Attic loop - this time it works!

[nanZor]

With outdoor space available, long ago I gave up totally on attic-loop antennas, but I tested again to give hope to those with no other choice.

I'll *try* to get to the point quickly ... maybe...

The two key points are:
1) Use a small vertical loop rather than a horizontal loop, either plain wire or a nice amplified commercial unit. Horizontal loops are a waste of time (20db or more loss on low frequencies compared to a small vertical loop.) The small vertical loop is obviously much easier to rotate, or in my case slighty relocate based on empirical listening tests.

2) Much of the so-called attic noise is really just "whole house" noise being transferred via the common-mode of your coax. Off-the-shelf coax chokes do great to be able to make the loop sing, rather than listen to "two" antennas at the same time (your loop and the unchoked coax)

Details:

The small vertical loop itself is a simple wire loop about 16 feet in total circumference shaped as a circle, square, triangle (acute or right-angle). It is fed to the coax anywhere on the bottom wire of the loop that is convenient. Being so small and so close to ground, the near omni pattern is just about the same - so use what is convenient to attach to in the cramped space. But don't attach to the top of the loop.

I had plenty of zip-cord available, so I tested (and modeled), and like it predicted, horizontal loops, even small ones take a major hit in gain being so close to the ground. So small vertical loops in all these shapes was tested and found to be superior in all cases.

The 16 foot total circumference is not that critical. If you have the space a few feet more or less is not a big deal. And since we're not transmitting into it, simple stand-offs from roof studs can be used - including just tacking them directly if you don't have stand-offs - but if you can stand them off, do so.

CHOKES
Absolutely necessary. My attic is like most others with power, cable/sat tv, doorbell, heating and air-conditioning units and ductings etc etc.

But what was once first a totally unusable antenna, was quieted by using TWO chokes just like I do with my outdoor antennas. One choke off a small jumper at the back of the receiver, and another one up near the loop feedpoint in the attic.

Example: A quick example would be the use of the common MFJ 915 inline coax choke. This would go near the back of the receiver. Up at the loop in the attic, a similar version, the MFJ 918 which has two wires coming out at one end makes it ideal for use up there.

Plug - You might just want to bring out the big guns right from the start. What I use are Myantennas CMC-0510-R chokes instead. Big improvement over the MFJ, especially if AM BCB and 160m are important. Most amateur radio chokes have weak choking impedance at 160 meters, and don't go down to 500 khz like the Myantennas cmc-0510-r do. To make the transition to the two wires of the loop easier, I adapt to a BNC red/black double-banana-jack, which are commonly used for multimeter connections.

Whatever you do, do NOT use any sort of "voltage balun" up near the loop to make the transition to coax, UNLESS you follow up with a choke. If you only have one choke available, then you can use any sort of "current balun" to help keep that coax feedline quiet. Absolute impedance matching is not critical - I use a 1:1, but if you have a 4:1 or even 9:1 available, try that too up top.

This antenna like my outdoor versions was inspired by KK5JY's work in this area:
Small Loop Antenna for HF Reception

But I can't emphasize this enough - without choking you are fighting a losing game not knowing if your loop is picking up noise, or if your coax braid is the culprit. In my case, I'd say 99% of the noise was my unchoked coax.

If you have an amplified loop in the attic, I'd be sure to also use a choke at the receiver end, to help make sure you aren't rotating the loop to try and null out your coax noise! See what I mean?

Operations:
Now that my feedline is properly choked and out of the equation, low and behold, the attic loop performs as it should. Bands sound normal, my S-meter isn't pegged, but YES, I am in a new rf neighborhood. New little gremlins are heard on various frequencies. When my heater motor kicks in, a brief S9 spike on the meter jumps up and back down. So maybe I'll try to relocate it a little bit.

Signal gain - despite the superior gain as compared to the horizontal loop, I am still under a roof which isn't totally opaque to rf. But now that the majority of noise is controlled, I CAN use my preamp, and crank the rf-gain down just a little bit to make it just right. This is very similar to my on-ground log and dog antennas.

I guess the moral of the story is not to give up. Choke your coax - at BOTH ends - before you assume that the attic is unusable.

