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

[Steve Thompson]

The attenuator is probably there to protect against overdriving the
transistor with whatever tx that the other guy has. Overdrive is a good
way to kill that transistor. You need to get the input circuit to give a
good match otherwise the power does not reach the transistor.

I think you will struggle without a meter to monitor the forward and
reverse power at the input, it's a vital part of designing/tuning a PA.
I suggest you buy or make one before going much further, it will repay
you very quickly. Your VNA is limited here as the matching changes with
power level so an inline meter is much better.

Bird throughline meters are well known in the Ham world, personally I
love my R&S NAUS3, they are great if you can find one. Many products
targetted at the Ham market are rubbish so chose carefully. Anything
which claims to cover 2-500MHz should be avoided, also anything which
has 259 type connectors.

One unit which seems better is the SW-102N which is popular in the PMR
world.

73, Steve

On 29/03/2025 14:41, Lukasz wrote:
> I have a certain suspicion... I found one more person who described their
> experiences (it wad also, "put it together , got 70W, no tuning etc
>
> However , that person drew a schematic of their amp and they had an
> attenuator in the front. I don't have that. My pcb has pads but no markings
> what to put there so I thought it is optional. Also I didn't see it
> populated.
>
> I wonder perhaps that attenuator is the answer? Perhaps the input is never
> meant to match 50 ohms fully, but that's not a problem because the device
> that sends the signal sees the attenuator. This is something I saw in a
> tube amp once.
>
> Now, another piece of info is "how" I sent the 1,2.5 and 5W to the amp. My
> function generator tops out at 7dBm (and I did get +18dBm from the amp).
> But to send 1W and more I used a little baofeng set to min/medium/max power.
>
> I suspect perhaps the baofeng saw the horrible swr and limited the power to
> 1W. This would explain why I got roughly the same 20W of output on all 3
> power settings.
>
> So the key question now is... Are we supposed to make a close to perfect
> match on ldmos inputs? Or just add an attenuator?
>
> I'm not sure, because the original diy author showed photos of his device
> and there was no attenuator there.
>
> 73, Lukasz
>
>
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