I don't see any mention about the dummy loads rating. Since I'm operating at $13.8~V$ the power output is about $4~W$. I'm looking to use what I have on hand as I have a field day approaching soon and if I order a dummy load online it won't get here by the time the event begins
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$\begingroup$ shop.qrp-labs.com/qcxp $\endgroup$– Mike WatersCommented Aug 20, 2020 at 23:24
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2 Answers
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I'm assuming your nanoVNA came with just a small SMA "terminator" load. I'm seeing various specs in various listings for various similar parts, but most of them are in the 0.5 to 4W range. This partly depends on the duty cycle/duration: one I found is rated for 2W "average" power at 125ºC [presumably continuous usage] but also claims a "peak" power handling of 1kW … for 50µs!
If you look at the type of loads rated for 10W continuous power you will see they are quite a bit heftier than these little 50Ω terminators! So I certainly wouldn't do extended (continuous unmodulated carrier) tests at 4W with your load.
But if it's all you have, you could probably get away with some short tests giving it time to cool in between — but be particularly careful because it may become hot enough to burn you long before it's hot enough to damage the resistor itself. [Come to think of it this may the main reason why the higher-power dummy loads have such large heatsinks!?]
answered Aug 20, 2020 at 19:48
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$\begingroup$ yup! A dummy load's goal is to convert 100% of the input power into heat (as opposed to RF radiation). $\endgroup$– webmarcCommented Aug 20, 2020 at 20:08
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1$\begingroup$ Assuming these little terminators are 0.5W, how much more power would they be able to dissipate if immersed in a cup of chilled oil? $\endgroup$– hotpaw2Commented Aug 21, 2020 at 3:51
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I wouldn't recommend it.
The NanoVNA SMA calibration kits for sale online are quite small. They don't seem to have specifications for power, but similar dummy loads do, and they're rated for 1 W or less. You may get away with 2 W for short times, like a series of Morse dots, but not key down for long enough to adjust anything.
At HF you can easily make a dummy load - and especially if you have a NanoVNA. Size and shape aren't critical - 10 or 20 small quarter-watt resistors in series and parallel will easily take 5 watts.
20 x 1k, arranged in a radial pattern, would work well up to VHF at least. If you have more 10 or 100 ohm resistors, then you'll need series and parallel arrangements. Check the load with the VNA, to be sure the resistors aren't too inductive.
Here is one from VK2HHS, you could do it with just an offcut of coax, no PCBs:
Here are some more photos of various construction methods.
