[Amps] LDMOS and VSWR

[Jim Miller]

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

73

Jim ab3cv

On Oct 29, 2016, at 11:37 PM, jeff millar <wa1hco at wa1hco.net> wrote:

Hi Jim...

IIRC, the amplifier in those video are using a pulse mode with limited duty cycle.  The transistors have a high enough voltage rating to with infinite VSWR with a phase that causes open circuit voltages, but they don't have the thermal capability to handle the equivalent of short circuit impedance...thus the limits on duty cycle.

If the spec mentions 20 ms pulses, then the VSWR trip should respond in less than 20 msec.

jeff, wa1hco

> On Wed, Oct 26, 2016 at 7:17 AM, Jim Miller <jim at jtmiller.com> wrote:
> From several Youtube videos it appears that the LDMOS FETs.are quite
> capable of withstanding opens and shorts indefinitely. If true then
> agonizing over VSWR and the need for a tuner for the typical case of 3:1 or
> less would seem to be unwarranted at least for the FETs.
> 
> The next question would be what about other components in the finals and
> lowpass filters and how would they react to continuous 3:1? Coils,
> capacitors, etc.
> 
> How would the power supply to the FET output stage be affected? Would the
> excess voltage or current at the amp LPF output cause the signal to degrade
> due to starvation of either?
> 
> 73
> 
> jim ab3cv
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