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I have a QMX that I'm operating at 10V, giving me about 2 W output power. I'm trying to drive a QRP Labs 50W PA, powered at 13.2 V. I'm operating in FT8, which has an operating cycle of 15s TX and 15s RX, so the rig has a 50% duty cycle and 15s ON time.

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Several measurements with different cables indicate that the best VSWR I can achieve into the PA is about 2.5. (I get about 15W out of the amp into a dummy load.) My external SWR bridge and the QMX's internal SWR display agree. Assuming that I built the amplifier properly, I'm guessing this is about what I can expect from this setup.

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Question is, how high an SWR can the QMX safely drive without overstressing the output transistors? Obviously, the answer here will vary with supply voltage. I'm already set up for a reduced QMX supply voltage because the 13.6V from my LiFePo battery is a tad high. I'm not troubled by only getting 15W out of the 50W PA, as 15W into a decent antenna should be quite adequate for FT8.?

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Thanks,

Steve

Get two Power supplies
Set them both to the recommended voltage in the instructions?
If you have too much power going into the amplifier buy some 5W metal oxide film resistors to increase the input attenuation (designs/calculators online)? by replacing R2, R3 and R4?

"Too much power going into the amplifier" wasn't the question. The question was, "What is maximum safe VSWR for TX in digital modes?"

I'm not worried about overdriving the PA; I'm concerned about cooking the QMX output transistors.

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Thanks,

Steve

Hi Steve,

I would say

2.0:1 max at +12V DC,

2.5:1 max at +11V and

3.0:1 max at +10V.

This is my suggestion. I stick to these rules set by myself and so far no harm to my QMX+ PA.

But I must say I am conservative in this respect.

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73 Bojan S53DZ

- to be clearer

That PA will have better performance and slightly higher gain at a higher supply voltage
The QMX will work best at its intended voltage?

While overdriving the PA should be avoided my point is that if there is then drive to spare increasing the input attenuator components R2,3,4 from its current '3.5dB ish' value to nearer 6dB (say 150, 39, 150? - you could? play with 39 ohm alternatives of 33R and 47R? ) will improve the input match considerably?
2 or 3 Watt metal oxide film resistors will be fine?

True.

I am using the 50W PA at +20V. But QMX at 10 - 11V on 20m and 17m.

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73 Bojan S53DZ

On Thu, Aug 1, 2024 at 10:15 AM, Steve H wrote:

I'm operating at 10V, giving me about 2 W output power

Something doesn't seem right with that.

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I'm running 9Volts into a QMX built for 12V. Output was measured "north of 5Watts" at 12V. When running 9Volts in, I get a measured 3.75-4Watts out.

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How are you getting 2Watts out from 10Volts?

Here's a way of knowing of any semiconductor device is being stressed

(regardles of SWR blah blah).

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And it' the temperature of the device when operating into the load

in question.

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Typically any semiconductor is best used at what we might call room

temperature, and that tends to be 25 Centigrade.

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Above that we have to de-rate the device.

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40 Centigrade is usually a good judgment to make for continuous

operation.

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You could measure the temp using your preferred measurement devices,

or use the human method.

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The Human method uses..well a human ;-)

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Humans feel pain above about 40-45C, so of you stick your finger on a device

and can't hold it on a device for more than about 10 seconds then you are

not in the safe operating area of a semiconductor. If you are above that then

you need to de-rate a device.

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A device that is being used beyond it's safe limits exhibits the following

characteristic.

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Power output such as a PA stage may give for example 5.000 watts upon

initial keydown.

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Then wait 20 seconds, does the power drop ?

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If so back off the drive, or bias, or achieve a better input match to the

next device, be it another semiconductor stage or antenna.

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Safe operating ranges (SOAR), look it up on the spec sheet or Google.

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Humanoid test equipment, try this for starters -

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Read and learn ;-)

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- 73 de Andy -

Alas, in its sandwiched arrangement in its nifty anodized case. I find it hard to access the transistors to assess their temperature. I've been hoping others have explored this space before, and I think the replies here will help.

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Thanks,

Steve

Could be that my cheap'n'cheerful Chinese SWR bridge isn't especially accurate down at the low end, I suppose. (Or my series-pass regulator isn't doing what I think it is when transmitting.) I'll run some more measurements, and cross-check it against net power draw from the DC supply.

Andy?

As there are beginners here I would like to say that sticking your finger on anything with substantial RF power going through it will not just burn but cook it.? ?My 'best' RF burn took about 4 months? to return to pink and stop aching.? (High power UHF )?

Any 'finger tests' should be done quickly after powering down but personally I'd buy a thermocouple thermometer and some blu-tac?

Hi All

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If you are worried about fingers getting cooked.?

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Get yourself a laser thermometer they look a bit like a pistol. You squeeze the trigger and point the laser dot at the device and read the temp.

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I use my finger inside a QCX or similar. I would be very careful touching the driver devices or output devices inside a PA when first powering up and aligning the bias.?

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