 1 ppm
Voltage 3.3 - 5.0 3.3 - 5.0 3.3 - 5.0 3.3 - 8.5 3.3 - 5.0 3.3 - 5.0 3.0 - 5.0 3.0 - 5.0
Size mm 19 x 36 19 x 36 28 x 41 21 x 59 20 x 38 45 x 90 24 x 40 39 X 53

All of the modules are designed to be soldered onto other larger boards, containing additional components. The products above marked as boards, are somewhat larger than the modules and are ideal for evaluating or building simple projects. The boards contain SMA antenna connectors, channel switches, audio amplifiers, PTT switch, microphone and larger solder pads for wiring.  However, you must include a low-pass filter to eliminate the transmitter harmonics. A low-pass filter kit for VHF can be found at qrp-labs.com for under $5 or look at MiniCircuits.com for other filters like the PLP-550+ for under $15. 

The DRA818M above is similar to the current modules. This gives you an idea what is inside.  The RDA1845 chip is 0.197" square.  All modules have a metal shield enclosing most of the assembly. This will provide reasonable protection for spurious radiation from the transmitter, but it is inadequate as protection for the receiver.  The use of a small tin metal box such as those available from PaperMart.com like the tiny 656070P  or the larger  651641321P tin can with cover, to house the module, filter and other parts.  If you go to the trouble to use a metal box, be sure to use feed-thru caps to isolate the power, audio and other leads coming out of the box. Otherwise, unwanted signals my enter or exit the enclosure.   

There are many of these modules in use by Radio Amateurs [Hams] as remote receivers. If you intend on using the receiver in a high RF environment like a large city or somewhere with any TV channels nearby, use a pre-selector or bandpass filter to limit excess off-channel energy from overloading the receiver.  Many Hams strip the front-end filters from older Motorola or GE radios that have several helical resonators. These really work great and are usually several MHz broad.

It would be advisable to do some extended temperature testing if you plan installing these modules outdoors and if transmitting, extended test at your desired power level. The specifications are rather sparse and you may encounter some modules with poor temperature compensation or unable to transmit at full power without failing. The NiceRF brand is the only manufacturer who claims to use a KDS brand TCXO that is specified at 1PPM. However, they do not specify an actual temperature range for 1PPM.  

Below is the digital version which includes support for many DMR functions.  Keep in mind the module was designed to be a user terminal, otherwise a handheld or mobile. Therefore attempting to use it for a base station or repeater may require some improvising in the digital mode.
If the reader intends to use one of these various modules for some commercial application, the receivers should be no problem. However, only the DMR series modules can be Type Approved under FCC Part 90 rules, because in early 2000 the FCC decreed that only digital capable transmitters would be approved, even if the transmitter was to be used for analog FM only.

Two of these modules would work well as a repeater, if they were placed in individual metal boxes. The typical receiver standby current is only 60 ma, so you could easily operate on solar. The transmitter should be powered through a timer so if no activity occurs for maybe 5 minutes, the transmitter is powered down completely to save battery. The first key-up of the repeater might take another 1/3 second, or less.  If the antennas were far apart, or the frequencies were far apart, then you would not require a duplexer. If you wanted to separate the antennas, instead of running coax, simply place the modules with the antennas and connect with  a pair of wires to carry the audio and power between them. Simply reverse polarity to transmit.  Vertical separation of antennas always provides vastly greater isolation than horizontal separation.  But at 1 watt, it may not be noticeable.  

Last revision April 25, 2018 by Fred Daniel