Cylinder Wound 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 (19x16 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 (5x11 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 (10x3.8 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.
Tuning & Connections.
Six Spoke BCB Loop Antenna
Hexagon Sprial 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.
Marking Masking Tape
Antenna Spoke Hub
Brad & Wire Detail
The Penta-Loop Antenna
Pentagon Sprial 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.