BAOFENG BF-888S LOW POWER MODIFICATION FOR BRIAN

BAOFENG BF-888S LOW POWER MODIFICATION FOR BRIAN

BRIAN Construction Manual 1

A low power modification to the Baofeng BF-888S board in BRIAN is suggested for a number of reasons.

1. It ensures that there will not be a “self generated” RFI issue with BRIAN.

2. It reduces the heating of the final power amplifier FET and board

3. It reduces the required DC current when transmitting,

4. In most cases, it complies with FCC rule requiring you to use the minimum power required.

Here is a detailed explanation of the RFI problem and the process involved in investigating and resolving

the RFI issue.

The RFI problem exhibits itself in two ways.

1. Interference on the audio heard by the user on the radio he is using to listen to his node.

2. Interference to the proper operation of the node such as unintended keying of the node radio

The first two BRIAN prototypes we built in early 2018 used the Baofeng BF-888S radios with the full

length PC board. In these prototypes, we sanded the bottom of the BF-888S aluminum cast chassis flat.

The Baofeng PCB was reinstalled in the chassis and mounted to a prototype motherboard in the

Hammond extrusion currently used for BRIAN. These prototypes worked great and had no RFI issues.

Later in 2018, since these prototypes worked so well and our investigation of the alternative choice of

the SA-818 for the node radio confirmed the issue of distorted transmit audio at the beginning of a

transmission discussed on the HamVOIP website, we decided to design a PCB board which would

contain the required interface electronics and serve as a motherboard for the BF-888S and Sanwu FOB

as daughterboards.

This motherboard PCB was procured and we built several test units using various ½ length Baofeng

boards and various Sanwu FOBs (CM108A, CM108AH, CM108B). All of these test units exhibited RF

interference issues to some degree when using the rubber duck antenna attached to the antenna

connector mounted to the enclosure. If we used a remote antenna and a coax feedline, the RFI issue

was cured or greatly reduced. Further investigation revealed that we could cure the RFI issue by adding

additional capacitors for RF decoupling, ferrite beads and relocating interface components and wiring.

But we also found that what worked on one unit did not necessarily work on another unit. We also

discovered that changing the FOB and or BF-888S boards had varying effects. Additional testing using an

E-field probe determined that the most sensitive area on the board was the Sanwu FOB and the

wiring/traces connecting it to the rest of the circuitry.

Wealso found that the RF power amplifier was getting very hot on some units, even though we had

improved the heatsinking above that used in the Baofeng chassis by connecting the power amplifier

heatsink foil to our motherboard ground plane via an aluminum spacer. Testing of additional new

Baofeng unmodified HT’s revealed that the power output of 10 BF-888s varied from less than 27 dBm

(1/2 watt) to greater than 33 dBm (2 watts) with the 2 watt unit getting a lot hotter than the ½ watt

unit. Moreover, we found that there is a transmitter timeout timer in the BF-888S that is nominally set

to 3 minutes because the radio was not meant for 100% duty cycle operation at these power levels.



Thus we decided to investigate ways to lower the transmitted output power. It has been pointed out on

the web that although the Chirp and Baofeng programming software has a LOW POWER option, it has

no effect. So a hardware modification of some type was required.

We could not find a published schematic of the BF-888S but did find the following by studying the PCB.

Here is a photo.

Yellow indicates the RF signal path, Red indicates the collector/drain voltage paths and Cyan follows the

base/gate bias voltage paths.

The transmitted RF is generated by the RDA1846 IC at a level which, according to the part data sheet,

can vary from -2dBm to 8 dBm ( 5 dBm nominal). Immediately one realizes that this large variation

is probably the biggest reason that the output level varied by over 6 dB on the 10 unmodified units we

tested. Furthermore, the data sheets reveals that this output level is not programmable at the IC level.

The output from the RDA 1846, pin 18 is coupled via a capacitor to a 2SC3356 NPN RF Transistor

(Identified as R25) serving as a driver for the power amplifier (PA). It’s output at the collector is

capacitively coupled to the gate of an RQA009 N-channel MOSFET (H8 5043 in the photo). The drain of

the FET is then routed through a coupling capacitor to a low pass filter to remove harmonics and

ultimately to the rubber duck antenna.

Working on a board with a nominal output of 33dBm (1.05A total current), we first tried to reduce the

output power by replacing the 0402 SMD inductor to the left of the FET gate with a 10 ohm SMD

resistor. This worked well. The output power was reduced to 23.73 dBm but the total current went to

1.31A. We also tried moving the 10 ohms to the drive collector voltage feed of the driver transistor.



This resulted in an output of 6dBm but the total current increased to 1.31A. It appears that the PA is

operating class A and thus the power input requirement is not changed much although the output

power is decreased. So this type of mod to reduce the drive to the power amplifier does not reduce

dissipated power to permit 100% duty cycle operation.

A web search suggested removing the output FET (PA). This would have the advantage of reducing the

required current significantly since no class A bias current will flow through the FET. We found several

suggested methods to perform this modification. After lots of testing we settled on a slightly different

method used in BRIAN which is:

Remove the final power amplifier FET

Remove the inductor supplying DC voltage to the FET drain

Remove the 0402 SMD inductor feeding the RF signal to the gate of the FET

Connect a wire from the capacitor matching circuit on the collector of the driver to the

low pass filter.

Removing the FET drain inductor removes DC voltage from the FET drain and low pass filter. Removing

the 0402 SMD inductor removes the gate bias voltage from the circuit.

There is a picture of the completed modification on the next page. The output power will be from 13 to

17 dBm (20 to 50 mW) depending on the RF level from the RDA1846 IC. The current during TX increases

less than 50 mA over the current during RX operation of the board so 100% duty cycle operation will not

be a problem. At this RF level we encountered no RFI issues.

We have found that these power levels work well for operation around the shack, throughout the house

and even out into the yard.

If one wants a node for wider area coverage, we suggest the use of a mobile radio and a radio interface.

We created our DINAH (DIN connector based Allstar Node for Hams) for this purpose. Of course there

are many other choices for a radio interface from Masters Communications, DMK, Repeater Builder,

HamVOIP and Custom Comms to mention a few.



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BAOFENG BF-888S LOW POWER MODIFICATION FOR BRIAN