The Umbrella Loop

The 5 foot umbrella loop in its open "deployed" configuration.

The umbrealla loop folded for transport.

Detail of the folding mechanism of the umbrella loop.

Introduction

Those of you who have read my previous articles know how I have "learned by doing". I started over 30 years ago salvaging loops out of old radios. About 20 years ago I used a simple formula based on the area and number of turns, and produced a 5 foot planar loop that was good for transatlantic listening - but included no tuning capacitor (it was internal to the radio). I then shelved the project for 20 years.

I still had my 5 foot planar loop, which was soon adapted to a 4 foot edge wound model with a tuning capacitor. Unfortunately, the wood is actually getting rotted, so it sits in my father's storage shed 300 miles away. It has such a low Q on the high end of the band that I considered the performance unsatisfactory.

If you don't mind, I will digress a bit before starting this construction article. I have learned a lot of things in the process of building loops. Among other things, I learned that:

If you have read my other articles, you know I am driven to make a practical - and portable - loop. I have a scenario of the "swimming pool", where I want music from a distant station with a minimum of hassle. The loop is impractical if it is large, heavy, delicate, or takes over 30 seconds to deploy. It is also impractical if it is hard to transport or store in its collapsed form. I wanted to make a really large loop practical to take and use almost anywhere. The folding loop was impractical because it folded to a flat shape that was too long to fit in a car trunk. The three foot ribbon loop was impractical because it was too delicate and took too long to deploy.

The Umbrella Loop Concept

I saw the answer in an everyday object that just about everybody owns - an umbrella. Like my "ideal" loop, it needs to have a large deployed shape, yet needs to be lightweight enough to carry, and needs to fold to a compact form that is easy to store. It needs to deploy quickly. It has many characteristics in common with what I want for loop! But - could I make a loop antenna based on the umbrella design? I cannot use umbrella hardware directly, because it is metal. But - I found a way to use wood instead of metal for the supports, and a minimum of metal used near the center of the mechanism.

Like the 3 foot ribbon cable loop, this design makes use of ribbon cable. This is almost a necessity, because separate turns would get tangled with this design.

Materials

  1. Three 8 foot lengths of 2 by 2 lumber. This is usually the smallest size available at the lumber yard. Some warpage is almost inevitable, but does not affect the construction of the loop. Water sealed lumber will make your loop heavier. I suggest painting the finished pieces as an alternative to water sealed lumber.
  2. 2 wooden yardsticks. I found these in the checkout lane of Home Depot for 69 cents.
  3. 4 gate hinges with screws. Often times, gate hinges are not provided with wood screws (see next item).
  4. 24 1 inch by 1/4 flat head wood screws.
  5. 9 1 inch furniture hinges.
  6. 12 4*40 screws, nuts and washers.
  7. 12 sheetrock screws.
  8. one 2 inch by 2 inch by 1 inch slide assembly (see text).
  9. 20 feet of 25 conductor ribbon cable. 25 conductor is the way it is commonly sold.
  10. a 9.6 to 365 pF tuning capacitor.
  11. a SPDT switch.
  12. hookup wire.
  13. eight plastic push pins (used during construction).

Construction

This is a complex construction project (but not too bad). You need to be careful to do each step carefully. There will be some degree of customization and judgement involved - I will try to explain the rationale when you need to do this. I will also break this up into sub-assemblies to make it easier to follow.

Constructing the Slide Assembly

The slide assembly is one of the most demanding aspects of this design. I spent many hours thinking about how to design this part of the umbrella loop. If you unfold an umbrella, the slide assembly is the thing that slides along the center support of the umbrella. I discovered that it needs to be quite strong - as it takes a lot of abuse. This made the selection of material critical. I searched for something commonly available, strong enough, and a material that can be worked equally. I finally settled on crate strapping metal. There are at least three types in use. One type is not metal at all, but plastic of some sort. Plastic is completely unsuitable - it is not strong enough. There is also a very flimsy metal type of strapping metal, as opposed to a thicker metal strapping. It is the thicker metal strapping that you are looking for. It is about 10 mils thick. It should be easy to beg some from somewhere - as it is usually discarded anyway - and people are anxious to get rid of it because it can cause injuries. I got mine from a computer room retrofit, where it was used to support 6 foot tall racks of equipment. I suspect that refrigerator and washing machines are also shipped with the same type of strapping metal, so stores that sell them may be a good source.

WARNING Hold the strap metal tightly or secure in a vice for the next step. Don't let it get away from you, it can cut you! If you slice your finger, don't come crying to me about it with some slimeball lawyer. I warned you!

You now should have a nice little square metal frame with hinges on it. Next, we will take this assembly and add the "spokes" to it - similar to the spokes that attach to the sliding part of an umbreall. Only we are going to use yardsticks.

Constructing the Spoke Assembly

Constructing the Frame

Mating the Slide Assembly and Frame Assembly

This step involves completing the basic "umbrella" mechanism. It will involve a bit of trial and error to get it right, so you are enouraged to play with the mechanism as much as you can to verify that it will work. Now is the time to fix any problems - not when the fragile loop winding is on the frame and can be damaged.