 

[nanZor]

Large horizontal loop revisited:

Now I understand why this worked for some in the past - talk about taking the long way around to fix a problem. I had to physically wire it up, even knowing beforehand to prove it to myself again:

The massive approx 50db loss with a large low to the ground horizontal loop certainly pushes down your noise floor as well as your signal. BUT, if the only thing you listened to was flamethrower SW stations, or high-powered amateur stations under overhead short skip conditions, yeah, I guess. Oh, and throw in roof covering attenuation too.

But what a waste of wire. And for the attic, placing your wires ever closer to other noisemakers or inductive field generators. In the past when all you had was perhaps some power and a doorbell, you could get away with it. Not realizing that "less is more" with a small vertical loop, I suppose stringing out hundreds of feet of wire close to the ground was attractive. And in some cases worked - but for the wrong reasons.

That's a really bad way to emulate pushing the 60db attenuator button at the receiver.

So again that's my advice - don't string up a large immoveable horizontal loop in the attic - even if you have the perfect ground or have perfect coax choking. It's a waste of wire and energy.

And of course the added danger of putting your foot through the ceiling hopping around up there, when a small vertical loop is higher performance and much more manageable.

 

[prcguy]

I have not measured, but I'm told by a loop mfr that the impedance of a small loop around 1m dia is not far off from 200 ohms across a very wide frequency range. I would use a good 4:1 balun for the feedpoint of the loop instead of just attaching coax. it should give a little more signal level.

With outdoor space available, long ago I gave up totally on attic-loop antennas, but I tested again to give hope to those with no other choice.

I'll *try* to get to the point quickly ... maybe...

The two key points are:
1) Use a small vertical loop rather than a horizontal loop, either plain wire or a nice amplified commercial unit. Horizontal loops are a waste of time (20db or more loss on low frequencies compared to a small vertical loop.) The small vertical loop is obviously much easier to rotate, or in my case slighty relocate based on empirical listening tests.

2) Much of the so-called attic noise is really just "whole house" noise being transferred via the common-mode of your coax. Off-the-shelf coax chokes do great to be able to make the loop sing, rather than listen to "two" antennas at the same time (your loop and the unchoked coax)

Details:

The small vertical loop itself is a simple wire loop about 16 feet in total circumference shaped as a circle, square, triangle (acute or right-angle). It is fed to the coax anywhere on the bottom wire of the loop that is convenient. Being so small and so close to ground, the near omni pattern is just about the same - so use what is convenient to attach to in the cramped space. But don't attach to the top of the loop.

I had plenty of zip-cord available, so I tested (and modeled), and like it predicted, horizontal loops, even small ones take a major hit in gain being so close to the ground. So small vertical loops in all these shapes was tested and found to be superior in all cases.

The 16 foot total circumference is not that critical. If you have the space a few feet more or less is not a big deal. And since we're not transmitting into it, simple stand-offs from roof studs can be used - including just tacking them directly if you don't have stand-offs - but if you can stand them off, do so.

CHOKES
Absolutely necessary. My attic is like most others with power, cable/sat tv, doorbell, heating and air-conditioning units and ductings etc etc.

But what was once first a totally unusable antenna, was quieted by using TWO chokes just like I do with my outdoor antennas. One choke off a small jumper at the back of the receiver, and another one up near the loop feedpoint in the attic.

Example: A quick example would be the use of the common MFJ 915 inline coax choke. This would go near the back of the receiver. Up at the loop in the attic, a similar version, the MFJ 918 which has two wires coming out at one end makes it ideal for use up there.

Plug - You might just want to bring out the big guns right from the start. What I use are Myantennas CMC-0510-R chokes instead. Big improvement over the MFJ, especially if AM BCB and 160m are important. Most amateur radio chokes have weak choking impedance at 160 meters, and don't go down to 500 khz like the Myantennas cmc-0510-r do. To make the transition to the two wires of the loop easier, I adapt to a BNC red/black double-banana-jack, which are commonly used for multimeter connections.

Whatever you do, do NOT use any sort of "voltage balun" up near the loop to make the transition to coax, UNLESS you follow up with a choke. If you only have one choke available, then you can use any sort of "current balun" to help keep that coax feedline quiet. Absolute impedance matching is not critical - I use a 1:1, but if you have a 4:1 or even 9:1 available, try that too up top.

This antenna like my outdoor versions was inspired by KK5JY's work in this area:
Small Loop Antenna for HF Reception

But I can't emphasize this enough - without choking you are fighting a losing game not knowing if your loop is picking up noise, or if your coax braid is the culprit. In my case, I'd say 99% of the noise was my unchoked coax.

