The simple Coax Loop works

[lanbergld]

I think there was a thread, somewhere on here, about making a simple loop antenna from coaxial cable. I'd been plagued by harsh QRM and electrical buzz at night time, which hampered LW/MW Dx during these hours. My homemade loops had been made out of standard insulated electrical wire. I've seen many nice "shielded loops" for sale on Ebay & other places, but could never part with the $150-200 needed to buy one.

So last night I built my own simple loop out of RG-6U coax, which I braced onto my apartment window. I'm talking real Fred Flintstone simple: No variable capacitor, no amplifier, no special connectors -- just the coax. The results were fantastic; almost breathtaking. ALL QRM & electrical buzz < 2000 kHz was completely removed. I was back to logging NDBs from all over, and back to MW Dxing across the country. Also, my reception on 160-meters was better than it had ever been on any receiver. Quiet, whispering noise floor but decent gain of signals.

The loop was rectangle, about 7x3 ft. Basically the inner periphery of my window.
I did make a 5/8" break in the outer braid of the coax. The inner foil shield I left intact.
The coax was also its own feeder.
One center conductor went into my (+) Hi-z; the other end went into my (-) terminal.
The outer braid of one end was connected to my receiver's ground screw.

Interesting that all noise was also removed > 15000 kHz, in addition to < 2000 kHz that I mentioned. But NOT between 2000 - 15000 kHz. There was no effect there. But signal strength on all HF was very good.

Larry Lanberg
Richmond VA

FRG-100
homemade antennas
too poor to buy expensive things

 

[zz0468]

Since you left the inner foil shield intact, the entire thing is still shielded. I presume it's working because of how you have it connected. Normally, the shielding would be intact at the feed point, and broken in the elsewhere. But it works...

I use a shielded loop installed outside on a tv rotator. The noise level here is brutal, but a loop makes it quite manageable.

 

[lanbergld]

I left the foil shield only because I was unable to remove it (without also taking the center insulator at that point). My method of connection & ground was unorthodox, but that's typically the way I do things.

But yes I am extremely pleased with the results. My FRG-100 now, all of a sudden, has a hushed noised floor (from 50-2000 kC and also >15000 kC) yet the distant signals come punching through the speaker.

Larry Lanberg
Richmond VA

 

[raisindot]

This sounds like a great idea.

Knowing absolutely nothing about antennas, cables, connectors, grounding etc. (I usually just string a bunch of wire around the ceiling, and get tongs of noise), I'd be very grateful if you'd be willing to show what this whole thing looks like, including how it's strung up the window, connected to the radio, grounded, etc. if you're able to. I'm getting a new HF rig (R-75), can't stick up an outside antenna, and would like to be able to get the best indoor antenna system possible, and yours sounds like a great solution

Suzie

 

[lanbergld]

Dear Suzie,

Like you I also do a lot of trial & error experimentation with wires to make indoor antennas. I was actually surprised that this simple coax contraption eliminated my noise problem like it did. Totally eliminated it <2000 kHz, while also making a great antenna there.

But anyhow, hopefully the batteries in my cheap digital camera are still good. If so, I will take pictures. Then I will figure out how to post them here, or, send them to you. BTW I use 4 (plastic) suction cup hooks to form a rectangular wire brace on my window. Or whatever shape I want to make.

Let me try taking these photos in a little while. Maybe I can get them to you later tonight. If not tonight, then tomorrow evening sometime.

Larry Lanberg
Richmond VA

 

[ka3jjz]

This is an interesting subject. Sad to say, one of the better websites disappeared some time ago - however, most (although not all) of the site can be restored via the Wayback machine.

Coax Loop Antenna for 160 - 10 meters

I'm trying to figure out how the heck I can put this on my wooden porch and get the feedline into my second bedroom....would be a killer, since I can get the loop somewhat outside the electrical field of the condo....73 Mike

 

[lanbergld]

Very nice link Mike. Thank you. This person does a great job describing his. (Looks like he has an FRG-100 also).

