70 MHz 25 W Power Amplifier

This purpose of this page is to give a brief description of a 70 MHz power module PA that has been published in the Danish amateur radio magazine "OZ." This way hams outside Denmark may also benefit from the design.

Click here for transverter kit.

PA kit

There are no more kits available.

The kit consisted of the following parts:

Here is a list of the delivered components etc.

70 MHz PA


The circuit is quite straight forward and consists of the following subparts:

The published article in OZ has not been translated. However, the design and build is quite straight forward.


Drain bias current, IDDQ, vs. gate voltage, VGG.

4,0 110
4,5 520
5,0 1550

More IDDQ measurements here. I recommend to set IDDQ to 700 mA for linear and mixed modes (SSB, CW and FM) and for FM only to 500 mA.

Fig 1. Power module output power (solid lines) vs. input power and drain current (dashed lines, IDD ). Red lines are for IDDQ = 1,5 A and blue lines are for IDDQ = 500 mA.

As fig 1. shows the efficiency is quite low in comparison to a discrete transistor PA design. This is quite normal for power modules. As a consequence the heatsink size should be like the ones used for a 100 W brick PA and not as if it was a normal 25 W PA.

Selected dBm and Watt levels.

dBm Power
-10 100 µW
0 1 mW
10 10 mW
20 100 mW
30 1 W
40 10 W
44 25 W
47 50 W

Low pass filter

Fig 2. Low pass filter characteristics.

The aligning of the low pass filter is done by squeezing or pulling L3 and L4 but only one at a time. Adjust L3 until the dip is at 210 MHz and similarly for L4 on 140 MHz. The inevitable attenuation in the pass band should not be affected by the tuning of L3 and L4. Otherwise there is something seriously wrong with the coils. In case you do not have the necessary instruments to align the filter apply a small but know signal level at 70 MHz and measure it before and after the low pass filter. The maximum attenuation should not exceed 10%.

For those with access to no instruments - almost

If you are really short of instruments you can get away with just a simple mA-meter to set the bias current.

If you want you can ask a very nearby station to listen for your 140 MHz and 210 MHz signals and adjust L3 and L4 accordingly.

Alternatively you may contact one of the voluntary alignment supporters listed on the main 70 MHz kits page.

Output spectrum

Fig 3. Output spectrum of the power module itself at VDD = 13,8 V, IDDQ = 1,5 A and 25 W.

Fig 4. Output spectrum of the power module PA at VDD = 13,8 V, IDDQ = 1,5 A and 25 W.


Fig. 5. Intermodulation distortion at VDD = 13,8 V, IDDQ = 500 mA and 25 W PEP. Third order IMD is 24 dB below first order.

Fig. 6. Intermodulation distortion at VDD = 13,8 V, IDDQ = 1,5 A and 25 W PEP. Third order IMD is 29 dB below first order.

Fig. 7. Intermodulation distortion at VDD = 13,8 V, IDDQ = 1,5 A and 33 W PEP. Third order IMD is 26 dB below first order.

Parts list

If you have bought a kit please see the list of the delivered components etc.


Resistor Value
R1, R2, R3 Optional Pi attenuator (see table below)
R4 390 Ω
R5 10 kΩ
VR1 100 Ω, trim pot
VR2 10 kΩ, trim pot

All resistors are 0,25 W/0,4 W and metalfilm type.

You have to select the value of R1, R2 and R3 according to your application. Under no circumstances may the input power level to the module exceed 100 mW. If used together with the 70 MHz transverter a 10 dB attenuation is required.

Attenuation R1, R3 [Ω] R2 [Ω]
0 Do not mount Short
1 820 5,6
2 470 12
3 270 18
4 220 22
5 180 33
6 150 39
7 120 47
8 120 56
9 100 56
10 100 68


Capacitor Value
C1, C2, C7, C8 1 nF
C3, C4, C6, C9, C10, C18 10 nF
C5 10 µF, 25 V, electrolytic
C11 1 mF, 25 V, electrolytic
C12 4,7 pF
C13 15 pF
C14 27 pF
C15 56 pF
C16 18 pF
C17 1 pF

All capacitors are ceramic types unless otherwise stated.


Component Description
D1, D2 1N4148 or equivalent
D3, D4 1N5711, HP5082-2800 or other schottky diodes
IC1 78L05
IC2 RA30H0608M
Q1 BC177 or similar


Inductor Description
L1, L2 Ferrite beads
L3 122 nH, 5 turns, 6 mm inner diameter, 1 mm enamelled wire, 12 mm long
L4 85 nH, 5 turns, 6 mm inner diameter, 1 mm enamelled wire, 13 mm long


Component Description
"Tin box" 111 x 74 x 30 (or 50) mm3. Schubert Weissblechgehäuse type
BNC sockets Two x UG290/U or UG447/U
Copper bands Two pieces each 10 x 30 x 0,5 mm3
Copper foil Mounted under the power module in the 90 µm gap and soldered to the PCB ground
Heat sink Find one yourself suitable for the need
Screws Four M4 8 mm long
Feed through capacitors Four x 1 nF, solderable

Documentation etc.

Power module PA


  • 78L05: IC1. (Warning PDF file!)
  • RA30H0608M: IC2. (Warning PDF file!)
  • BC177: Q1. (Warning PDF file!)

Bo, OZ2M, www.rudius.net/oz2m