If you have an amplified loop in the attic, I'd be sure to also use a choke at the receiver end, to help make sure you aren't rotating the loop to try and null out your coax noise! See what I mean?

Operations:
Now that my feedline is properly choked and out of the equation, low and behold, the attic loop performs as it should. Bands sound normal, my S-meter isn't pegged, but YES, I am in a new rf neighborhood. New little gremlins are heard on various frequencies. When my heater motor kicks in, a brief S9 spike on the meter jumps up and back down. So maybe I'll try to relocate it a little bit.

Signal gain - despite the superior gain as compared to the horizontal loop, I am still under a roof which isn't totally opaque to rf. But now that the majority of noise is controlled, I CAN use my preamp, and crank the rf-gain down just a little bit to make it just right. This is very similar to my on-ground log and dog antennas.

I guess the moral of the story is not to give up. Choke your coax - at BOTH ends - before you assume that the attic is unusable.

 

[ka3jjz]

I take some issue with your notes about horizontal loops. The Carpet Loop is nothing more than a horizontal loop that is tuned by a L network. I had this in my attic on the 4th floor of my condo and it almost consistently did better than any of the rat trap antennas I had constructed. There would likely be common mode issues at some point down the line, surely. I used it on a TenTec RX320D, a Drake R7A and a Yaesu FRG7. Better than a good outdoor antenna? Hardly. But I heard a lot of DX with that antenna. Here's the website for it...

The Carpet Loop -- antenna special on hard-core-dx.com

Regardless, the loops you are working with indoors, along with this, are likely to be miles ahead of some random piece of wire strung around the room. Indoor antennas are always a compromise, no matter what design you are using.

I made mine roughly 10 foot square. I could have gone larger, but I figured that this is around the size of a good sized bedroom in an apartment or condo. Not like what you are using, true, but heck - it works.

 

[ka3jjz]

A few random comments on the Carpet Loop. Every installation has its own issues, of course, but in my case, noise from all the wiring in the attic started to be an issue around 3 Mhz and down. It was still usable at 3 Mhz, but I ran the audio (in the RX320D's case) into DSP software called SR5 (don't know what happened to it...) to clean it up a little. Below that, no matter how much fiddling I tried (even tried using an old MFJ active audio filter), the noise was just too much of a hassle. Worked fairly well at 60 meters (4.7-5.1 Mhz), managed to hear a few Africans and a couple of Central Americans on 49 meters (6 Mhz) that were almost not there on other antennas. Remember we're looking at signal quality, not strength. I scrounged an old capacitor from a Grove Tun-2 - in fact, I just jumpered it into the circuit just to see if it would work. It did.

This antenna is tuned; the antennas that Hertz is experimenting with are untuned. You can't really compare the frequency response between the 2. However (and this is brought out in the article) the tuning MAY help if you are using this with a portable (PL-880, PL-660, etc.) and you have 1 or 2 MW powerhouse stations nearby. If you have the parts, sure beats buying a filter. Note, however, that this won't help if those powerhouse signals are coupling into things like the cord from the wall wart, or worse, just bleeding thru the case.

It's not a miracle worker by any means, but it is a good learning experience. You get to learn something about L networks and a little about loops, too. And if it works for you, you now have another tool in your toolkit to play with. To me, that's a win-win all around.

 

[nanZor]

Prcguy - nah, no need to get fancy with impedance matching with such a small loop for rx only. No need for big tubing either.

Whether 1:1, 4:1, or even a 9:1 balun ratio, the most important issue is common-mode control. That means that voltage baluns, which have no intrinsic common mode attenuation, are either out, or followed immediately by a good resistive ferrite choke. So if you have any ratio of *current* balun around, feel free to use it. My preference is to use just mostly ferrites chokes.

What I mean by all this is that for this project, there is AMPLE signal strength provided by the loop, and fine-tuning the impedance ratio on this project was undetectable in use. I pulled them all out of the bag - LDG's, MFJ's, W2AU (with choke following), Radiowavz 9:1 isolated, (choke following), etc. Rx only - no difference really. In fact, in many cases I'm *still* turning down the rf-gain even when the S-meter shows nothing at times.

