13cm PA for es'hail-2 NB-Transponder

Overwiew

Deutsche ‹bersetzung von DL1RNO: DOWNLOAD als pdf.

There are a lot of power amplifiers for the 13cm band, you might think, but it's not that easy. The PAs built in DL are ATV power amplifiers for 2.3 GHz and usually not adapted for 2.4 GHz, and if so, then priced in the upper class segment. There are other PAs from different European countries, all not really cheap. Therefore, many OMs order "WLan Boosters" from China, which are advertised with 4W or 8W. Apart from the fact that these power figures are only for optimists, the import becomes more and more difficult due to new regulations of tax office and customs. After two orders never arrived, we decided to become active and build a PA by ourselfs, publishing the results to the HAM radio community for free.

Finding a suitable concept:

We are using a 2-stage concept. Both stages are completely independent are can be used alone or chained in series for higher gain.

Stage-1: 

* output-power (CW) max. 10 watts, or SSB 2,5 watts
* gain +17dB
* PTT circuit
* reasonable efficiency
* in-/outputs tuned to 50 ohms
* 24V power supply
* cost of material < 20 Eur
* DIY (do it yourself) must be possible without expensive test equipment

Stage-2: 

* output-power (CW) max. 30 watts, or SSB 7 watts
* gain +15dB
* PTT circuit
* reasonable efficiency
* in-/outputs tuned to 50 ohms
* 24V power supply
* cost of material < 50 Eur
* DIY (do it yourself) must be possible without expensive test equipment

Examples:

the gain and power levels are very important to get good results. It may be required to place a simple attenuator at the PA's input to avoid overdriving. Here are a few examples:

1) Stage-1 PA at full output power

Input (drive power): 200mW (23dBm)
Gain: 17dB
Output: 10 Watts (40 dBm)

2) Stage-1 driven with 100mW (i.e. Amsat Upmixer)

Input: 100mW (20dBm)
Gain: +17 dB
Output: 5 watts (37 dBm)

3) Two Stage-1 amplifiers in series (i.e. for SDR transmitters)

Input: 2mW (3dBm)
Gain: 2 x 17 dB = 34 dB
Output: 5 watts (37 dBm)

4) Stage-1 followed by Stage-2, maximum gain

Input (stage-1): 20 mW (13dBm)
Gain (stage-1): 17 dB
Output (stage-1): 1 watt (30 dBm)

Input (stage-2): 1 watt (30 dBm)
Gain (stage-2): 15 dB
Output (stage-2): 30 watts (45 dBm)

5) Stage-1 followed by Stage-2, driven with 100mW (i.e. Amsat Upmixer)

Input (attenuator): 100mW (20 dBm)
Gain of the attenuator: - 7 dBm
Output (attenuator): 20mW (13 dBm)

Input
(stage-1): 20 mW (13dBm)
Gain (stage-1): 17 dB
Output (stage-1): 1 watt (30 dBm)

Input (stage-2): 1 watt (30 dBm)
Gain (stage-2): 15 dB
Output (stage-2): 30 watts (45 dBm)

in this case the input attenuator is required or stange-1 will produce too much power resulting in overdriving stage-2

there are many other possible combinations of stage-1, stage-2 and attenuators. Simply add/substract the gain to the dBm power values and take care not to overdrive an amplifier stage:

maximum allowed drive levels:

stage-1: 400mW (26dBm)
stage-2: 1 watt (30 dBm)

if these drive level are exceeded then the transistors will get damaged (refer the data sheets) !

 

Choosing the transistors:

after some searching I found a very cost efficient Mosfet transistor for stage-1, the MHT1008N from Freescale.
This transistor is specified with 12.5w peak power at a price around 10 Eur and a reference design is available.

For stage-2 we need a more powerful transistor with better termal contact to a cooler. The choice is the LDMOS transistor BLF2425M9L30U from Ampleon (digikey).
It is specified with 30 watts at a price around 40 Eur.

 

Input and Output matching:

The adaptation of the transistor input and output to 50 ohms is usually done with strip lines, which are copper tracks in the layout. By appropriate choice of the impedance and length of the stripline, the desired transformations can be produced.

Fortunately, we did not have to start from scratch (which probably would not have worked), but Freescale and Ampleon have a reference layout with the correct dimensions. However, this layout requires Rogers 4350 or Duroid 6035HTC board material, which is very expensive (for small quantities). However, for our expected maximum power, a PA can also work with normal FR4 board material. To keep the losses as low as possible we choose the thinnest board material offered by our PCB manufacturer: 0,6mm thickness.

The next task was to convert the strip lines of the reference layout from Rogers to FR4 board material. DC1RJJ has agreed to do that because he has the required knowledge and the software. Based on his calculations, I modified  the reference layout and had the boards created and manufactured.

 

The final layouts:

Please see the menu for the description of the two amplifier boards, stage-1 and stage-2.