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Author Topic: Any issues making a mag loop out of strip rather than tube?  (Read 1503 times)
G7DMQ
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Posts: 40




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« on: February 11, 2013, 12:00:17 PM »

First post!

I've been thinking about building a Magnetic loop for 80/40m.  I'm not short of space for random wite antennas - so this is out of curiosity more than anything.

Most designs are based on using tube to give the max surface area - but it becomes increasingly difficult to bend.  So, I wondered if there was any negative (electrically) to using flat strip instead.

You can buy 50 or 100 wide, .6mm thick copper strip used for roofing fairly cheaply.  100mm wide, counting both sides gives the same surface area as 63mm diameter tube.

It's also rather easier to solder / bolt / weld to the capacitor.

I fancy the idea of TIG welding copper tubes to the ends of the hoop and making a trombone style variable capacitor.

Any thoughts would be much appreciated!

Si
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W5CPT
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Posts: 557




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« Reply #1 on: February 11, 2013, 03:18:31 PM »

Years ago AEA had an antenna called the Isoloop made of a flat metal band. It was similar to the MFJ-1782.  I don't remember if it was stainless steel or aluminum. The ads for it touted the flat band as being better at something.

Clint - W5CPT -
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N3OX
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« Reply #2 on: February 12, 2013, 09:17:17 AM »

You can use strip... if you size the strip to match the loss resistance of the tube alternative, there are still a couple of disadvantages.

One likely disadvantage is that you'll probably get lower voltage handling at high power, but that can be mitigated by rounding and smoothing corners and edges near the capacitor.

Another disadvantage is supporting the strip so it doesn't move at all... if it moves, the tuning will probably change.

But you can make an effective magloop out of flat strip.

Something to remember about the losses is that it's not as simple as matching *surface area* with the tube equivalent. Because of the current distribution on the strip you need more surface area of strip to match the performance of a round tube. I suspect that weight for weight, you can often get equal or better performance with a strip b/c you can't get tube with a thin enough wall.


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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
G7DMQ
Member

Posts: 40




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« Reply #3 on: February 12, 2013, 05:35:19 PM »

Thanks guys - sounds like its worth an experiment at the very least!

I'll keep you posted.

Si
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K0JEG
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Posts: 646




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« Reply #4 on: February 13, 2013, 05:58:25 PM »

Years ago AEA had an antenna called the Isoloop made of a flat metal band. It was similar to the MFJ-1782.  I don't remember if it was stainless steel or aluminum. The ads for it touted the flat band as being better at something.

Clint - W5CPT -

Aluminum. I have one and it's fairly good. Don't know if there's any difference between it and tubes, but it is riveted to the tuning capacitor, something that would be next to impossible with tubular stock.
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KC4MOP
Member

Posts: 731




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« Reply #5 on: February 14, 2013, 03:22:24 AM »

You can use strip... if you size the strip to match the loss resistance of the tube alternative, there are still a couple of disadvantages.

One likely disadvantage is that you'll probably get lower voltage handling at high power, but that can be mitigated by rounding and smoothing corners and edges near the capacitor.

Another disadvantage is supporting the strip so it doesn't move at all... if it moves, the tuning will probably change.

But you can make an effective magloop out of flat strip.

Something to remember about the losses is that it's not as simple as matching *surface area* with the tube equivalent. Because of the current distribution on the strip you need more surface area of strip to match the performance of a round tube. I suspect that weight for weight, you can often get equal or better performance with a strip b/c you can't get tube with a thin enough wall.




There is always a catch. One other ham has had pretty good success with a spiral wrapped Loop on PVC pipe.

http://www.eham.net/articles/26572

Here is the article that was featured on eHam.
The Nay sayers will come out of the woodwork to shoot this guy down, but I would build it, if I wasn't strapped for cash right now. Present administration has me worried about my IRS Tax bill due on Apr 15.
Fred
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