1. Build a 1500 Watt LDMOS Solid State Amplifier, the Easy Way
John Eisenberg K6YP

2. Why consider such a thing?
✔Solid state amplifiers are broadband and require no tuning.
✔I hate the 180 second warm up time associated with “modern” ceramic tubes. The pileup can grow from 1 or 2 guys to hundreds in 3 minutes.
✔What a great learning experience for me as I have never worked with LDMOS.

3. Goals ✔Pout 1500W min 1.8-50 MHz ✔50V dc operation
✔Better than -30 dBc IMD
✔-50dBc Harmonics (FCC requires -43dBc)
✔Compact rack mount unit < 4 RU
✔Complete protection (Temp, VSWR, Current, Overdrive)
✔Easy to build

4. Result

5. Easy to build much mo’ better.
✔Make use of available printed circuit boards, kits and pre-built modules
✔Stay away from having to install large numbers of SMT parts. Thru hole is
much mo’ better.
✔Commercial panel and chassis
✔Standard parts, nothing exotic
✔No difficult machining

6. W6PQL, the best kept secret in amps
W6PQL, Jim Klitzing is a retired HP engineer
He is an avid VHF, UHF and microwave operator
He has introduced a series of LDMOS amplifier modules and a complete family of 1kW amplifiers covering 160m through 23cm
Located in Fremont
His pricing is fair and reasonable
Many of my illustrations are from his web site.

7. My Amp Uses W6PQL Modules
2 1kw MHz PA modules
1 power splitter and 1 power combiner
W6PQL controller board
One 10dB 100W attenuator board
2 high current switches
1 low pass filter board
1 input and one output RF SPDT relay PCB
2 directional couplers / VSWR sensors
2 bar graph indicators
1 ALC board

8. And some other stuff as well
✔1 Unified Microsystems BCD 14 band decoder board
✔A Chinese fan speed controller from EBAY
✔Several simple hand wired vector board assemblies
✔3 Surplus 50V, 25A power supply units
✔1 Surplus power supply rack

9. Parts is parts
✔The power supplies were very cheap but only 2 out of 6 worked, 2 still don’t
✔I let W6PQL solder down the LDMOS transistors. Voids are not your friend!
✔Watch out for fan noise. Think through your thermal design carefully.

10. Simplified Block Diagram
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11. Under the hood at about 80% complete

12. BLF188XR LDMOS MOSFET BLF188XR LDMOS Device
XR means extremely rugged. You can draw a continuous arc on the open output connector of a BLT188XR amplifier without failure!
The BLF188XR device soldered down to a W6PQL machined copper heat spreader

13. W6PQL 1 kW LDMOS 1.8-54 MHz Amplifier Module

14. Ampl Module Current and Pout

15. 20 meter Linearity Pout vs. Pin

16. Harmonics must be filtered
Amp harmonics measured without LPF at 7.2 MHz
Amp harmonics with LPF in place meet FCC -43 dBc spec with margin.

17. Switched Low Pass Filter

18. Switched Low Pass Filter Board

19. In Phase Power Combiner and Power SplitterV V

20. Power splitter and combiner
High power combiner board
100 Ohm
500 Watt Resistor
Input power splitter board

21. Amplifier Bypass Relays
Input SPDT RF Relay
Output SPDT RF Relay
Pout =1500W, Zload =50ohms
V =273.9Vrms or 387.3Vpeak I =5.47Arms or 7.75Apeak
Yes these low cost relays can handle 2kW with low VSWR to >100MHz
External VSWR can induce large currents and voltages which the relay must handle

22. Input Pi Attenuator
The combined amplifier modules produce 1.5kW with only 2.5W of input power. Thus an attenuator
is needed to reduce the drive power by a factor of 10 or 10dB.
Caddock thin film power resistors work well in such an application to above 54 MHz.
10 dB, 100 W
Attenuator

23. ALC Detector
The ALC detector produces a negative voltage proportional to amplifier input power. This voltage is fed back to the ALC input on the transceiver to maintain constant output power from the amplifier.
Here is an SMT assembly job even I can handle
ALC detector

24. Dual Directional Detector
Dual Directional Detector Assembly
Coupling = -20LogN-10dB Coupling = -26dB -10dB Coupling = -36dB

25. W6QPL Amp control board
✔Sequences antenna relay, amp bias, power supply voltage and RF drive hold off
✔High temp and high VSWR protection
✔Turns on fans at a set temperature
✔Shuts down amp if wrong LPF band selected
✔It is ANALOG!!

