I had an old dummy load I built out of a ton of resistors to get 50 ohm and handle around 5-10 watts. It was pathetic and with a new AM medium wave transmitter I am building I managed to smoke the resistors out after a minute or two of being on. Mmmmm burnt resistor smell fills the air...
Either I could have purchased higher rated non-inductive resistors and built a dummy that can handle more power, or I could spend nothing at all and instead make what is known as a "salt water dummy load". So WTF is this? It's what it sounds like, a jar of water with two electrodes submersed into it that are soldered onto a PL239 RF jack. Of course you could use other types of jacks, but these dummies work best at VHF and below into the HF range. Above VHF care must be taken into consideration with shielding and electrode length.
The strange thing about this dummy load is that it will not measure 50 ohms on an ohm meter. It needs to be "tuned" to a SWR meter in practice to get it to 50 ohms before using it on other gear. I used a cheap CB radio with SWR meter to get the salt water dummy to read a 1:1 SWR by simply sprinkling in a little more salt a tiny pinch at a time and stirring, and occasionally dumping a little water out and replacing the water to narrow it down to the perfect dummy load reading. Why this doesn't measure 50 ohms on a ohm meter is a mystery to me right now, but as far as a purely resistive load without any inductance for testing HIGH POWER transmitters this thing rocks! For a tiny pickle jar filled with water and a few pinches of salt it performs better than most factory made dummy loads I have used back in the day.
The only two issues are that it will heat up the water a slight amount. With 25 watts into a small jar it was getting warm to the touch which did de-rate the SWR reading slightly until it cooled. Of course larger jars will allow more cooling for higher power, but this jar was all I had in the recyclables at the moment. Other issue is once in a great while (every few weeks or month before use) you need to shake the jar up to get the salt stirred back up. This can be messy if the jar is not water tight which is why I used RTV silicone around the PL239 under the jar lid to seal it up water tight. Small inconvenience for something that cost almost nothing to make and performs as good as expensive high wattage resistors.
Pictures...
So a few notes:
This can be used at VHF, including FM broadcast transmitter tests. Only thing needed is an SWR meter and a low power setting on your transmitter so you can add or remove salt/water until it reaches 1:1 before testing at high power. Once that is done, it will work exactly like any other resistive dummy load and may handle far more power. Be aware that it will radiate RF a bit compared to a resistor stack like a traditional dummy, but for low power work it is fine.
It takes only 1/16 to 1/4 teaspoon salt. Maybe even less or more depending on your tap water. Distilled water will be easier to work with if your city tap water is full of minerals and whatever else.
The electrodes can be anything but there are some metals which will corrode less fast in salt and water than others. I used some copper wire laying around and twisted them together and curled them at the bottom to make them thicker and more resistive. No need to be fancy here.
I've also read that sodium bicarbonate (baking soda) may be even better than salt because it has less of a capacitive effect. I may try it sometime soon and report back, but for now ordinary salt works fine.
I'd imagine for VHF and above it would be wiser to use a metal air tight container so that the whole outside is at RF ground to reduce radiation when testing transmitters, but at HF and below, like shortwave and mediumwave transmitters glass jars are perfectly fine and radiate little. It's not perfect but it beats using a stupid light bulb or something that isn't a proper 50 ohm match when testing transmitters above a few watts, and yeah even a small jar can handle dozens of watts, a pint or larger hundreds of watts, and a gallon+ probably a few thousand watts!
Post by ogrevorbis on Aug 13, 2018 19:45:00 GMT -6
Interesting idea!
This reminds me of a video I watched recently on a guy who made a salt water pump antenna. I doubt it works as well as this though.
There are many improvements you could make with this. I was thinking about having an inlet and outlet tube and connecting that up to a small pump and radiator. You could use a PC water cooling setup.
There are many improvements you could make with this. I was thinking about having an inlet and outlet tube and connecting that up to a small pump and radiator. You could use a PC water cooling setup.
Though I am sure that would work I don't really see the need for it. There are amateur operators using these salt water dummy loads up to a few thousand watts in nothing bigger than a cement mixing bucket.
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