HFU HF Underground
Technical Topics => Equipment => Topic started by: jackson_r on February 08, 2016, 1530 UTC
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Been absent with the broadcasting attempts for the past couple of months but finally getting back to work after a long vacation. I'm putting the finishing touches on the mag loop antenna I've been working on for awhile. I'll likely be able to give it a try in the next week or two. Here are some photos:
https://drive.google.com/folderview?id=0B1Sx8UoqolwldDZfOEdFT1dKOFU&usp=sharing
My calculations say it should be good between around 6400 and 11000 kHz, with a maximum transmitting power of around 150 watts. The 12 v capacitor drive motor works great and has adjustable speed. Both loops are about 1 meter in diameter and made from 3/4" copper pipe. If it works I'll paint the whole thing a flat black. The only thing that's keeping me from trying it out are two things. First, I'm waiting on a plastic enclosure to keep the capacitor and the drive motor out of the weather. Second, I'm scratching my head on the proper wiring for the air variable capacitor and I was hoping for some expert advice here. Take a look at photo 6956 as I've marked it up with some letters. The two screw posts marked with "A" both go to the "stator." The screw post marked "B" appears to make contact with the rotor. From what I've read, I believe I need to run one side of the loop to one of the A posts and the other side to the B post. Can anyone confirm this or provide alternate advice?
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Not sure what you have there?
Is it a 2 turn loop?
Is the cap 'Split Stator'? it needs to be!
Do you have a drawn diagram anywhere?
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Not sure what you have there?
Is it a 2 turn loop?
Is the cap 'Split Stator'? it needs to be!
Do you have a drawn diagram anywhere?
Hi Stretchy - Sorry, I should have taken some better photos! Here is the exact item:
http://www.ebay.com/itm/321880092119?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
It looked to be the same time of variable capacitor I've seen on other home builds. I suppose you're going to tell me I bought the wrong kind now. :D
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er well....
You're going to get a large loss across the cap as it has a wiping (resistive) contact.
Just connect the open ends of the loop across A & B.
With power higher than 10W (or so?) the rather large voltage present at the wiper will probably start arcing but you're going to have to see.
Still don't understand the loop, is it 2 turn?
How are you coupling in the RF I/P?
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Yes, two turn design. Tried to base it on this:
https://www.youtube.com/watch?v=OuaTCZG2EBs
The only difference is that I jettisoned the vacuum capacitor I was going to use because it was just too large and heavy. I can try to put it back in the design if you think the air capacitor is going to be unsuitable. I planned on making all connections pretty much like the guy did in this video.
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Hamfest season is just around the corner. Many old antenna tuners have split-stator caps inside. Take screwdriver with you so can check out the caps in the tuner. Hams sell old tuners dirt cheap in these days of built in tuners and no hassle auto tuners.
With a little digging you should be able to pick up one with a split-stator cap cheap. Pull the cap for your loop and use the box for something else down the road. A 300 watt used tuner can be picked up at hamfests for next to nothing as most guys don't want to haul them back home.
Of course you could fight the crowds digging in the good junk boxes for the split-stators, but cracking open a few tuners to find one is a helluva lot easier and cost about the same.
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Jackson r, thanks for the update, I was wondering how things were going. I'm not an expert, but I did read this great webpage by a guy who is one! On his link, there are close up pics of how he connected the capacitor to the loops.
http://gridtoys.com/glen/loop/loop3.html
I would call your loop a dual loop. It has 2 separate pieces of copper pipe that each make their own loop. And I would not call it a 2-turn loop. 2-turn loops use one piece of pipe that has been shaped like a coil to have 2 turns. (somebody correct me if I'm wrong)
The antenna on the webpage has 4 loops (does it make you drool too?). The more loops you have, the more efficient it is but with diminishing return.
It looks like you got most of the hard work done, good for you. Was it hard to bend the pipe?
P.S.
To camouflage such a crazy thing, you could put it inside a pop-up tent.
And don't get close to it when transmitting so you avoid radiation. There's lots of pics on the web of people right next to the things as they transmit with them.
And don't touch it because it has high voltage that can keel you!
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Yes, I had found that page myself. The guy's antenna knowledge gave me a thousand yard stare! I've found it somewhat hard myself to figure out the terminology when researching designs. My antenna indeed has two separate parallel loops. But some people call them multiturn, parallel multiturn, multiple conductor, or multiple parallel conductor. And yet others just refer to them as a regular magnetic loop antenna.
The build wasn't too bad, but also not without its moments. The copper tubing I used was 3/4" "soft" tubing such that is used in air conditioning systems. I envisioned something akin to refrigerator ice maker tubing. Turns out it was not quite that easy. I can't remember the last time I cussed so much over a garage project. ;D The spacers to keep the tubing the same distance apart were made on my 3D printer. From there I made silicon molds and then used casting resin to make all of the spacers.
Right now I'm going to follow Stretchy's (and others) advice and re-insert my vacuum capacitor in the design. I think it will work better and have fewer problems. The damned thing is so massive, however, I'm going to have to tweak a few things. Not to mention that my nice little 12 VDC motor that turns the air capacitor so easily might not be up to the task of turning the piston on the vacuum capacitor.
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The build wasn't too bad, but also not without its moments. The copper tubing I used was 3/4" "soft" tubing such that is used in air conditioning systems. I envisioned something akin to refrigerator ice maker tubing. Turns out it was not quite that easy. I can't remember the last time I cussed so much over a garage project. ;D
Hey jackson; sounds like you are on a good path.
I have had to make many coils out of copper tubing, large and small diameter, in both professional and hobby builds. You are right; even for large coils it is not as easy as it appears. Copper will work harden - become more brittle - as it is mechanically deformed, and this can complicate forming a coil. Manufacturers may say that the tubing is supplied in a "dead soft" state, but YMMV. I have found the following steps to be quite useful. In the event you need to make new coils, the following may help.
1) Uncoil the roll to a somewhat straight length a bit longer than needed. It does not need to be perfectly straight, but there should be no kinks.
2) Torch the tubing (ox-acetylene preferred but "MAPP" or propane OK) by working a torch slowly down the tubing from one end to the other in a dimly lit room. You would like to bring the copper to a dull red color if possible, and keep it at that temp for maybe 10 seconds at each part of the tubing. This will help to soften the tubing a bit. When done, you would like to see a nice dark oxide film on the tube's surface.
3) When you have torched the entire length, let it air cool until you can handle it.
4) Fill the tubing with dry sand, closing the ends up with tape. For larger diameter tubing, this really goes a long way in preventing kinks.
5) Bend the tubing to shape. I generally bend against a form made from scrap plywood cut with a sabre saw. I generally bend "against" any remaining curvature in the tubing.
6) Remove the sand, and trim the ends to length. The oxide will need to be removed at points of electrical connection. You can leave it on the rest of the tubing, as it will act as a pretty good primer for paint top coats.
Looking forward to your tests!
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Something to consider is to flatten the copper tubing, this will reduce the capacitance between them a bit and may result in higher efficiency.