Rev 1.03
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clockwise until you observe a very slight increase in the current consumption. The point where the
current just starts to increase is the zero bias point.
An alternative (and arguably simpler) method of bias adjustment is to transmit into a 50-ohm
dummy load, measuring power with a power meter or observing on an oscilloscope. Turn the R1
trimmer potentiometer clockwise until maximum power output is obtained, then back off slightly.
The bias setting using this method is higher than the zero idle current method. The power output
will be higher too.
The gain of the IRF510 amplifier is less at the upper end of the HF bands particularly 28MHz (the
10m band). The input signal is also typically lower, when using for example an Ultimate3S kit as
the driver. In these cases, adjusting the PA with a higher bias voltage can improve the gain/power
output at the high frequency end of its range of operation.
If using this PA in a multi-band transmitter and desiring maximum power output, you could adjust
the bias for best power output on the highest frequency that will be used (for example, 28MHz)
where the bias is most critical. The bias setting is much less critical at the lower end of HF.
4.3
Heatsink temperature rise; PA kit installation in enclosure
The desired output power of this PA kit in your application has a significant bearing on how you
enclose the project. Remember that the heatsink will dissipate a considerable amount of heat. The
heat dissipation will cause the temperature of the heatsink to rise. You need to take this into
account when deciding what enclosure to use. You may wish to bolt the heatsink to the back of a
metal box, for example; or consider a fan to cool the heatsink.
In operation the power modulator transistor will dissipate some heat as it is an imperfect switch;
the IRF510 final PA will operate at perhaps 50% efficiency or something like that (dependent on
many factors). For a power output of 5W you will therefore be dissipating at least 5W, maybe
somewhat more.
Another factor to take into account is the transmission mode you will be using. A continuous key-
down transmission using FSK modes such as WSPR, will dissipate a much higher average power
than an on/off keyed mode such as CW.
As an example, one prototype here was
operated on continuous key-down on
10MHz, with 13.8V supply using an
Ultimate3S transmitter as the driver.
The power output was set to 5W using
the maximum DAC value (meaning
excess power was also dissipated in the
power modulator). An overall current
consumption of 1.06A was recorded.
The assembly was lying on the
workbench with no enclosure, and no
forced air cooling. It took about half an
hour for the temperature of the heatsink
to stabilise. By that time it had reached
