I recently acquired a small generator (1850 watt) that has a 12 volt battery charge output.  Since this output can supply 15 amps, I decided to build charger for my 7 amp hour cells that would provide a regulated charge voltage for the battery and also limit the current to a discharged cell to prevent damage to the cell.

This charger was the answer, the current is limited to 650 mA maximum and the float voltage is adjustable through the 13.8 to 14. 4 volt range. The 100 uA meter was salvaged from the junk box and allows me to monitor the charge current and to visually see when the battery is charged.

The charger also works well with my solar panels, I was able to get 400 mA to a partially discharged battery from two of the 5 watt Harbor Freight panels.

Lacking a housing, and due to the fact that I operate in the field and go camping,I decided to use the Pelican 1050 enclosure. When closed, the enclosure is waterproof and the leads (complete with PowerPole connectors) and meter are protected. I made a thin aluminum divider to make a place for the leads to be stowed inside the housing.

NOTE: A schematic of the circuit is available here.  Many thanks to N5IB - Jim who drew up the schematic for me after emailing him a hand drawing of the circuit ! 

Since I live in a small house without much  yard,  I seem to be  always experimenting with antenna's. Physically small, hopefully effective antenna's are a big plus for me. 

One day while researching antenna's, I came across an article about Magnetic Loop Antenna's, also known as  a "STL" (Small Transmitting Loop). I became interested, and immediately knew that I wanted to build one of these antenna's. I didn't know where I was going to obtain the high voltage butterfly capacitor, or vacuum variable capacitor, or several other items that would be needed for the project.

I had also learned that very high voltages  are developed across the gap at the top of the loop across the capacitor, and that alternately, very high RF currents flow through the loop and capacitor. This makes it necessary that both the capacitor and the loop conductor itself have very low resistive losses to maintain efficiency.

The "Loop" at the left was completed during early Jan, 2004 and features coverage from 10 meters to 20 meters, a homebrew motor driven  butterfly capacitor, and a homebrew "trombone" capacitor that is manually adjustable. The manually adjustable "trombone" capacitor provides additional capacitance to allow tuning to the 17 and 20 meter band. The antenna can be used on the stand, or  removed from the stand and hauled up a tree with a simple  line thrown over the tree limb.

While no hard performance data is available,  the loop seems to be fairly efficient, has very sharp tuning and is very quiet on receive. It is also very easy to transport while fully assembled.  The ability to remotely tune the loop to a very low VSWR means that even with a long length of feedline, losses due to mismatch are minimized.

 Many  thanks to the other hams that have ventured to build these antenna's before myself and that have provided the data, calculations, testing results, and tips for building this type of antenna, a few of my references are listed below. Sorry if I missed some, there are numerous references with a simple search on "Loop Antenna", "Magnetic Loop", or "Single Turn Loop"

Back views of the Copper Loop.

The  loop conductor is constructed of 1" copper tube, This is the thin wall, hard drawn stick copper. This loop is built such that it's self resonant frequency is above the 10 meter band. According to  my calculations, 30" square was about the correct size.

The photo show the copper caps used to close the gap at the top of the loop. 1/4"-20 brass bolts were used to both support the end gap and as electrical conductors to the capacitors. The bolt heads are silver soldered to the copper caps and protrude through the front side of the  white plastic insulator.

Notice the plastic cutting board used as an insulator. The cutting board was cut into two sections, the top section insulates the copper from the vertical plywood support. The handle in the cutting board is useful for carrying. Two holes were drilled to use to attach a rope bridle for hanging the loop. 

The remaining half of the plastic cutting board is used as an insulator and as a foot at the bottom of the loop.

Notice the copper 1" tee at the lower center of the loop. This was to be originally used for supporting the loop, but was not used for that purpose. (The bracket for the SO-239 feed connector is soldered to the front side of the tee.)

In the photo the two 1/4"-20 bolts that hold the loop to the stand are are visible. The wing nuts and bolts can be removed and the antenna removed from the stand for transport or for hanging the loop without the stand.

Since it seemed that obtaining a suitable tuning capacitor would be difficult, I decided to build my own. The capacitor plate are made from .025" thick aluminum sheet obtained fro the local hardware store. There are 7 sets of stator (stationary) plates and 6 rotor plates. I started with just a few sets of plates and found that I needed more capacitance  to tune from the 10 meters down to 15 meters. (More on that later....) 

The shape of the rotor plates is not optimal, but I designed them to be easy to cut out using ordinary had shears. A drawing/template for the plastic insulator plates, rotor, and stator plates can be download in PDF format here. (Click the capacitor in the image at left for a larger image)

The leads from the capacitor to the studs on the copper loop were made from outer shield braid from RG-8 coax. The leads are soldered directly to two of the brass spacers between the stator plates.

