Welcome to W2BRI's Magnetic Loops
Custom made Magnetic Loops for HF

Creating your Loop's Coaxial Capacitor

Creating A Coaxial Stub:

1. Coax Type: First step here is pulling out the razor blade and getting ready to work with coax. Not all coax is created equal. Some are better, and a there's a lot of bad stuff out there too. So beware, and don't forget that I warned you. You're not going to get away using some cheap RG-8X, or any other small diameter coax -- unless you're running QRP. Your coaxial stub is going to carry a lot of voltage and current, so use quality products for your own sake. Or you might end up with a coaxial stub catching fire from the 'AM' I was running through it at 200 watts. Not a good idea unless you have a vacuum variable. However, coax is a heck of a lot cheaper and lighter, and pretty easy to work with. I would recommend not using double shielded coax, it's too hard to work with. I have tried using LMR-400 and those types of coax (which are usually excellent products, low loss, etc), but for the work we are about to begin, go with basic RG-8U or RG-213. Let's use a 3 foot piece. 

2. Cutting the Outside Covering: You can use whatever tool you like to get the outside rubber off. I find the delicate use of a razor blade makes a nice tool. We are going to start with the part of the coaxial stub that gets connected to the loop, so we need to seperate the braid from the center conductor. Measure about two and half inches from the end of the coax piece and with the razor blade and cut into the rubber lightly. Keep cutting till you have a complete circle. Try hard not to nick the braid underneath, but don't worry if it isn't absolutely perfect. You'll get the hang of it. OK, now take the coax length wise and cut a nice long slice down the center of the coax from the end to the point on the coax where you made your circle. If you have made the cut deep enough, it should now be easy to peel off the rubber coating. 

3. Preparing the Braid and Center Conductor: You should now have the peel off the coax, slightly pull back the copper braid from the coax so that it bunches up. Take your scissors and cut along the braid as pictured, leaving about 1/4 inch of braid intact towards the bottom. This will give your braid more strength when you pull it away from the center conductor of your coax. Pull the braid away from the center conductor and place some tape on the base of the braid where it seperates. Once you have this re-enforcing tape on the braid, tape up the rest of the braid as shown leaving a half inch exposed. This will give you a nice strong Y at the end of your coax for attachment to the loop. Also tape of the outside white foam dialectric of your cable too. I beleive each layer of electrical tape has a 1 KV tolerance, so let's beef up the coax a bit anyway. I have seen the white foam dialectric melt at high wattage and burn and vaporize -- so it can't hurt! This is important, leave a half inch or so of center conductor without tape so you can cut off the white diaelectric and expose the copper inside for attachment to the loop. I would recommend using a wire cutter for this, and gently pull off the white covering on the center conductor. You now should have a stub with a nice Y with both braid exposed and the an exposed center conductor.  This is the part of the coaxial stub that will eventually be attached to the loop. 

4. Preparing the End of the Stub: In general, the way these stubs work are as follows: the more coax you have, the more capacitance you have. The more capacitance you have, the lower the resonant frequency of the loop. The less coax you have, the less capacitance, the higher the resonant frequency. So we want to start with a longer coaxial stub than a shorter coxial stub and keep trimming the bottom end of the stub until we reach the frequency patch you want to work. I'll go into a lot more detail on this topic in later chapters on acutally tuning the loop, but I think this important infomation to mention. For example purposes, I have demonstrated in a picture what the end of the coax should look like when your done. A exposed center coductor with its surrounding braid exposed. We accomplish this by using our razor blade and cutting a circle in the coax again. This time about one to two inches from the end of the coax. Let's cut a horizontal slice in the rubber so we can peel off the coating and once again have the copper braid exposed. Now pull the braid away from the center conductor just slightly and tape the end of the center conductor. That's it. You now have a pieve of coax that will work as a capacitor for you loop. Congratulations. The last picture in the series to your right shows how the coax feed is attached to the  loop. More information to come! Let's move on...


 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Side Bar

 Continue to Step #3: Assembling the Loop's feed -->




 

 [ Return to how to Build Your Own Magnetic Loop  ]
Return to Home Page