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Re: Newly assembled QMX, SWR=1.5 with dummy load
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I agree.
Never use those dinky 50 ohm calibration references as a transmitter dummy load.
Unless your transmitter is sending out a max of a tenth Watt or so.
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Hans has a nice 20W dummy load for $8.50,? perhaps include that with your next order.
https://qrp-labs.com/dummy.html
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A cheap dummy load can be made with a bunch of 3 Watt metal film resistors in parallel,?
For example, four 200 ohm parts in parallel can deal with 5 Watts all day long, and cost about $0.25 each.
Mouser part number 603-RSF3WSJR-73-200R? is a good choice.
For 50 Watts, perhaps use twenty 1000 ohm 3 Watt resistors, Mouser 603-RSF3WSJT-52-1K.
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If you already have a dummy load and want to use it as a 30 dB attenuator, you can do something like this:
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in ---+--------R------+------- out to coax ? ? ? | ? ? ? ? ? ? ? | ? ? 50ohms? ? ? ? ? 50ohms ? ? ? |? ? ? ? ? ? ? ?|
? ? ?GND ? ? ? ? ? ? GND
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The first 50 ohm resistor is your dummy load, it must be high power to deal with the transmitter.
R should be 1/2 Watt for a 30dB attenuator, can be 1/8 Watt for a 40dB attenuator, value of R calculated below.
The second 50 ohm resistor at the output can be 1/8 Watt.
I assume you are sending the sample out to something like a tinySA over coax.
The second 50 ohm resistor is the source termination to absorb any reflections coming back from the coax
There is a third 50 ohm resistor inside the tinySA that is in parallel with the second 50 ohm resistor,
together they make a load of 25 ohms.
For 30dB of attenuation we have a power ratio of 1000, so a voltage ratio of sqrt(1000)=31.62.
Those two parallel 50 ohm resistors in series with resistor R form a voltage divider.
To get a voltage ratio of 31.62, the value of R should be around (31.62 - 1) * 25 = 765.5 ohms for 30dB of attenuation.
For 40dB of attenuation, the value of R should be (sqrt(10000) - 1) * 25 = 2475 ohms.
Anything within 5% or so should be fine for our needs.
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On the 30dB attenuator, having (765.5+25) ohms in parallel with the 50 ohm dummy load brings it down to? 47 ohms.
So in that case we might be better off with four 220 ohm resistors instead of 200 ohm resistors.
The transmitter now sees 55 ohms in parallel with 765.5+25, or 51.3 ohms, which is good enough for most of us.
If we then add a 2000 ohm resistor in parallel with that 51.3 ohms, we then have the transmitter seeing 50.02 ohms.
On the 40dB attenuator that additional parallel load of (2475+25) is so small it won't matter, so use four 200 ohm resistors.
At 40dB, you need to start being careful with how it's built to avoid unwanted coupling from input to output.
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Check out that attenuator before using, as you don't want to burn out your tinySA.
I'd just hook the transmitter input port to a 12v battery, check for the a voltage of 12v/31.62 at the output
with a voltmeter.? Add that third 50 ohm resistor before measuring.
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For US residents buying stuff direct from China:
In summary, the de-minimus exemption from tariffs for packages worth less than $800 expires on May 2,
at which point the minimum customs fee will be $100.? On June 2 it rises further to $200.
So you could be spending $210 for that $10 attenuator.
If you place an order now, it will probably arrive at US customs around May 2, so place your bets.
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Jerry, KE7ER
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On Mon, Apr 21, 2025 at 08:26 PM, Donald S Brant Jr wrote:
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