Helical STL Scratchpad
A place to store/share construction data. Helical loaded magnetic loop, or “STL” (small transmitting loop) antenna designed by Rich K8NDS http://www.hlmagneticloopantennas.com/
The blue (magenta) circle dimension is 39″ diameter. The circle is the mean center-line of the 2″ tubing around the octagon. The 40.44″ dimension is the PVC elbow centerline. to centerline distance.
Using PVC supplies from Lowes hardware store- the 45deg bend has 0.5″ insertion allowance on each end, and pipe end-to-end gap is about 2″ fully seated. That gives me a pipe length of 12.7″. I’ll see how close that estimate is and report back.
Later that day…..
I made the frame, all glued up and dry. It came out within half and inch of the drawing, and plus/minus half inch symmetrical. I’ll make up the difference when wrapping the copper.
Tomorrow I’ll bring it inside and wrap the copper. I started gluing the PVC in the basement, but had to move to the freezing garage as the solvent fumes were building up quick in the house.
Here’s some pics of the connector end of the gamma match. Brass tubing soldered to the N connector, heatshrink covered, and clearance hole in the copper. Copper is .028″ thick.
Soldering the connector base copper(.028″) to the copper wrap (.010″) Not the prettiest, but it’s the contact area underneath that counts.
March 6 2015:
The vacuum variable cap arrived from Russia. Here’s the start of the cap mounting plate. I hijacked a kitchen cutting board and cut it to 16″ by 5.5″. Lowes hardware supplied everything else. Plastic electrical conduit clamps to mount the board on the pvc antenna, and steel 2″ pipe hangers to hold the cap. (part# 302045) I don’t know how they measure 2 inches, but it’s a near perfect fit on the 60mm vvc cap ends. The copper foil ribbon will be folded and wrapped between the cap and clamp. No extra soldered connections.
March 7 2015
The motor drive has reached a stall because I can’t find a suitable shaft coupler that adapts 12mm to .250″.
In the mean time I couldn’t wait to try this out. I started to manually tune the cap for highest signal level at 14.250 MHz, then I played with the gamma match for the lowest SWR, and back to cap adjust- – – -back and forth. At ground level I got 1.4:1 and heard stations a couple S units below my dipole. Can’t wait to get this thing outside and motorized!!.
March 9, 2015
Still motor-less. I spent the first sunny above freezing afternoon trying to tune the Gamma Match feed. I tuned the cap to best swr at 14.2 mhz. Like others said, the match is different on the ground vs 6 feet high. Many times up and down on the tripod, tune and test….. the best I could get is 2.3:1 at any 20 meter frequency. But the great news is it’s easily one S unit above my 20 meter wire dipole. Once I get things locked down below 2:1 I think it will just get better.
Finally locked down and soldered the Gamma match. I can get 1.5:1 or better SWR from 40 to 15 meters. I bought a RigExpert AA170 S11 analyzer and that really helped the process. Below are some screen captures from the Rig Expert AntScope app.
March 27: The Mini-6
I wanted to try a mini loop for portable and marine 6 meters. Here are a couple of 1st version prototypes. I first made a solid core loop, just shooting in the dark for size, feed, and cap value. Ended up with a 10″ diameter of #6 wire, and a Johanson 22pF cap. It’s a low voltage cap for receive only, but I did get 2 watts into it to test SWR and pattern without it popping.
I then wound a helical loop on 3/8″ PEX tubing using copper tape of .500 by 1.5 mil dimension. Still 10″ diameter, but now with 50 inches of copper tape (25 per side).
Both of these loops are tune from 40 to 90 MHz easily with the 22pf cap. The usable bandwidth of the helical wound loop is about 40% better being about 100KHz wide at under 2:1 swr !
Field testing in receive mode will be nex week out in the cornfields. I’ll compare it to a tripod mounted Buddypole dipole setup with the help of another portable operator.
My next goal is to get some 3/4″ plastic tube,and 5 mil copper to wind a new one, and search out and find either a 1KV standard variable or a small VVC and run it remotely. I only plan to run 25 watts when this is done.
Keep experimenting and 73