I know that I'm continuing to enjoy excellent, clear reception on 160. And as I already mentioned, every last vestige of noise & buzz has been completely eliminated from MW and LW, which was the most pleasant surprise for me.

Suzie: My digital camera is dead until I can get new batteries for it. Just go to your local big box hardware store & have them cut you 50 ft or so of coaxial cable. I use RG-6U. Use suction cup hooks or screw hooks for supports. Duct tape if necessary. Shower curtain rings can help too supporting these simple loops.

Larry Lanberg
Richmond VA

 

[ka5lqj]

&quot;Outside Loop&quot;.....

Good Morning,

I remember seeing an article about building an outside "loop" some time ago. It was fashioned from pieces of 3/4", white pvc (painted to prevent UV damage). There was, I think, a 12"x12" piece of 3/4" plywood for a '"backboard".

The antenna consisted of 4 pieces of 3/4" of pvc, 5 feet in length, glued to an "X" pvc connector. At each end of the pvc pipe a 90 degree "elbow" was glued facing "outward", but in the same direction and additional pieces of pvc were glued to make it easy to wrap the coax loop too. The "X" was positioned so the pvc went out at the corners of the wood "square". A piece of 2" white (schedule 40) pvc was mounted with "U-bolts" for a vertical support. An "X" of 6' pieces of wood 4x4, notched, so they were level with each other (Wished I had a "drawing" :-/ ) The center of the "X" was drilled to accommodate the 2" pvc and "outrigger" pieces of wood were attatched at each end of the "X" and secured to the 2" pvc....like a Christmas tree stand. I hope I'm explaining this so you can visualize the concept.

The "Loop" was wound around the forward "supports" and the coax was attached to some sort of "amplifier" or "matching device" and coax run from the antenna to inside the "shack". The whole thing wasn't over 10' tall and could be hidden from the neighbors behind a tall bush (for those who have "jerks" for neighbors, LOL!) The article SAID it worked well, with testimonies from those who had built and used it.

I kept the article, as I want to build it some day, now that we are moving just outside Shreveport, LA. When I locate it, I'll see if I can get it
"scanned" to send out. I don't have a digital, flatbed scanner.

Respectfully submitted,
73,

Don/KA5LQJ

Grid Square: EM-32

 

[nanZor]

Ah, I LOVE small vertical loops for receive-only. I've spent quiet a few years with them, and would be happy to help. Although I've used coax loops, I get the same results with simple wire and a balun - or tubing if you go hard core - I can explain why if you want to know, but I'll try to keep it short here.

Whether you use a wire/balun combination or coax for the antenna element, the key is to keep the circumference of the loop at roughly one-tenth (1/10th) wavelength for the center of the band of interest. It will operate at half the frequency or twice the frequency with some limitations. Outside that, and you'll have to build another loop with different dimensions. (we need only ONE current node in the loop for directional purposes, otherwise it becomes a multi-node antenna in what is considered a "large loop" which has a circumference greater than 1/10th wavelength)

With a 1/10th wavelength small vertical loop, the horizontal directional pattern is just the opposite of a regular horizontal dipole - that is, the null is looking through the loop as if you were holding a hula-hoop in front of you. typically 30-50db! Great for killing local noise like that tv across the room! Basically the noise arrives on both sides of the loop at the same time, cancelling out so no current in the elements are created, yet signals crossing the loop slightly out of time from above do create a current/voltage feed. (I won't go too deeply into it now...)

The vertical pattern leaves a little bit to be desired as it is very vertical - not as bad as an nvis antenna, but low-angle dx is not the loops strong point - however, with an extremely low noise-floor, you just might hear what you thought impossible. Thing is, you have to keep the loop mounted in the same plane on some sort of support and rotate it for best effect - you can fall out of the 50db notch pretty quickly. Once you find it, you could just lock the whole thing down - assuming you don't have other noise sources as you roll across the band.

Thing is, one can get needlessly sidetracked into all the technical details and never put one up trying to make it textbook-perfect. I can get close, but if the objective is to null a noise signal and still allow for good general reception, it can be done without going crazy about it - especially since we are just rx-only.

See - you guys got me started!