Going Magnetic Loop

<< < (5/12) > >>

K5WLF:
Quote from: Ed/KB1HYS on October 18, 2011, 10:11:42 PM

How big is your lot? Any trees or just the house/roof?  I have the same problem with our 100x100ft lot, But I have some very tall trees (65 ft or so white pine and a few oak) that make great ant supports.  One day I will mess around with actually using the Tree itself as a vertical ant on 160 meters.  It's been done and is supposed to work, tested by the Army. 


Ed,

My lot is about 100' (E/W) X 70' (N/S). No trees that'd do any good and the roof of the 24' X 36' house has the Hustler 5-BTV in the geometric center for 80-10. The homebrew coil-loaded 160 dipole is about 20 feet off the deck along the south fence. It has lousy bandwidth, but I made S-9 into IA with it a while back on 100 watts SSB. I'm planning on getting an amp and I need something that'll take the power and give some decent bandwidth and coverage from a fairly low elevation.

ldb

W0BTU:
Quote from: N8UH on October 19, 2011, 03:36:56 PM

... My question is about doing multi-turn loops. I read the QST article this month about mag loops, and the author made it sound as if doing a multi-turn loop gives you roughly the same efficiency as a single turn, larger loop using the same amount of material. ...

I got kind of excited about that, too, until I saw that the two-turn loop in the 11/11 QST actually had significantly more loss than the single-turn one. But maybe I misread that.

Multi-turn loops with conductors too close together have the "proximity effect" issue. The conductor in the front may have a certain current value at a given point on that conductor; but the turn behind it next to that point may have a current that is quite different. This causes increased current in a given area. You can spread the turns out to reduce this effect, but the bottom line is that losses can be much higher than in single-turn loops.

W0BTU:
Quote from: flintstone mop on October 19, 2011, 07:20:50 PM

... Mike W0BTU, the helical winding has a purpose. It is not to reduce the weight. ...

I realize that, Fred. I've studied K8NDS's material about his loops extensively, and have his info printed and bound in a 3-ring binder along with a lot of other mag loop info. I know what he's claiming. I was just saying that I added up the cost of the copper and the pipe, and for a reasonable efficiency, Type M copper water pipe was not only cheaper, it was easier to build.

There's a lot of controversy about Rich's design. He wrote an article about his helical loop on eHam, if you're interested. The comments to his article there points out that his helical design may have additional losses, but I think that's a little overblown.

W0BTU:
Quote from: ke7trp on October 19, 2011, 08:04:10 PM

... Get a Childs hoola hoop at Walmart.  ... Buy some Tinned copper braid.
Slip braid over and around hoola hoop pull the braid tight. ...

That would be fine for a receiving loop, but it would have way too much resistance for a transmitting loop. Small loops like this have been made from coax like RG-8, and they get extremely hot at 1500 watts. Almost all the power is lost in heating the copper braid. (Depending on the loop diameter, the loop current can exceed 100 amps.)

The reason coax braid is not a good choice for an STL is that the radiation resistance (Rrad) of an STL like this is in the milliohms, and the resistance of the braid can exceed Rrad of the loop itself.

That's why we have to use what might seem to be a ridiculously large conductor to have a loop with any kind of reasonable efficiency.

Even people who have made small receiving loops from, say, #16 copper wire find out that the signal level is waaaay down from one made from a much larger conductor. The Rrad is so small that the losses in the wire are enormous.

There was a good discussion on this at http://forums.qrz.com/showthread.php?311422-HF-magnetic-Loop-antenna-questions.

ke7trp:
Interesting.  I can see why Rich used that thick copper strap!

C

Navigation

[0] Message Index

[#] Next page

[*] Previous page