Article by N2DS Dave
Welcome of my homemade loop antenna or loop aerial pages. All the loops described here were designed and built by myself. The features are not real different from loop to loop. These loops are made to enhance radio reception in the broadcast, or medium wave (MW) band range. Some of the loops are just one coil, some have tuning capacitors and some have an extra coupling coil. Read on and discover the fantasy of loop antennas. Or fetish in my case.
A Word About Loop Antenna Design
All these loops involve a resonant tuned circuit made up by a multi-turn loop coil and a variable capacitor. If you are using one of my designs shown on these pages, you are all set to go ahead. But if you are designing your own loop antenna, please read on.
As just mentioned, a resonant circuit is made with the coil and variable capacitor. The old standard configuration to tune the medium wave band is a 240µH coil and a 365pF variable capacitor. These numbers are fine when using a small diameter coil, but are grossly inaccurate with a loop antenna coil.
This inaccuracy is due to a relatively large amount of self capacitance (also called distributed capacitance) built in to large diameter wire coils. Increasing the wire spacing doesn’t do a lot to reduce this extra capacitance.
The tuning range is determined by the ratio of minimum capacitance to the maximum capacitance. As an example, the ratio of a 365-14pF variable capacitor is 26. But if you have 20pF of self capacitance of the coil, then the effective value of the capacitor is 385-34pF or a ratio of just over 11. (This difference isn’t as bad as it sounds as all coils have self capacitance and is factored in to the tuning range outcome.)
To use the “standard” 240µH coil value in a loop would cause the tuning range to be much less with a good portion of the top frequencies not being tuned. The loop coil is built using somewhere around 160µH in value, which allows for the top of the band coverage.
But with the smaller maximum to minimum capacitance ratio, a 365pF variable capacitor and the 160µH coil won’t tune to the bottom of the MW band.
There are a couple of ways to tune the whole band. One is to add a fixed capacitor and a switch which places an additional 200pF or so capacitance in parallel with the existing variable capacitor. Another fix is to use a higher value variable capacitor which will restore the higher ratio maximum to minimum capacitance values.
Knowing this information saves the new builder from an unpleasant surprise and the necessity for a redesign. Now on to my loop projects …
Loop For My Old Sparton Radio
Here it is friends. I made a loop specifically for one radio. This my loop #6. I was using my “test” loop (which is shown below) to see how my Sparton radio would perform. I was impressed and since I use that my Sparton a lot, it deserved it’s own loop.
I decided that this antenna had to be lightened up a little. The two Garolite® squares had to go. I cut a notch in both pieces of wood so that they would interlock. The pieces are glued and fastened with a single flat head wood screw.
The wire is wound with some litz wire. Litz isn’t really necessary but the wire is more flexible than the magnet wire. This makes a nice looking wiring job when finished. You will need 70 feet of wire for the tuning coil and about 14 feet for the outer coil. I thread the wire for my loops outside where there is room to pull the wire. The drilling and other details are contained in the drawing shown below.
I used litz wire between the loop terminals and the base box. Earlier I had used 18 gauge PVC covered wire. This was too stiff and the loop would not stay in the position it was turned to. Just one of those little learning things.
A base box isn’t necessary. All the parts could be mounted on the loop itself, or a small board could hold the loop with a small panel for the other parts. But I like those basswood boxes that I have.
The Sparton radio is a multi band set and the shortwave bands require a different antenna. I added a switch and some terminals to accommodate this need. The radio antenna is switched between the loop coupling coil, or an external antenna and ground. The other switch is a high low band switch and adds a 300 pf capacitor across the 365 pf variable capacitor. A larger value of variable capacitor would tune the whole band in one chunk.
This loop is not real expensive to build, but they require a lot of building time. Use your imagination and you will be as proud of your loop as I am of mine.
R & D Loop
This is my 4th loop. Sorry for leaving #2 and #3 off the site, but those were so much like my first that I decided I would wait for something worthwhile to offer. This is worthwhile. I wanted a loop with lots of bells and whistles to do my radio experiments. Look at what this loop has to offer.
This loop has two windings. The larger winding (15 turns) is connected to a variable capacitor for a large tuned circuit. Since a single 365 pf capacitor will not tune the entire broadcast band, more capacitance has to be added. In the case of my dad’s loop (shown below), he switched a 300 pf fixed capacitor across the 365 to allow tuning down to 540 khz. I am using a ganged capacitor and a switch. Included is a third position of having no capacitor across the coil. I did this as some of my radios tune the antenna loop by a ganged capacitor inside the radio.
