[Amps] LDMOS MRF186 100W UHF amp from a kit. Need help finding tuning resources.

[Steve Thompson]

When you're simulating circuits at these frequencies you need to include
the parasitic inductance of the capacitors as it can have a significant
effect - for the typical ATC100B type it's around 0.5nH, probably
several times that for a trimmer cap depending on the type.

To check or adjust matching with transistor amps using a VNA the usual
method is to put 50ohm terminations on the in/out and measure the
impedance at the transistor end. At UHF and above it can be tricky as
you're probably working at <10ohms impedance and positioning probes to
avoid stray inductance needs a lot of care, fractions of a mm matter.

You mentioned the 20W power you were getting out of your amp. It's very
helpful to also give the voltage/current as that gives useful
information about how the amp is working.

73, Steve G8GSQ


On 29/03/2025 12:13, Lukasz wrote:
> I've been trying to simulate the 930MHz circuit from the datasheet as a
> starting basis and I had to increase the trimmer by 5pf - otherwise numbers
> don't work.. Then it seems to work? Can someone tell if this is correct or
> not?
> https://ibb.co/DgrFS77z
> https://ibb.co/h1shgvYj
>
> Also, with some help of (hopefully not hallucinating) AI I've managed to
> establish the input likely has 1.3nH of parasitic inductance in addition to
> 177pF of gate capacity (this gives the 2.5+7j at 945MHz) and these values
> calculated at 440MHz give 2.5+1.55j. Not sure how far I'll get with that.
>
> 73, Lukasz
>
>
>>>
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