The insulating end plates for the capacitor are made from 1/4" thick clear acrylic, again, a cutting board was used for material. The stationary plates are supported 1/4" apart using electronics brass spacers, and lengths of 6-32 sections of threaded rod, nuts, and nylon locking nuts. Even stacks of brass washers could have been used here for the spacers.  The rotor plates are also supported by a section of 6-32 threaded rod that is also used as the shaft for the capacitor. The rotor plates are also 1/4" apart, but washers were used to shim these plates such that they are centered between the stator plates.

A pair of cone bearings were made from brass to support the shaft, see picture below. The bearings were glued into the plastic ends plates using cyanoacrylate (super glue).

Two 1/2" long sections of brass 1/4" hex stock were bored through the middle and tapped for 6-32, then tapered at the end.  These are used as the adjustable cones that allow for adjustment and smooth operation of the capacitor shaft.  A nylon locking nut is placed behind these "cone bearings" to lock the adjustment.

At the right, the small 40:1 gear reducer and servo motor can be seen. Below, the servo motor is shown, amounting bracket was fabricated of PC board material.  A shaft coupling had to be fabricated to make the connection between the capacitor shaft and the the gear reducer. Since an insulating coupling was needed, a section of silicone rubber tubing was used between the servo motor and the the shaft of the gear reducer. There is not much torque here, so this worked very nicely. 

The servo motor was leftover from my radio control flying days. The electronics and feedback pot were removed from the servo case.

Also, a plastic stop on the output gear of the servo had to be cut off, this allows the servo to act as a gear motor without stopping at a limited shaft rotation. This, and the fact that the capacitor itself has no end stops eliminates the need for any kind of limit switches.

The combination of the gearing in the servo motor and the 40:1 gearbox give me a final output speed of 2 RPM, this is just about the right tuning speed for the capacitor.

A small mini box with NO pushbutton switch, 2 AA battery holder, and DPDT reversing switch was fabricated. This is my "Tuning Box" for remote tuning of the loop. (PDF file Schematic Here).

Tune up is simple with this antenna, just tuning for the loudest noise in the receiver gets you very close. Then, transmitting into the antenna with a VSWR indicator in line allows for tuning to lowest VSWR in the  1.1:1 to 1:1.3  range. The matching loop tap position has some affect on the tuning, but mostly the position of the tap affects impedance matching and VSWR.

I used Anderson connectors to have the ability to used short or long extension leads between the tuning box and the loop. The Anderson connector and leads wound through a  ferrite bead to reduce noise can be seen at the left.

A FEW NOTES:

These antenna's are very tuning sensitive to people and objects in the proximity of the antenna. A loop could be built for operation on one frequency using only a coaxial stub capacitor. This greatly simplifies the antenna, but since these are very high Q antenna's the the useful bandwidth is very narrow. Tuning is very critical.

Even if a motor drive is not used with a variable capacitor for tuning, it is necessary to have a long shaft affixed to the capacitor shaft so that the antenna can be tuned without being to close and having stray capacitance affect the tuning. 

Originally, the plan was to use only the butterfly capacitor to tune from 10 all the way down to 20 meters. This didn't work out in practicality, the butterfly capacitor would have had to have many more plates to have enough capacitance for tuning the 17 and 20 meter band. This would have made it very sensitive and harder to adjust to resonance.

My decision was to build the sliding "Trombone Capacitor" shown in the pictures. This capacitor is about halfway engaged for course tuning to 17 meters, and almost fully engaged for 20 meters. The capacitor is manually set fully extended for 10-15 meters, or as described above for 17 or 20 meters, then the remote tuned butterfly capacitor is used for fine tuning at any point within the band.

The trombone capacitor is wired in parallel with the butterfly capacitor. Note that the flat braid leads for this capacitor are cut from the outer shield braid from RG-8 coax, and are soldered directly to the top end of the outer capacitor tubes.

The capacitor tubes are fabricated from 12 " sections of brass tubing available at many hobby stores. The outer tubes are 11/16" OD and the inner tubes are 1/2" OD. Inserted into the top ends of the inner tubes are machined Teflon spacers that center the tubes in the outer tubes to provide an insulating air gap. These bushings are cut for a sliding  fit.

 A Teflon cap was made for the lower end of the outer tubes having a hole that is a sliding fit for the inner tubes. This centers and guides the inner tubes at the lower end.  Brass plugs with a 10-32 tapped hole were soldered into the lower end of each inner tube, this is for attachment of the brass 1/2" by 1/2" wide shorting strap shown in the picture. At the center of the shorting strap, a brass 1/4-20 nut is soldered over a 5/16" hole, this serves to provide a thread for the leadscrew adjustment system described next.