The other winding (2 turns) is to connect to the antenna input of the old BC radio. I picked two turns as that emulates the single turn that my dad had on his larger loop.
This loop has 3 wood pieces. The arms are 12-1/2 inches long, and the main pole (vertical part) is 30 inches tall. This is a little larger than my earlier loops so that I would have room for that extra winding. After the holes are drilled, I stained the oak with Minwax Red Mahogany stain. After the gloss deft dried, assembled the loop using two 3×3 inch square pieces of Garolite®. Thin wood, plastic or anything you have around will work. Before you mount everything, fit it together and take a look to see if it looks right.
Another change from my first loop is how the ends of the wires are handled. So that the wire wouldn’t work loose, I drilled an extra hole close to each end hole on my loop. This makes 2 extra holes I had to drill. What the heck, holes are cheap so I splurged. The extra holes are about 3/16 inch from the end holes. The wire can be then looped around a couple of times. This holds the ends secure and looks neat too.
On the top of the mast pole and each side arm, I drilled 17 holes. I used a 3/32 inch drill but it isn’t too important the size of the hole. Each hole is 3/8 inch from each other and I started 3/8 inch from each end.
Then measuring down from the top of the pole 25 inches, I drilled a series of 21 holes going back up the pole, 15 holes, one for each turn, one for the end winding. Then two extra holes close spaced for ending the tuned winding. . When the wire is wound on this loop, it will look like a square.
Three more holes are drilled for the untuned winding. I looped the both wires once through the middle hole as with this winding, having the ends so close wouldn’t matter. I think that comes out to 21. Lets see…. 15+1+2 +3 = 21. Good thing I stayed awake in the first grade!
You with me so far? Good. After I wound the wire, I made a little 4 terminal connection block from another piece of Garolite®. I also drilled a 1/4 inch hole in the bottom of the pole and inserted a dowel rod in the hole. This is the weak point of the loop. If the loop falls over the dowel will break and need replacing.
Next I built the box that serves as a base for the loop and holds the capacitor. I used a cut down basswood box, a piece of 3/4 inch thick oak as a base and some 1/8 inch Garolite® as a panel. I took a 1x1x3 inch piece of wood and screwed it to the Garolite®. This is so the 1/4 inch dowel will be stable in the loop base.
Using that big thick 14 gauge wire, I make the connections to the capacitor and link switch. In my case, I used a 3 gang capacitor. That made it easy to use a single link switch to do what I wanted it to. You could make two spst link switches, one to add a fixed or second gang and the other to disconnect the capacitor from the circuit. I built mine the way I wanted, and you can build yours any way that is handy for you.
So I can use this loop several ways:
Connect a radio to the untuned 2 turn loop and tune the main coil with the capacitor.
Hook an antenna and ground to the two turns and a diode and earphone on the tuned side for a cool crystal set. Or if I was really close to the station, forget the antenna and ground.
Use the untuned winding as a tickler coil in a regenerative set.
Use only the large loop without the tuning to connect to a radio. This loop could be used to replace a built-in loop on a radio.
Connect the coupling loop between a wire antenna and a crystal set for a wave trap. Make sure the loop is turned to null the station you want to trap.
Maybe I could use this loop to strain my spaghetti. Anyway, below are some pictures of how I constructed the loop. If something is unclear or you have a better idea on something, please e-mail me. These loops are very cheap to build but take a long time to make. But they sure look cool sitting on top of your old radio. Best of luck with yours!
The “Harpo” BCB Loop Antenna
That is what this loop aerial reminded me of, Harpo Marx’s harp. I hope you don’t think I am harping on this loop thing, I just like to build them. This type is similar to the one my dad built.
In some respects, this antenna is easier to build than my other ones and in some it is harder. The arms are 27 inches (68.5 cm long and are made out of 3/4 x 5/8 inch (19×16 mm) oak. The crossover point is notched and one wood screw placed in the center. This insures that the legs continue straight and are at 90 degree angles from each other.
The mast is a 16 inch piece of the same size lumber as the arms. All these pieces of wood are held together by the two pieces of 4 (10 cm) inch square Garolite®. These pieces of Garolite® are 1/8 inch (3mm)thick. The front piece of Garolite® has ten screws and the back has 5. Make sure to offset the screw holes so the wood screws from the other side won’t hit. Another piece of 1/8 inch Garolite® 2 x 4-1/4 inch (5×11 cm)holds the capacitor and terminals.
