Salt Water Dummy Load

Build a Saltwater Dummy Load

A great first project for the new ham.

Background
How would you like to make a useful piece of ham radio test equipment for virtually no cost?

There are numerous plans in books, magazines and on the internet on how to build a dummy load out of noninductive resistors and some fabricated hardware, and maybe with a can of oil to stick it in so it doesn't overheat and burn up. Where is the fledgling ham to find "noninductive" resistors? What if they don't have a workshop or tools to fabricate the metal? Here is a simple solution- the saltwater dummy load. Instead of a physical part or parts to provide the resistance, here we use plain water with a little salt added. The salt allows the water to conduct electricity, so the volume of water in the container becomes the resistive element. All we need is a container to hold the water and a connector to get the RF from the radio into it.

I can't recall where I learned of this first, but I'm sure the idea has been around a long, long time. I built my first saltwater load as a new ham back in the early 1980's. I've built them in containers as small as a baby food jar all the way up to a gallon pickle jar. The size (volume of water) will determine how many watts the load will dissipate before the water gets too hot. I found out the hard way glass jars will shatter if the water gets *too* hot, and plastic jars will get soft. So keep this in mind when sizing the container for your version.

There are some compromises using a dummy load like this. One, is it's not shielded. For most practical purposes, shielding isn't really necessary. Most of the RF will go into heating the water and very little will be radiated. Another issue is frequency. These loads appear resistive until you go up high enough in frequency to where the electrode lengths approach a significant portion of a wavelength, and the stray reactance becomes significant. Certainly any reasonably sized load will work up to 30 MHz, up to 50MHz is possible if care is taken in it's construction.

Construction
You can have fun with this part. About the only requirement is that the container hold water and be non conductive. I standardized on glass jars but pretty much anything you can come up with to hold water and an SO-239 connector will work. I have used both bare solid copper wire and tin plated copper wire for electrodes. The tinned electrodes seem to be a bit more resistant to the corrosive effects of the saltwater but either one seems to work equally well. Romex is a convenient source of solid 12ga copper wire, but any solid wire you can find will probably work. Mount a chassis-mount SO-239 or BNC in the lid or cap of the container. Solder one electrode to the center pin, the other can wrap around one of the mounting screws and tighten the nut down on it. Bend the electrodes so that they are as far apart as possibe inside the jar and parallel to each other. Cut them about 1/4-1/2 inch above the bottom of the container. It might be a good idea at this point to goop up the connector underneath with silicone seal to prevent liquid from leaking out, and salt from corroding the connector and hardware.

ALL PHOTOS CLICKABLE FOR A LARGER VIEW


This is the general idea of what the finished container should look like.	Use your imagination when selecting a suitable container!

"Alignment"

To "align" the saltwater load you need a transmitter capable of generating a few watts CW at the highest intended frequency of use (say 30MHz for a load intended for HF), an SWR bridge and some patch cables. If you have or can borrow an antenna analyzer like an MFJ-259 it makes this task a little easier. Fill the container up with water. Put the lid on and make an SWR measurement-- it will measure pretty high. Now, using the tip of a knife as a spatula, add a tiny amount of salt to the water. Stir it up, secure the lid of the container and make another SWR measurement- it should be incrementally lower. Keep adding salt until the SWR reaches 1:1. You want to "sneak up" on the 1:1 point without going past it. If you put too much salt in and go past the 1:1 point you'll have to dump some of the solution out and add more fresh water. The actual amount of salt you'll need depends on how counductive your water is coming out of the tap, but in my load it worked out to be less than 1/4 teaspoon per pint of water. That's it! The saltwater load is now ready to use. Unless your container leaks or water evaporates out of it, the load's resistance won't change.


Alignment taking place on the 'bench'.	MFJ-259B Analyzer Display after Alignment.

OK, So Whaddaya Do With It?

A dummy load is an indispensible tool for the novice as well as expert ham. For starters, it's a safe way to test your transmitter without worrying about QRM'ing the bands, and you know it's a perfect match for the transmitter. Unlike an antenna, which could have mismatch and is reactive when off-resonance. Additionally, it's very useful for testing coaxial cable, especially if you doubt the integrity of the antenna installation or suddenly something with the antenna doesn't seem to be working right, and you want to eliminate the feedline as a variable. Terminating a coax cable with a dummy load, then testing power at the transmitter end and power at the dummy load end will tell you how much loss your cable has at that frequency. Many new hams set up an antenna and discover their SWR is high no matter what they do, or suffer RF feedback and aren't sure where it's coming from. Putting a dummy load at the end of the cable and seeing if the symptoms change can go a long way to isolating and solving these problems.


For the skeptics, here is a network anaylzer plot of my sample peanut butter jar load.	Graph showing the temperature rise with 100W input. Load tested was approx 22 fluid ounces.

Other Resources
Believe it or not, these are the only other links I could find
on the internet pertaining to Saltwater dummy loads:

http://hamradio-online.com/2000/apr/w6bky-26.html
http://www.hamradio-online.com/2000/jun/w6bky-27.html

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