Sorry, this ended up being quite long winded! I'm currently building one from a Russian Vacuum Variable Cap from ebay. Pure DC resistance is the biggest challenge as I see it. Even little additional resistance just turns into useless heat in the loop so R needs to be as low as possible. The MFJ mag loop helps in that dept by welding the loop to the variable cap. Since that is out of reach I am going to use one solid piece of copper pipe bent into a circle and then a length of 1" solid copper strap will be soldered to each pipe end and wrap around the ends of the vacuum cap. I figure once a year I should disassemble the copper/vacuum cap connection and clean any oxidation off. I found 6cm aluminum automotive fuel pump mounts on ebay that fit onto the ends of the vacuum caps perfectly providing a mount and allowing the copper strap connection to take place. Email me directly if you are going the vacuum cap route and want to see how I'm doing it. There are two lessons I have learned so far: 1). That Russian vacuum caps are way heaver then I thought. 2): The screw device needs more torque then I originally anticipated. The weight factor just needs to be addressed with a beefier mount. Since it will reside at the top of the loop it will have a high center of gravity and I don't want the wind to knock over a >$200 glass cap! The torque issue just means I need a stronger NEMA17 stepping motor and a 2 amp easy driver board to give it extra current. I imagine when it is cold the grease will stiffen up requiring even more torque. It may come to pass that I will use a gear reduction stepper motor to get the torque I need. I'm tending to figure out the little problems as they crop up! I'm building this one for 100 watt operation as the vacuum cap should be able to handle the voltage. For QRP operation I have found a 2Kv air variable on ebay that (according to the 99pacific calculator) should handle the 10 watt KX2 for portable operation. I'm already starting rev.2 and I haven't even finished the first one yet! Anyway I'm having a hard time finding someone to bend the copper pipe. It takes a rolling tube bender and I don't want to invest $150 at Harbor Freight to use it a few times! Most local garages can't bend 1" pipe. Too small. I need to find a fabrication shop. I could use soft copper and try to make a good circle but I'm a little too OCD for that! Well that's about all I can say so far. I'm sure I'll have more advice as I finally get this thing actually working! ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected]
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