The wire combs are made from 1/16 inch (1,6 mm)Garolite® and are 4 x 1-1/2 inches (10×3.8 cm). I cut 15 small notches on the edge of each comb. The notches are 1/4 inch (6 mm) apart. These combs fit in to slits cut in the end of the arms. I used a scroll saw and my little 1 inch belt sander for these operations. After I cut the 4 pieces, I taped them together so as to make the cuts at once. Two brass #6 fillister head screws and nuts hold the combs in place.
The outside dimensions are 20 inches (51 cm) per side. 105 feet (31.5 m)of wire will be just enough, including the leads to the capacitor and terminals. Wind the big coil first, just in case you run a little short.
I used 22 gauge magnet wire. This looks like a good size. 24 can be also used. There is one turn of wire wound around the center notches and that connects to the receiver. There are 14 turns of wire wound around the loop and a 365 pf capacitor is across this winding. The inductance measures 230 µH. This came out very close to what I was looking for. The entire broadcast band can be tuned without other capacitors being switched in.
I am impressed with how this loop works. The wood is beautiful too. The electrical characteristics are as good as my others and maybe it has better directional abilities.
Six Spoke BCB Loop Antenna
Here is a little twist on my loop designs. This is a 6 sided loop antenna. From what I can tell, there isn’t any difference between a square and a hex loop besides the shape. I like to build things that look nice around my house (or someone else’s house). I had help with this project. My friend Fred Wise from Glen Burnie, MD was kind enough to make the round center portion of this loop for me. The block of wood is 5-1/4 inches (13.4 cm) in diameter and has six 1/2 inch holes expertly drilled an inch deep. The 5 spokes are 12 inch (30 cm) long, 1/2 inch (12 mm) hardwood dowel rods. The 12 inches allowed me to use an entire 4 foot long dowel. The mast dowel is 22 inches (56 cm) long. A little shorter is fine also. I used what was left from a dowel I had.
I didn’t want to thread wire through lots of holes on this loop. That is a summer outside project. I decided to use some 5/8 inch (1.6 cm) long brass escutcheon pins The diameter of the pin is about 1/16 inch. 98 pins are required. 16 on each of the 5 arms and 18 (one for the end of each coil extra). I used a V block (also made by Fred) attached in my drill press to drill the pilot holes with a 1/16 inch drill bit. I set the depth gauge for half way through the wood.
Before I drilled, I stained and finished the dowels. I then took 6 strips of 3/4 inch wide masking tape and carefully stuck them to a piece of plastic. The measurements were made with an adjustable square. I marked in 1/4 inch (6 mm) for the first hole. I then marked 17 more holes on each piece of tape, in 3/8 inch increments. The mast will have all 18 holes while each arm will be missing the second hole from the outside. This allows for extra spacing of the pick up loop. I then marked a line down the center of each piece of masking tape. The picture shows 7 tapes as one was used for a test run.
The masking tape is then moved to each arm. This provides the drilling location for all the holes. One idea is to elevate the V groove block slightly to when the pins are nailed, they will angle slightly away from the center. This would keep the wire against the dowels. That will happen on my next loop.
After the holes are drilled, the pins can be nailed in, leaving a 1/4 inch or a little less showing. Then the spokes can be glued or otherwise fastened in the block. The dowels can’t be allowed to turn or all the wires might fall off the pins. That would be quite a mess!
The wires are now ready to be wound around the pins. About 90 feet of wire will be required for the 16 turns. 100 feet (30 m) will give you a better safety factor, just in case I goofed up the math. I started with the single outside turn. That takes two pins on the mast spoke and one pin on each of the others. Leave enough wire length to go to two binding post connectors. The other winding is wound with 15 turns, beginning and ending at the mast spoke. (See why the extra two pins were needed in that spoke?) The ends should be wound several times around the pin to hold the wire in place. On the 15 turn winding, I wound an extra turn around each pin on the mast spoke to further aid in holding the wires on the loop. Pulling the winding tight and pushing all the wires towards the dowel after each turn is recommended. This step is pretty easy. Check your work every couple of turns to make sure you didn’t jump a pin.
A 365 pf variable capacitor is connected across the 15 turn winding, while the single turn winding ends were terminated at some binding post type terminals using knurled nuts to fasten the wires going to the radio. A good support base should be made to keep the loop standing upright. Care should be taken to keep the loop from falling over as this could damage the mast.
