BAOFENG BF-888S LOW POWER MODIFICATION FOR BRIAN

BRIAN Construction Manual 1

A low power modification to the Baofeng BF-888S board used in BRIAN is recommended 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 interference problem and the process used by us to investigate

and resolve RFI issue with BRIAN.

The RFI interference 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

3. When the node is receiving, there is no RFI problem

The first two BRIAN prototypes that 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 designed and procured. We built several test units using various half 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 on the RFI

interference. Additional testing using an E-field probe connected to the output of the node 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.

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

improved the heatsinking betond 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 ten BF-888 radios varied from less than 27

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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.

In other words, if you happen to purchase a BF-888S that has high power output (like 2 watts), use it in

an Allstar node radio and mostly listen to an active node, the radio well get very hot!

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 discover the following by studying the

PCB. Here is a photo of the PCB which includes the RF transmit chain.

The traces marked with the color Yellow are the RF signal path, Red indicates the collector or drain

voltage paths and Cyan follows the base or 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 ( nominal is 5 dBm). Immediately we suspected 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 sheet reveals that this output level is not programmable at the IC level.

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

(labeled R25 in the picture) serving as a driver for the power amplifier (PA). Its output at the collector is

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

BAOFENG BF-888S LOW POWER MODIFICATION FOR BRIAN

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the FET is then routed through a coupling capacitor to a low pass filter to filter 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 collector voltage feed of the driver transistor. This

resulted in an output of 6dBm but the total current still was 1.31A. It appears that the PA is operating

class A and thus the current 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) as a method to reduce output power. 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, for use in BRIAN, we settled on a slightly different method than those found on the internet.

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 voltage divider 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. After the modification, the output

power will be from 13 to 17 dBm (20 to 50 mW), mostly 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

(www.hamprojects.com/DINAH ). 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.

BAOFENG BF-888S LOW POWER MODIFICATION FOR BRIAN

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