26. W6PQL Control Board
I bought this board assembled!

27. High current switch and bar graph display
✔Two high current switches are used to activate or kill PA +50V on start up or in the case of a fault. One for each PA module.
✔The bar graph displays read out forward power in ~200W increments to 1800W and reflected power in 25W increments to 250W. It is driven by a directional detector.

28. High current switch and bar graph display
High Current Switch (40A) It takes two of these, one for each amplifier pallet
Bar Graph Display One for forward and one for reflected power

29. Band Decoder
✔The United Microsystems BCD14 band decoder takes input from the accessory output on my K3, and selects the appropriate LPF band. If the K3 is not connected the LPF bands are selected using the front panel band switch.

30. Band Decoder This came assembled as well
United Microsystems BCD14 Band Decoder board

31. Fan Speed Controller
✔The 48V dual fan speed controller was found on EBAY and came with two thermistors for sensing temperature.
✔It worked great once I was finally able to decipher the Google translated instructions the vendor supplied.
✔It contains the only uP in the amplifier

32. Fan Speed Controller 48V Fan Speed Controller (Less than $10 on EBAY)
The fan speed controller monitors and reads out the heat sink temperature and controls fan speed to maintain
a constant heat sink temperature

33. 50V, 75A power supply Assembly
✔The modules and rack were obtained on EBAY for peanuts.
✔The units are server power supplies and are power factor corrected and are designed to be hot swapped and easily paralleled
✔The units sound like a Boeing 747 on takeoff and will soon be replaced with commercial Meanwell telecom supplies

34. Power Supply Assembly Under Test
Power Supply with 4 five ohm resistors in parallel.
50V/1.25ohms = 40A or 2000 watts.

35. Amplifier test data
Measured with a Bird 30 dB, 4kW attenuator and a dual channel HP438A power meter.
Dual channel capability allows the HP438A to also provide gain information. Gain=Pout/Pin
The accuracy of diode type power meters like the Bird 43 and nearly all “ham radio” power meters is severely degraded by harmonics
IMD at 14.2 MHz with 2 375W tones (1500W PEP), spaced 20 kHz is -32dBc

36. K6YP Amplifier Test Frequency 1.9 MHz Pin dBm Pin Offset Pout dBm
Pout Offset
Pout (W)
Gain
Left PA
Right PA
Total PA
Pdc
Efficiency
Measured dB
Actual
Current A W %
-7.9
38.93
31.03
4.44
49.57
54.01
251.8
22.98
9.7
10.5
20.2
1010
24.9
-5.3
33.63
7.25
56.82
480.8
23.19
13.0
14.0 27
1350
35.6
-3.3
35.63
9.43 59
794.3
23.37
17.0
18.0 35
1750
45.4
-1.8
37.13
10.86
60.43
1104.1
23.3
20.5
21.5 42
2100
52.6
-0.9
38.03
11.36
60.93
1238.8
22.9
22.0
23.0 45
2250
55.1
1.5
40.43
12.21
61.78
1506.6
21.35
25.0
26.0 51
2550
59.1
4.5
43.43
12.89
62.46
1762.0
19.03
28.0
29.0 57
2850
61.8
3.7
-6.5
38.95
32.45
4.50
49.56
54.06
254.7
21.61
10.0
11.0 21
1050
24.3
-3.0
35.95
7.92
57.48
559.8
21.53
15.0
16.0 31
1550
36.1
-1.3
37.65
9.56
59.12
816.6
21.47
18.5
19.8
38.3
1915
42.6
0.0
10.66
60.22
1052.0
21.27 43
2150
48.9
1.4
40.35
11.45
61.01
1261.8
20.66
24.0 48
2400
3.8
42.75
61.77
1503.1
19.02
27.2
53.2
2660
56.5
8.0
46.95
12.63
62.19
1655.8
15.24
30.0
2950
56.1
7.2
-5.0
39.00 34
4.11
49.55
53.66
232.3
19.66
22.1
37.2
7.64
57.19
523.6
19.99
15.7
31.7
1585
33.0
-0.3
38.7
9.13
58.68
737.9
19.98 37
1850
39.9
1.3
40.3
10.48
60.03
1006.9
19.73
21.8
43.6
2180
46.2
3.0
42.0
11.41
60.96
1247.4
18.96
52.0
5.0
44.0
61.76
1499.7
17.76
26.2
52.2
2610
57.5
7.4
46.4
62.44
1753.9
16.04
28.2
56.2
2810
62.4
14.2
-2.6
38.98
36.38
4.61
49.66
54.27
267.3
17.89
12.0 24
1200
22.3
-0.2
38.78
7.34
57.00
501.2
18.22 32
1600
31.3
1.7
40.68
9.08
58.74
748.2
18.06
19.0 38
1900
39.4
3.4
42.38
10.44
60.10
1023.3
17.72
23.5
45.5
2275
45.0
4.8
43.78
11.33
60.99
1256.0
17.21 50
2500
50.2
6.5
45.48
12.14
1513.6
16.32
27.0 55
2750
55.0
46.38
12.77
62.43
1749.8
16.05
31.0 60
3000
58.3