A note on "notching" - it is OK to notch the frame slightly to accomodate spokes and their screw heads, although I did not find it necessary. The spokes and screw heads just barely touched the frame in the open position when I built my loop.

Constructing the Loop

This series of steps involves making the actual loop. The simplified schematic is shown above - the actual loop has 6 turns, but that would be difficult to show so I simplified it. The schematic reflects the fact that I needed to switch one turn in and out to cover the whole AM band. I found a mice little plastic box in which to house my circuit - I suggest you do the same. Even something like those fake "chewing gum pagers" have nice plastic boxes in which you can mount the switch and a small tuning capacitor. Attach to one of the supports with a couple of small wood screws.

Hanging the Loop on the Frame

This section also involves a bit of "trial and error". It is important that the open (deployed) frame not stretch or otherwise put stress on the delicate loop. Tolerances during assembly make it impossible for me to give you a really good measurement here, but you can get close. If you make a mistake and are an inch off on one support of the frame, the loop will not be a perfect square - but that will not affect it to any degree. Also - the loop must remain a little bit slack. This will mean that the loop will not produce as good a null as some other loop designs, but the large size of this loop makes it optimum for reception of very distant stations - not nulling.

Attaching the Leg

This last step attaching the leg that will allow the loop to stand on its own, without leaning against anything. Be very careful not to damage the loop you just hung in the last section as you work!

This (finally) completes assembly (WHEW!)

Deploying the Antenna

If you still have the antenna face-down on the work area and haven't figured out what is going on just yet - lift it up and support it on its side by the leg. The figure below is a pretty close (though admittedly not perfect) representation of what is going on. The leg is the right length to allow the loop to rest slightly tilted against it - but in a wind it will blow over. The leg also serves to hold the slide assembly in place when deployed. When the leg is folded up, in line with the loop's center support, it allows the slide mechanism to slide freely, collapsing the loop. It is normal for the loop to be slightly out of "square", due to loose tolerances in the gate hinges. This won't hurt anything.

Once the frame is up, you can then take the loop and hang it on the four legs (you might have to slip it under the support leg). Slide it in the notches and if you put popsicle stick pieces to hold the loop in the slot slide them across the slot. The function of the loop is NOT to support the top two support legs at 45 degrees!!! Just let them stay where they are and hang the loop. The loop wire is not strong enough to pull them up so the loop looks like it is a perfect orthogonal structure.

Deployment notes ----

Well, there you have it. The umbrella loop won't keep you dry, but it is an elegant way of having a large loop that stores and transports easily.

Test Results

When I took the thing out to my remote test site at my father's house, there was a week of thunderstorms. As any AM DX'er will tell you - that is the END of your DX'ing! Nevertheless, I have found a good test station in the Dallas / Ft. Worth area. It is KKYX 680 kHz, a 50 kW (daytime) station from San Antonio. The format is classic country - so it is nothing I would be interested in receiving - but it is a good test candidate. Even on a GE Superadio 3, the signal is barely listenable - a 2 by the "1 to 7" criteria I set up in the 4 foot loop antenna article. The transmitter is located at 29 30' 3" N 98 49' 54" W, making it about 274 miles from my listening location in the DFW area. Reception of this station improved to a 5 to almost 6 using both the GE Superadio 3 and the Optimus 12-603. As expected, the loop did not produce deep nulls, although reception of the 550 kHz station from San Antonio improved markedly when the loop was tuned to 550 - even with the presence of a strong 540 kHz signal in the area.

I found that the loop would tune from 640 kHz to 1700 kHz on one switch position, and about 540 khz to 1500 kHz in the other - full coverage of the band.

I have tested this loop at my remote test site. Results were similar to those obtained with the 4 foot loop on the lower part of the band, but vastly superior on the upper part of the band. I can sum it up in one word: HOT!!! Also very high Q - I spent a couple of hours DX'ing every 50 kW station within a thousand miles of Lubbock. Reception varied from barely readable on WSB Atlanta to loud and clear on KOA Denver. Daytime DX of this nature is characterized by very deep, very slow fades. A station barely readable one hour may be booming in an hour later - so reception tests are not included here due to this variability that I suspect weighted earlier test results. At some time in the future, I may assemble a ten foot loop out there and attempt to characterize distant (more than 1000 mile) daytime DX. I suspect that stations much further may also be receivable, if you can catch them between deep fades. One of the surprises of the day was reception of an extended band station in the 1640 to 1660 region from due North of Lubbock. 45 minutes of listening yielded no station ID, but reception was between 6 and 7 by my original criteria. Without the loop, there was not a trace of the station. Perhaps I will try to figure out where it was coming from at some point in the future. But one thing this pointed out - sensitivity of the large loop was greater on the high end of the band. MUCH greater. From zero reception to loud and clear. If the sensitivity of a large loop is indeed proportional to its aperature (as we all have basically known), then the aperature of the loop is much greater to stations on the high end of the band. This has profound implications for shortwave listeners - and to those of us who want the 50 kW monsters on the low end of the band. Or even the longwave DX'ers: It is much more important to get a large aperature on the low end of the band than the high. And even a very small loop may be a good performer for shortwave.