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Scrapyard loop




 
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December 04, 2022, 07:52:07 PM *
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Author Topic: Scrapyard loop  (Read 535 times)
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k7mdo
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« on: November 21, 2022, 04:50:33 PM »

Well, here is first try at a loop antenna.  I was at the local metals scrapyard a week or so ago and found this 5' diameter SS hoop (3$ a pound)....  have been interested in making a hoop to experiment with and picture is what I came up with.... the small hoop is foot in diameter and the large is 5 feet.  The material is stainless steel and is 1 inch in diameter. 

I cut the hoop at the weld and inserted a variable cap at the top.... then at the bottom I robbed the wife's plastic container lid as a support for the lower loop made from coax.


At first I was concerned that the stainless steel might not be the best conductor but wow does it work! 

Initial tests show it to be very sharp in SWR and very sensitive to variable capacitor adjustment (touchy)....  however, the settings are very stable after adjustment and don't seem to fluctuate with my old body approaching the hoop.

So far I am happy to report the 40 meter band seems to be near the center of tuning range though 10 megahertz WWV signal is almost as good.  We have a noon time net on 7284 which seems as good as my other antennas but with significantly less background noise.

It is the drop in background noise that is the most impressive result.

I am a little fearful of transmitting yet but will give it a try soon.


* Loop Antenna.jpg (119.27 KB, 483x644 - viewed 138 times.)
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W1NB
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« Reply #1 on: November 21, 2022, 08:26:44 PM »

That SS loop is a great find.
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km6sn
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« Reply #2 on: November 22, 2022, 10:42:08 PM »

Please be wary of fire potential...with Q factor like that monsterous voltages appear on the cap, and
arcing can happen. Yes, as your body approaches it will have little effect: at proximity, you are in the magnetic field of the antenna, and at 7 MHz the electric field has not yet been developed at that distance. Your body is mainly an electric device, therefore "Invisible" to the antenna. Try putting 10 watts into it and approach it with a fluorescent bulb! A similar 20m version with 50 watts into it and the bulb would light full brilliance just a couple of feet away. (in  retrospect, not sure why--electric field emanating from the tuning cap??)

I did a fascinating experiment at 2 GHz or so: two loop antennas, broadside to each other spaced maybe 20 inches apart. I took an electrically-lossy piece of sheet material, held it orthogonal to the propagation path, and moved it from one loop to the other. When the sheet material was within about 1/4 wave of either loop, little path loss was observed (still in the magnetic-field-only region). Further away and the path loss became immediately and profoundly obvious.
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K4RT
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« Reply #3 on: November 27, 2022, 11:01:04 PM »

Tom,

Nice work. Looking forward to your report on transmitting with the loop antenna.

73,
Brad
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Patrick J. / KD5OEI
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« Reply #4 on: November 28, 2022, 02:53:45 PM »

Kind of opens a door toward a visit to the automotive exhaust pipe bender, though the steel used there is usually ferrous and it's all weird to me..  Also want to know about the results!
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Radio Candelstein - Flagship Station of the NRK Radio Network.
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Making AM GREAT Again!


« Reply #5 on: November 28, 2022, 07:53:46 PM »

We use rigid aluminum conduit at work almost religiously
 

I'd like to get a few pieces and bend them to make a loop.  They screw together via a threaded coupling so electrical contact shouldn't be an issue, I figured I would use NoAlox at every joint.  Or possibly get our welder at the plant to weld each seam.

Then I look at my assortment of capacitors here.....   I'll ha e to wait until the connex box gets here next year.

I wonder how a mag loop would perform in regards to being next to the salt water?  We have a 1200 foot pier over a very salty Caribbean sea that would allow me Iota and POTA since it's at Ft Frederiksted.

Do loops have any help from saltwater?


--Shane
WP2ASS / ex KD6VXI
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k7mdo
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« Reply #6 on: November 28, 2022, 09:58:09 PM »

Interestingly enough, one of my biggest problems is to explain to the neighbor "how this loop works"!

I don't know if anyone has seen a lay explanation for the functionality of the loop versus the wire dipole.  For me the explanation always evolves to Maxwell's partial differential equations and the right-hand rule.  The neighbor simply glazes over and I get the feeling that it is nearly beyond me to give a straight answer.  My own understanding is too weak to provide the answer that the inquisitive neighbor demands.

Does anyone know of a "lay" description of the loop's functionality? 

For the neighbor, long wires and wavelengths suffice, they make sense but a hoop standing in free space is not as simple.  Particularly when it is not elevated nor seemingly related structurally to wavelengths of signals.

Tom

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