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Hermes Lite 2 safe driving a 400W PA. 20210410
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==>> Always FIRST press F5 <<==

When a linear power amplifier is driven with this HL2, it is wise to connect the HL2 to the PA via a 5W attenuator.

I use a "magnetic loop antenna". 10kHz beside its resonance its VSWR is already to high.

When using SDR software (like SDRconsole) it is easy to click beside the wanted screen location, causing an unwanted jump to an off-resonance frequency, causing to high antenna VSWR. 

My 400W PA needs about 1W drive. PA input voltage is then 7.1Vrms//50 Ohms.

AS the input transformer has 2:1 turns ratio, the MRF300 gate voltages will be about 7.1 x 2.8 / 2 = 10Vpp or +/- 5Vp.

Adding +2.7V gate bias voltages results in gate peak voltages of  -2.3 / + 7.3 Vpp.

The MRF300 in my PA have absolute max. gate voltages of -6 / +10 V. This equals about 2.6W drive.


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The HL2 can deliver 5W drive. The PA gate drive voltage will then be 15.8 Vrms .That is 44.7Vpp at the primary of the input transformer. At the secondary of input transformer T1 it is half that value : 22.35Vpp or +/- 11.2Vp.

Gate peak voltages are then (including 2.7V bias) (-11.2 + 2.7) and (+11.2 +2.7), or -8.5V / 13.5V pp. 
5W input injected to the MRF300 will blow the MRF300 gates !

    Easy solution.
   Inside the PA :
At the antenna relay board, disconnect the SMA plug of the coax to the PA input. Insert three cheap 2dB / 2W SMA attenuators connected in series. With 5W drive, the input to the PA is 1,25W, and 11.2 Vpp at the gates of both MRF300.

With 4W drive my PA will generate 400W output. 5W drive will not harm the PA nor the attenuators. However, the output will be 500W, with a higher chance of IMD, and higher cooling profile temperatures. Not recommended.