Understanding HamShield1 Transmit Power

Understanding HamShield1 Transmit Power

The HamShield1 has a two stage power amplifier (PA). Those two stages boost the transmit power of the radio IC that is the heart of the HamShield. The amount that they increase the output power by is called their gain, and the gain depends a little bit on frequency.

HamShield1 Transmit Chain Block Diagram
To change the transmit power of the HamShield, we use the 'setRfPower(value);' function. That function actually writes to an RF power control register in the radio IC, changing the power that the radio IC puts out. The radio IC can put out between .06 milliWatts (-12dBm) and 5.6 milliWatts (7.5dBm), depending on what you set the RF power to.​

The first stage power amplifier is a wide band chip. It's matched to the radio IC in all three transmit bands, so it doesn't need much coupling circuitry to connect it up to the IC. It has a gain of about 12dB, which means that if the radio IC puts out 1 milliWatt (0dBm), then the first stage PA would put out 16 milliWatts (12dBm)

The second stage PA is not a wide-band input, but it has circuitry connecting it's input and output to match it across all three transmit bands. That allows us to use just one second stage PA for all transmit frequencies, but it does lead to a little more variation in transmit power depending on frequency. The gain of the second stage PA is between 10dB and 15dB, depending on frequency. That means that if the first stage PA put out 16 milliWatts (12dBm), then the second stage PA would put out 158 milliWatts (22dBm) to 0.5 Watts (27dBm).

After the second stage PA, we have a set of filters that we use to reduce harmonics. Harmonics are higher frequencies than your transmit frequency that are generated by the amplifiers themselves. If they're too high, you'll interfere with other radio users. We have a different filter for each band the HamShield can use, and hardware selects the right filter automatically based on what you're transmitting.

What all of this means is that when you set the power in your HamShield software, you can expect results similar to the following:

frequency (kHz) pwr = 0 pwr = 7 pwr =15
136000 13.8 15.7 22.5
153000 17.4 19.7 29
170000 16.9 19.3 27
201000 16.9 18.3 28.4
230000 16.7 19.4 24.5
260000 15.7 18.5 28.8
400000 18.6 21.3 25.5
460000 19.1 21.3 27.3
520000 18.5 21.7 29

The power measurements in the above table are taken in dBm, so if you want it in watts you can use our converter. The column indicates the value set using radio.setRfPower().

In general, the antenna that you use (and any cables you use between HamShield and antenna) can degrade your power output. We recommend measuring the output power in the setup you plan to use if output power is an important requirement. You can use our PowerMeter to make those measurements.

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