37. Frequency
21.2
MHz
Pin dBm
Pin Offset
Pout dBm
Pout Offset
Pout (W)
Gain
Left PA
Right PA
Total PA
Pdc
Efficiency
Measured dB
Actual
Current A W %
-1.8
39.05
37.25
4.24
49.73
53.97
249.5
16.72
11.5 23
1150
21.7 1
40.05
7.3
57.03
504.7
16.98
16.0 32
1600
31.5
2.4
41.45
9.09
58.82
762.1
17.37
19.0
18.0 37
1850
41.2
3.7
42.75
10.26
59.99
997.7
17.24
22.0
20.0 42
2100
47.5 5
44.05
11.22
60.95
1244.5
16.9
24.0 48
2400
51.9
7.1
46.15
12.21
61.94
1563.1
15.79
27.5
27.0
54.5
2725
57.4
8.2
47.25
12.77
62.5
1778.3
15.25
30.0
29.0 59
2950
60.3
28.2
-1.4
39.10
37.7
4.22
49.87
54.09
256.4
16.39
10.2
9.8 20
1000
25.6
1.6
40.7
7.17
57.04
505.8
16.34
14.0
13.5
1375
36.8
3.3
42.4
8.89
58.76
751.6
16.36
17.5
33.5
1675
44.9
4.8
43.9
10.21
60.08
1018.6
16.18
21.0 40
2000
50.9
6.5
45.6
11.11
60.98
1253.1
15.38 46
2300
8.0
47.1
11.91
61.78
1506.6
14.68
51.5
2575
58.5
48.9
12.56
62.43
1749.8
13.53
26.5
56.5
2825
61.9
50.2
0.2
39.40
39.6
4.01
50.04
54.05
254.1
14.45
12.0
13.0 25
1250
20.3
3.2
42.6
6.97
57.01
502.3
14.41
17.0 35
1750
28.7
5.0
44.4
8.77
58.81
760.3
22.5
44.5
2225
34.2
6.9
46.3
9.97
60.01
1002.3
13.71
27.2
53.7
2685
37.3
9.5
10.9
60.97
1250.3
12.07
32.5
33.0
65.5
3275
38.2
50.3
11.2
61.24
1330.5
10.94
34.0 68
3400
39.1
SAT
Freq
Offset dB
Pout W
1.9
49.57
53.98
250
38.93
49.56
56.99
500
38.95
7.2
49.55
58.75
750
39.00
14.2
49.66
60.00
38.98
61.76
1500
Output Attenuator
63.01
Input Coupler & 10dB Pad
P(dBm) from P(Watts)

38. Conclusion
✔The result of this work is a solid, reliable W LDMOS solid state amplifier that looks OK and is a pleasure to use.
✔LDMOS MOSFETs are capable of more than 1kW from a single device and offer excellent IMD performance and efficiency
✔The cost was half what others charge for a W SSA in today’s market.

39. Conclusion continued
✔Most importantly the amplifier was a fun learning experience
✔The amplifier can be further linearized using Pure Signal pre-distortion running on an Anan 100D or Flex 6X00 transceiver
✔Finally the design and building process kept me entertained and out of trouble for several months worth of evenings.