KA3JJZ - no disprespect to the horizontal carpet loop. For first floor dwellers, that is *almost* like the loop-on-ground sorta. But from your descripton about having noise at 3 mhz or down in a 4th floor condo indicates to me that you had common-mode problems. So your loop (if unchoked) was most likely part vertical with the coax run up into the attic, and not to mention that 4th floor up is a lot better than the typical single-story structure.

I think had you had a good ferrite choke either at the loop feedpoint, the receiver, or both, you may have not had the < 3 mhz issue - unless of course your loop itself was coupling.

I know that I will not put up with anything but normal band noise on AM and 160 meters from now on - since I know it can be tamed with proper ferrite choking in my case. The MFJ 915 / 918's were able to bring it back to normal, but the Myantennas cmc-0510-r put the icing on the cake.

But yeah, this is all empirical indoors - you do what you gotta do. Me, I'd only try the carpet loop on the 1st floor only, and choked as well. But maybe being 32 feet up on the 4th floor - yeah ok.

 

[ka3jjz]

I quite agree about the common noise issue. I should have tried at least 1 choke on the feedpoint (and 1 on each of the receivers) to see whether that would have helped. Heck, I was happy enough with the performance. Indoor antennas are always a tough way to go and anything is better than just a hack of wire.

 

[nanZor]

A loop in ANY form or orientation is THE way to go indoors!

But both of you guys bring up a great point that needs a bit of clarification - we're crossing the lines between categories of antennas.

This loop is very small compared to wavelength, unlike most of my larger amateur antennas. At 16 feet max circumference, it is only about 1/4 wavelength on 20m, 1/8 wl on 40m, and so on down the line. So for the most part it could be classified as a "small" loop, as opposed to the normal full wavelength or even half-wave loops.

And since we aren't transmitting, efficiency isn't the goal, but S/N. That means that strict impedance matching, or resonance tuning isn't really necessary. Receiver gain covers up a LOT of sins that would be a problem when transmitting into a small loop, or tuning a much larger antenna. The difference between a 1:1 and a 4:1 current balun at the feedpoint may make a nearly undetectable difference to the volume control.

To prevent it from being a larger antenna that benefits from tuning on receive, one needs to make sure that the SMALL loop is sufficiently choked so that it doesn't electrically appear as a random wire with a tiny capacity hat.

Wearing my amateur hat, I was kind of blown away since I had made many many antennas that appeared to be something cool, but were just common-mode random wires in disguise.

Hence my admonition that if one is going to try one of these small loop things - don't sling one up without doing proper choking of the feedline. Without adequate choking, it can quickly turn into something else - an end loaded random wire.

But yes, at some point transmission line loss will become an issue. Fortunately, the small loop doesn't need to be high off the ground, and I'm unlikely to put it more than 50 feet away, so the feedline loss is manageable too.

I'm glad you guys brought these issues up. I couldn't believe these small loops for rx only are so hands off - provided you actually have one by ensuring you are choked. I don't need to worry about things that would be a project killer if it was intended for transmit like small coupling loops, huge tubing, capacitor tuning and so forth...

 

[nanZor]

Another note about choking ...

In comparison to KK5JY 's project site, where he only needs to use an isolated transformer at the feedpoint, (great idea if you can incorporate it), my own situation needed much more.

Specifically, despite using an isolated transformer myself, AM BCB and 160m were still an unusable S9+ buzzsaw. Only when I put a ferrite choke inline just after the receiver on a jumper, did those bands come back to life.

So use what you have. If chokes make no difference to you, then congrats. But just beware that just because you hear no noise commonly associated with common-mode problems, it may be hiding the fact that you have a non-optimal pattern!

Unless you can tell, you may just want to CYA it with a choke at the receiver and feedpoint anyway. Unless you damage them, they are a lifetime purchase with not much to go wrong.

For me, having AM BCB and 160 be unusable without them actually helped identify an issue with a non-optimal pattern on the higher bands where I couldn't hear any apparent noise problem.

And there's the catch for SWL'ers and qrp amateurs. Unlike a high powered amateur station op that gets bitten by RF on the mic or key, swl'ers and qrp'ers might not notice. Oh sure, you may hear something on the bands, but without any telltale noise, you may not be aware of the pattern issue caused by common mode right away.

This is what prompted me to apologize to iMonitor for not showing a demonstrated need for a choke, when I realized my own glass house wasn't clean.

I hate to say it, but it's cheap insurance to help take the coax feedline out of the picture so you can be relatively sure that noise (or pattern) issues are actually antenna related.