Let's build a PA for 2,4 GHz

I have prepared a couple of board, tomorrw we will measure if the input matching is the same for all boards.

Taking measurements on 13cm is a realy challenge One SMA <-> N adapter has 2dB loss, another identical adapter only 0.5dB.

Also the temperature of the transistor has some influence on the input match.

But these are only details. The most important thing is, driven with the Amsat-TX mixer it generates a couple of watts output and can be used for QSOs.

I found one important issue: the maximum input power should never exceed 300 mW.

With 400mW at the input I measured a degration of the transistor gain, so we have to be careful not to overdrive it.

In the short wave Amps, I simply put Zener diodes on the gate for protection. For 2,4GHz this is not possible due to the diode capacitance.

Any idea how to protect such a circuit from being overdriven ?

vy 73

Kurt

(Board have been sent to DO1CTL, DD0KP and ON6AJ)

The last measurements were take in my Shack with very simple equipment.

This week we have been invited by Rohde&Schwarz to measure the PA with calibrated and expensive instruments.

The measurements were take by Alex, DG5RAQ and Johannes, DC1RJJ.

This picture shows the max. output power with 12 volts supply.

this is the spectrum (12 volts)

12V was measured just for fun. The datasheet specifies 28V for this transistor, so the next pictures shows the 28v operation.

The PA was driven with 25 dBm:

and generates 10 watts output power. Supply: 28 volts and 0,709 amps (=19,85 Wdc). This is about 50% efficiency, thats not bad for a 2.4 GHz PA.

this is the spectrum at 10 watts output. The measurement was taken using a power attenuator.

the requirement for spurious emissions (> 1Ghz) is -50dB. We measured 40,88. If the antenna has at leat -10dB at 4.8 GHz then we can use it without any filter. Otherwise some filtering is needed.

vy 73, Kurt, DJ0ABR

Before doing the final layout, we started some experiments with the BLF2425M9. Mounted on a homebrew prototype PCB this Mosfet generated 22 watts out from 500mW input with almost 50% efficiency.

(nice for use in portable 60cm dishes).

The plan for the final PCB is now as follows:

1) 2.4 GHz SAW filter at the input (as recommended for the Amsat TX board)

2) MHT1008 amplifier

3) LP filter

4) --- CUT ---

5) Attenuator

6) BLF2425M9 amplifier

7) LP filter

at step 4) there are simply a couple of holes in the PCB, the PCB can be cut here into 2 parts. So if only the MHT1008 is needed, the PCB is broken at 4) and the other part is thrown away (the PCB is very cheap ).

If one needs both stages (i.e. for higher gain), then he uses the complete board with a gain of up to 34 dB.

Some applcations (i.e. the Amsat Mixer board) need a gain of >17 but less then 34 dB. To adjust the gain to the needed value a simple attenuator (0,5 watts SMD resistors) can be mounted.

This week we are doing the measurements with the BLF2425M9 board and also design the low pass filters.

vy 73
Kurt, DJ0ABR

it took some time to get the new PCBs, but they arrived this week.

The first board which is finally released is the 10W amplifier.

10W means, compression begins at 10W output. In SSB we can expect an average power of 2,5 watts and if a speech compressor is used up to 4 watts, which works well with many QO-100 antennas. Gain is 17 dBm. We built a couple of board and no additional tuning was required, all worked immediately, so you usually don't need any measurement equipment except a DC voltage/current meter.

The board has a PTT circuit now. The quiescent current is only ON during TX. The requirements for a cooler are very low (in SSB only), a piece of aluminium or any other cooler is usually sufficient.

The complete development documents are available for everybody for free. Please see here: www.dj0abr.de .

A member of our local club, Ernst, dl1ev@gmx.de, has empty boards available. If you want to get a board, please write an email to Ernst and ask for board number: 10821.

The next step is a 30 watt booster which can be connected between the 10W PA and the antenna. The prototype is already working, currently I wait for the final boards.

vy 73
Kurt, DJ0ABR

you can get it from digikey or mouser:

search for: MHT1008N

today we built the full PA equipped with all boards,

the board in the middle is the (already released) 10W-PA. Right of it is the prototype 30W booster and at the left is a small universal board with an attenuator to fit the input to the 50mW of the Amsat TX mixer.

All boards can simply be screwed to a cooler, but we did an aluminium box, just for fun. The red plug is the 24V supply (2A max) and the black plug is the PTT input.

The 10W-PA followed by the booster has these data:

Output: 26 watts Peak

Input: 10mW (for full output, nice to be used with an SDR)

Gain: 34 dB

Efficiency: 48%

in my case I have put an 10dB attenuator in front of the PA, so I need 100mW for full output.

This small board (in the picture on the left) has pads for up to 5 SMD components (0603 or 0805). It can be used as attenuator, filter or whatever is needed.

The RF lines of all boards are at the same position, so all boards can be easly stacked together with a solder bridge. Alternatively SMA connectors can be soldered to each board's input and output.

I have ordered a couple of these filter boards and also the booster, should arrive end of this week, as usual you may contact Ernst (dl1ev@gmx.de) if you need empty boards.

Best 73
Kurt, DJ0ABR