1. Introduction - 2. Development - 3.
- 4. Links & more
5. Liquid metal Elastic Stretchable wire - 6. Practical issues
|The animated diagram on the left demonstrates the basic function of
the Liquid Metal Antenna (LM-Antenna).
Air is pumped into the sealed holding chamber of the liquid metal, forcing
the metal up the tube (the gray in the diagram). A contact in the
liquid metal is run to the center lead of your coax for the RF, and the
shield is connected to radials (the solid metal is red in the diagram).
In making a prototype LM-Antenna the first problem was that mercury is toxic, so I investigated a liquid mental called Galinstan. It is considered non-toxic and is used in Geratherm thermometers. Galinstan is composed of Gallium and other metals.
I actually bought some Gallium from the following website, then added
some Galinstan from a Geratherm thermometer to lower the melting point.
You can also buy what appears to be Galinstan from the same website.
(Breaking open Geratherm thermometer's is messy because of broken glass.)
I obtained what appeared to be about 0.5 oz (14 grams) weight of Galinstan
for $40 from this website:
You can ask for larger quantities Galinstan of from the distributor of Geratherm Thermometers
People have made mixtures of liquid metal similar to Galinstan for up
to 1/10th the cost. This requires having large amounts of Gallium,
Indium and Tin mixed for you. Look at the internet on Google, etc:
Thin Plastic Tubing
The Liquid metal is expensive, so I used a thin tubing to make it go
further. I bought a Sullivan "Gold-N-Cable" # 507 at a local hobby
store in order to get the 3 ft thin tubing (about 1/16 inch id).
You can also buy thin plastic tubing at:
A picture of the prototype antenna is to the middle left.
I read that the liquid metal would slowly dissolve into solid metals*, so I did some experiments first. I made several test "cells" to measure resistance through the liquid metal using different conductors. Stainless steel stared out with almost 200 ohms resistance then dropped to about 12 ohms a few days later. Brass coated stainless that came with the Gold-N-Cable mentioned above kept a resistance below 1 ohm. Copper wire also kept a low resistance that remained below one ohm.
Two copper wire contacts were placed in the container that kept the liquid metal. I also placed a copper wire contact at the top of the plastic tube. Because the liquid metal in my prototype was only adjustable for 2m to about 440 Mhz operation, the top contact would be used for fixed 6m operation! (I could add a separate, smaller, liquid metal tube at the end of the 6m part for fine tuning it's length.)
Caution: Gallium based liquid metals will quickly dissolve into
aluminum, and can ruin the aluminum. Interestingly a very small amount
can be used to "weld" two pieces of aluminum together.
The next problem was a easy way to add pressurized air for moving the liquid metal. I tried using a plastic medicine syringe for pressure to start with, but my container leaked around the seal. So I used additional tubing allowing me to suck the column of liquid metal up out of the container. This still was a problem because the syringe slowly leaked air and the liquid metal would slowly fall.
The final solution was to use duct tape to seal the lid of the container then I could add pressure. I used a cheap aquarium air pump for pressure. It allowed a maxim 14 inches of lift. You could regulate air from a high pressure air pump, but their is a danger of shooting the liquid metal out the top of column! (I added a "trap" to the top in case too mush pressure was used.)
To regulate the air pressure I used a 1 input, 2 adjustable outputs aquarium air valve. The output to the antenna is left fully open, and the other output is vented or closed. The reason for this is to help balance the slight air leakage that I was getting with the LM-Antenna. See the diagram on the lower left.
I did not use a pressure gauge because I can see the liquid metal in
the tubing that I am using. See the applications page for measurement
Keeping the Liquid Metal from Sticking
It appears that oxides (or "rust") actually cause the liquid metal to start sticking!
In the beginning I tried to make an oxide layer on the sides of the tubing by running a bead of liquid metal up the tubing with water. (It was suggested on the internet that oxide kept the liquid metal from sticking). After a few adjustments the liquid metal started to stick to the sides of the tubing.
Next I tried keeping some heavy mineral oil above the liquid metal, and it caused more problems. Putting RainX in the tubing then drying it by pumping air seemed hopeful. However the RainX only lasted for 8 to 10 adjustments before the liquid metal started sticking. I was starting to get annoyed and depressed.
Thinking about the "least sticky" substance that I could think of, Teflon came to mind. I had some PTFE plumbing thread seal tape and the liquid metal rolled over it like magic! The only Teflon tube that I could find was a tip for a desoldering pump. It seemed like the liquid metal flowed through it ok but it was hard to see because of the thickness.
I tried the "PTFE super lubricant" from a Radio Shack ’Precision Oiler’ to see if it would coat the plastic tubing and help. By it's self the liquid metal still would stick after 10 to 12 adjustments.
Note: I am no longer certain of the following claims. The Car wax made it harder to see though the tubing, so that sticking was harder to see. Close observation and cutting open some tubing revealed this to me. The wax still caused it to stick differently and after 120 adjustments there was less sticking than the result of other methods. Still, it looks like there was enough sticking to effect performance. I have considered testing to see what effect *small* amounts of sticking metal have. Teflon tubing still might be a solution. Other liquids in the tube may help, or hurt, also.
(see above note): Success came by pushing liquid "Du Pont Teflon Car Wax" through the tubing, then blowing it out and letting it dry with the pump pushing air through the tubing. I made over 120 adjustments over the same area without any significant sticking! The only reason that I stopped was that my fingers were getting worn out adjusting the cheap aquarium valves that I had bought.
There was still a little "PTFE super lubricant" left in the upper part of the tubing, so it may have helped. I will start with clean tubing and test again in the future. Knowing that something worked was enough for me. (If you read about me on my KD8CIK.com you will see why all this work was difficult. I had to stay awake enough to get it done.)
2-22-06 Update: Preventing Oxides (or "rust") may be the best way to prevent the liquid metal from sticking. I am currently experimenting with oil. Another problem is that oxides may have mixed with the liquid metal that I am using, so I am trying to figure out how to make it pure again.
Cautions for experimenting
Warning: Respect the liquid metal,
and waste, like it was lead based solder. Gloves and breathing protection
may be advisable to prevent breathing any oxides from "old" liquid metal.
(Like solder, avoid injecting liquid metal into the skin.) - For more information
1. Test the function using water before risking your liquid metal.
2. If you end up with "scrap" droplets of liquid metal, you can re-combine them and re-use them. Pushing together the droplets under water seems to help. Because of the oxide layer and surface tension, you almost have to smash the droplets together. Scrap liquid metal almost looks and acts like melted waste solder. Except the liquid metal is liquid at room temperature!
3. You might want to use a high power attenuator between your transmitter and the antenna, in case there is a contact or SWR problem. For example, you might accidentally force all the liquid metal up the tubing so that none is left in contact with the copper wire(s) in the container. A 3db or greater power attenuator can help protect your transmitter. Bad contacts with any antenna could be dangerous to your transmitter. There needs to be more study of how well the contact lasts.
4. The Gallium based liquid metal is known to dissolve into some metals. Instead of making a direct contact to the liquid metal, you could capacitively couple the RF. I considered using a mica wafer or other method to make a capacitor between the input and the liquid metal.
Caution: Gallium based liquid metals will quickly dissolve into aluminum, and can ruin the aluminum.
Next Page: Applications
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