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Covid-19 Lockdown Experiments – eBay 1-1000MHz 3W Wideband Amplifier Power Test

Since the entire country is on Coronavirus lockdown at the moment, I figured it was time to get round to finishing off a couple of small blog posts while waiting for the restrictions to be lifted šŸ˜‰

3W 1GHz Amplifier
3W 1GHz Amplifier

This is a small two-stage linear amplifier module available on eBay fairly cheap for SDR operation.This unit claims 3W (34.8dBm) power output at 0dBm input, however not surprisingly, this amplifier isn’t quite flat across the frequency range.

Frequency Response
Frequency Response

Here’s the readout from an R&S FSV7 spectrum analyser. The amplifier is being driven from the analyser’s tracking generator at 0dBm, and the output is fed back into the input via 60dB of external power attenuation. The span here is 1MHz-1GHz, and at the top end the frequency response is already beginning to drop off a cliff – the 1GHz rating appears to be the 3dB down point. The rated output power of 3W appears to only be attainable below 100MHz for the rated 0dBm input, after that it drops pretty quickly to about -3dB.

FrequencyOutput - dBmOutput - W
100MHz34.742.97
144MHz34.42.75
200MHz33.812.40
315MHz32.591.81
433MHz31.461.39
500MHz30.911.23
600MHz30.871.22
700MHz31.041.27
800MHz30.951.24
900MHz30.241.05
1000MHz26.860.48
Extended Span
Extended Span

Extending the frequency span of the analyser shows the roll off at high frequency – this module really isn’t usable above the rated frequency range.

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Fake Nagoya NA-771 Dual Band Antenna

NA-771
NA-771

eBay yet again doesn’t disappoint when it comes to fake kit. Here is a claimed dual-band antenna, a Nagoya NA-771. This is supposed to be good for both 144MHz & 433MHz.

Base Logo
Base Logo

The first clue is in the logo printing on the base, it’s not straight, or even. The base isn’t aligned correctly with the connector either, being glued on at a janky angle.

165MHz
165MHz
433MHz
433MHz

Beyond looks, a network analyser really shows the problems. Here’s a scan from 50MHz to 500MHz. There’s a fairly decent SWR of 2.01:1 at 164MHz (the tuning is still off, it’s supposed to be at 144MHz), but the SWR at 433MHz is 13:1!

544MHz
544MHz
920MHz
920MHz

Extending the range to 1GHz does show another couple of resonant points, at 544MHz (1.99:1) & 920MHz (2.83:1), well outside the band that this antenna is supposed to work to.

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QYT KT-8900D Teardown

Front Panel
Front Panel

Here’s a miniature-sized mobile rig, which I purchased from eBay to fit in the car – my main Wouxun radio is far too big to fit under the dash! This unit is not much bigger than a UV-5R and cost less than Ā£60! The front panel has a colour TFT LCD for the user interface, with a standard pot for volume, and rotary encoder for menu actions. Most of the controls are actually on the DTMF PTT mic, but some things are operable from the front panel.

Rear Panel
Rear Panel

The rear panel has a small fan to cool the internal RF power amp, and a PL-259 connector for the antenna connection. This will be the first thing to be replaced, with an N-Type. There’s also a headset connector on the back, along with a 3.5mm TRS jack for serial data – very important on this radio for programming, as the interface is abysmal in this department ;).

Internal View
Internal View

Popping the plastic bottom cover off allows access to the internals. There’s a single PCB in here with a double-sided load. Unfortunately the PCB is too difficult to remove from the casing without damaging anything, so only this side to be seen! On the top though is the main system microcontroller, the broadcast FM receiver, voltage regulation and the RF output stages.

RF Output
RF Output

The RF output filters are tucked into a corner of the board near the antenna connector, with a small relay to switch between VHF & UHF. My concern with this relay is that it’s not intended for RF use, and is in fact a general purpose relay. This would have been designed into the unit as a price reduction measure. Under the brass plate & thick SIL pad is the main RF output transistor. The external fan leads also pass very close to the RF output stage, so they may end up radiating some RF from the back of the unit, despite the ferrite bead on the leads.

FM Receiver & DTMF Receiver
FM Receiver & DTMF Receiver

Just behind the front panel is the broadcast FM receiver, an RDA5802, with it’s 32.768kHz clock crystal. Just above that is the DTMF receiver & decoder, used in most Chinese radios, with it’s ceramic resonator. There’s a couple of unmarked pots on the board, but I am not certain what their function is.

RF Output Stage
RF Output Stage

Here’s the RF output stage, with the “heatsink” removed. I’m concerned on several fronts with this part – the heatsinking provided by a small brass plate and thick SIL pad is going to be poor at best, but looking at the datasheet for the main RF transistor, an AFT05MS006NT1 RF LDMOS N-Channel Lateral MOSFET from NXP shows some alarming numbers. Grab the full datasheet AFT05MS006N.pdf (68 downloads) .
Remember that this radio is intended for mobile use in vehicles – the electrical systems of which can in a normal operational state rise to 14.8v.
This transistor is intended for handheld radio use, with an operational voltage of 7.5v, and absolute maximum ratings of 12.5v! Even when used on a regulated 13.8v PSU, the absolute maximum rating for the transistor is being exceeded.
At the very least, I would expect the life of the radio to be shortened due to this problem, and at worst the transistor may catastrophically fail in service, damaging the radio.

Main Microcontroller
Main Microcontroller

The main microcontroller is a Renesas R8C series device, with quite a few peripherals. It is accompanied by it’s clock crystal, and a programming header. An FFC cable vanishes off to the front panel PCB for driving the LCD & connecting up the user controls & mic connector. Just to the right is the main voltage regulation section for the electronics, minus the RF output stage, which is directly connected to the DC input bus.

Front Panel PCB
Front Panel PCB

There’s not much on the front panel PCB, so I won’t bother taking it out of the frame, this is going to mainly be interconnects for the SPI/IĀ²C driven LCD & analogue channels for the audio.

I’ll keep the blog posted with lifespan checks on this radio, as I’m definitely concerned about the power amplifier transistor, but other than that it seems to be an OK radio. The rotary encoder has zero debounce, so it doesn’t work properly, but this isn’t much of an issue when the radio is fully operable from the PTT mic.