[Top] [All Lists]

Re: [Amps] LDMOS and VSWR

Subject: Re: [Amps] LDMOS and VSWR
From: Manfred Mornhinweg <>
Date: Mon, 31 Oct 2016 20:33:26 +0000
List-post: <">>

The videos on YouTube were all running CW at various frequencies. No
pulse modes.

Unfortunately you are mistaken there. Jeff is right. They were pulsing the amplifier in that video.

If you look closely at the video, you will be able to confirm the following facts:

- When the 50Ω load is connected, they show the power supply only from a distance, so the exact current can't be read, but one can see that only one digit is on before the decimal point. So the current is less than 10A, and thus the average input power cannot be more than 500W. Yet the RF power meter reads 1200W. Unless the amplifier is magical, it must be operating in pulsed mode.

- During the mismatch test, the power supply is shown close-up. Depending on the phase angle of the load, the current varies between about 1 and 10A, limiting the average input power to 550W at most.

- During the same test, we can read the model and ratings of that power supply. It's limited to 1000W. More evidence that the amplifier cannot possibly be putting out 1200W CW.

- The MOSFET is mounted on a water-cooled copper block. Most ham amplifiers get less good cooling. Although the MOSFET is clamped down dry to the copper surface. Soldering it down would be better.

Summing it up: The worst-case power dissipation during the tests shown was around 500W, which that LDMOSFET can take rather easily, on that watercooled block. But if a ham mounts that MOSFET on an air-cooled heatsink, and drives it to 1200W CW, using a power supply that can deliver 2500W, it wouldn't survive those high SWR tests.


Visit my hobby homepage!
Amps mailing list
<Prev in Thread] Current Thread [Next in Thread>
  • Re: [Amps] LDMOS and VSWR, Manfred Mornhinweg <=