The loop works pretty well and much as expected. There is one major difference I found between the “harpo” and the spiral wound loops. The harpo will tune 540 to 1700 where this loop tunes only down to about 650. If listening below these frequencies are important, you should add a switched 150 pf capacitor. Overall, this loop turned out very well. It sure looks nice.
The Penta-Loop Antenna
This loop is much like my Hex Loop shown above but has a different geometric shape. I can’t believe that one shape receives any better than another. This one could be used to hear military broadcasts from Washington, DC. 🙂
This loop has a separate mast that holds the loop up and where the tuning capacitor is located. The shape is based on my Dodge Caravan hood decoration.
Along with the neat center hubs my friend Fred Wise made for me, he also made a little jig to help me drill holes in dowels a little straighter than usual. The holes aren’t perfect but a vast improvement over similar operations I have done with round stock. The piece of wood was clamped on to my drill press table. I wanted the escutcheon pins to be on a slight angle so the wire rests near the wood dowels, so I put a small piece of wood under one end of the jig. It worked perfect.
The pickup wire is wound around the outside pins. There is extra separation between this winding and the tuned windings. This is to reduce the coupling between windings and allow for sharper tuning. There is no scientific basis for the amount of spacing I selected.
The rest of the windings have a 3/8 (9.5 mm) spacing per turn and there are 16 turns total. This gives me tuning coverage from over 1600 khz down to about 650. A small capacitor can be switched in parallel with the tuned circuit to lower the resonant frequency. A 500 pf variable capacitor would probably tune the whole range, if you have one of those laying around.
I hope you like this little loop and I encourage to build one of this style. You won’t be disappointed.
The Loop My Dad Built
This is the loop aerial my dad built in 1958. Although I was only 8 years old at the time, I knew what this was for and how it worked. The radio is a Fairbanks Morse that my dad bought new in 1936, when he was a teenager. This loop measures 33 inches (84cm) on a side and has 8 turns of wire connected to a 365 pf variable capacitor. There is a band spread capacitor and a switch with a fixed mica capacitor that is switched in to cover the low end of the band. There is a single turn that is connected to the antenna and ground of the radio.
This is the original red paint. I never knew why he painted this red. After thinking about it I realized that the trim on our house was red and this was leftover paint. This loop is still in very good condition and one of my prized possessions. I have always been fascinated with this device and enjoyed tuning the radio and turning the loop. I never built a loop when I was a kid, but I was very influenced by it.
2004 Loop Update
I was not sure I was going to do this change to the loop. After all my dad built it and for me to modify it, was not taken lightly. But I had some good ideas. I wanted to make this loop into a crystal set for the upcoming contest, but I still wanted it as a loop. Not a problem. I took off the original front panel, which will be put away and labeled. I then made the new crystal radio tuner, complete with a “Hobbydyne” type detector and a Bogen audio matching transformer. The only change I had to make that would show, is two holes to mount the transformer.
I used a vernier drive, a dual 365 pf Jackson Brothers variable capacitor. The dial is calibrated in khz. Now I have true one control tuning when using this as a conventional loop. No band spread capacitor, no switch to change to a lower frequency range either.
I think my dad would be proud of me.
Update – September 2008
The 50th anniversary of the construction was in early 1958. Since this loop also serves as my DX loop crystal set, I decided to replace the 50 year old windings. The wire insulation was getting a little bad. The wire was starting to have that gummy feeling. Some red 660 strand, 46 gauge wire recently came on the market. While this Chinese wire was not of real high quality, one thing it had was a very nice double silk covering. The problem with the litz wire that I have is the covering was not rough and tough and would like fray while on the loop, or even while winding it. But the covering on this other wire is good enough to stand up on the litz. I now have reasonably low loss wire on my dad’s loop antenna. I tested it by connecting the headphones and seeing what I could hear. The volume was impressive, compared to what I remember it being. I am planning on entering some of the winter DX contests using this loop.
The new wire didn’t make that much difference in the normal loop mode, it gave my loop crystal set a new breath of life. I have enjoyed improving one of my fathers favorite toys.
Here are the specs as measured on my HP Q Meter:
L = 160µH
Q 1.6 mHz = 140
Q 1.0 mHz = 250
Q 0.6 mHz = 290
Distributed Capacitance = 35.5pF
Length per side = 33 inches (84 cm)
Turns = 8
Distance between turns = ½ inch (12mm)
This article was originally available at http://makearadio.com