 The manual adjustment system was fabricated from 1/4"-20 threaded rod, nylon lock nuts, and polycarbonate brackets cut, heated, and bent to shape. As the threaded rod is turned the lower half of the capacitor (inner tubes and shorting strap) move up and down, thus increasing or decreasing the capacitance. Note that the threaded rod only rotates in  and is not threaded into the plastic brackets. Nylon locking nuts are used at the top end of the threaded rod to prevent end play and to adjust clearance. This can be seen directly behind the servo motors in one of the photo's above.

Note that the 2 support brackets for the outer capacitor tubes are again made from 1/4" thick clear acrylic cut from ordinary kitchen cutting boards!

The matching loop was made from a section of RG-8X coax, the center and braid were tied together at both the center pin of the SO-239 connector and at the small adjustable brass strap.

The SO-239 connector is grounded and mounted to  the center of the bottom side of the copper loop with a small bent copper sheet bracket. The bracket protrudes through the white plastic insulator through the slot cut in the insulator as seen in the photo.

 The length of the matching loop (by the calculations) is 1/5 the length of the loop. The loop is 30 inches square, making the length (4*30) or 120". One fifth of 120 inches is 24 inches or 2 feet long.

The small brass strap clamp features a wing nut to allow loosening and sliding the clamp for VSWR adjustment. I have found that on different bands the position of the clamp varies somewhat across about 6" of movement along the horizontal section at the bottom of the loop.

Also observed is that objects near the loop cause the adjustment of the strap to be  further away from center, and that the Q of the antenna is lower, so objects near the antenna do affect tuning and VSWR and the final position of the tap. 

All in all this was a challenging project, but has been well worth the time.  I learned a lot from the project, and it has provided me with a useful antenna for limited space that works well even only a few feet above the ground. I have used the antenna at up to 100 watts, with only one incident. (I had excess glue on one of the Teflon plugs/spacers on the inner tube of the  trombone capacitor. This absorbed moisture and caused an arc over and carbon path. The tube had to be disassembled and the Teflon part cleaned. I since have center punched the parts in place  instead of using any adhesive and solved the problem.)

I hope to have provided some useful information for anyone wishing to build a Magnetic Loop Antenna. Please be sure to visit some of the links mentioned earlier, the information and calculation software provided there by others is invaluable.

Above all, take care using these antenna's, very high voltages are developed on the capacitor plates and around the top of the loop, even at QRP power levels !

This was a BRARC club project from late 2004. At last count, there were several more than a dozen of these built, and some of the group have been having great fun checking in with W0CH on  the 4 States QRP Club PSK31 net on Wednesday evenings with them.

I included an old SWR meter complete with the sensing bridge into the housing, this allows me to easily set the audio drive level for max power output (about 2 1/2 watts), then back off a bit to keep from overdriving. Also implemented is a VOX circuit designed by Jim - N5IB, the VOX circuit keeps one from having to use the serial connector shown on the back of the transceiver for T/R switching.

This project gave me the excuse to put up a better 80 meter antenna, the final implementation was a 282 foot loop stapled around the eves of the house. The loop is fed with 300 ohm twin lead and tuned with the LDG electronics AT-100 Pro Autotuner and a 1:1 balun.

 Please excuse the hand written front panel label, having so much fun with it that I never got around to a pretty front panel!

Here are a few views  of a  miniature paddle that I built for QRP use. The paddle features "magnetic springs", and guitar picks for finger pieces. Not quite a Schurr-Profi, but it has worked well on many trips, and seems to have held up pretty well.

A few details about the construction:

The contacts came from an old relay and are soldered into dimples that were machined into ends of the adjustment screws. The lock screws for the contact adjustment (seen in the second photo)  have weed eater line sections in the end of them to provide a soft locking action to the sides of the contact adjustment screws. 

Materials of construction were brass, aluminum, and the top plate was a piece of 1/8" printed circuit board. The base is made from 1" thick PVC plate, and hollowed out to provide room for some lead shot weight. The "non-slip" pad glued to the bottom was courtesy of Marshall - N1FN at Morse Express, (it came with a paddle that I purchased from him).  I pretty much used whatever was available.

I used my small combination lathe/milling machine to do the machine work.

Someone correct me if I am wrong, I believe that the original Elsie was a product of the American QRP Club. Our local QRP'r Jim - N5IB came up with an improved version, and with a little help from some of the local club members kitted it with individualized front panels for many of us to build.

The meter reads Inductance, Capacitance, and Frequency with CW audible readout of values.