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From the manual:

6.2 Input attenuator
The pi-network consisting of R2, R3 and R4 forms an attenuator. This is
necessary because the output of the QCX will be approximately 5W into 50-
ohms; this is a sinewave with 45V peak-peak. This will be too high for the
Gate-Source voltage specification (+/- 20V) of the IRF510 transistors.
To operate the transistor gates safely, and to correctly load the QCX at
approximately 50-ohms output impedance, and to ensure stability, this “3dB
pad” is a good practice inclusion in the circuit.

I'm not surprised that the input SWR is 2:1.

Hi,
This 3 dB attenuator at the PA input means that at the input the worse VSWR could be 3:1 in case of open or short. The PA input impedance is somewhere inbetween. If it was 50 Ohm, the VSWR at the 3 dB input would show 1:1.
So this 3 dB pad improves the PA R.L.

If it would be 6 dB the worse VSWR would be 1.7:1.

73 Bojan S53DZ

Nominal SWR without attenuator is not easily 50 ohms due to your driving the
gates of Mosfets (think 175PF caps from gate to ground).? Even with?
transformers its difficult to get 50 ohms on 40 though 15M as its reactive.
That and 5W is way too much drive anyway, 2-25W is more than enough.
So the attenuator is but for power and predictable SWR to the QCX.

--
Allison
------------------
Post online only, please no email.

Thanks.

One?thing still escapes me..

Is my 2:1 normal?

On Fri, Aug 4, 2023 at 1:03?PM ajparent1/kb1gmx <kb1gmx@...> wrote:

Nominal SWR without attenuator is not easily 50 ohms due to your driving the
gates of Mosfets (think 175PF caps from gate to ground).? Even with?
transformers its difficult to get 50 ohms on 40 though 15M as its reactive.
That and 5W is way too much drive anyway, 2-25W is more than enough.
So the attenuator is but for power and predictable SWR to the QCX.

--
Allison
------------------
Post online only, please no email.