Well, I finally have had time to sit down and put together part three of the DIY Magnetic Loop Antenna, sorry it has taken so long!
This post will cover building and coupling the loop to your transceiver. After reading through posts one and two you should have a good idea of the parts you’ll use and the physical dimensions of the main loop.
Most magnetic loops have the capacitor at the top of the main loop and the gamma match or matching loop at the bottom, this arrangement avoids running the feed-line through the center of the antenna.
You can assemble the main loop from continuous copper tube or from eight straight sections and 45 degree joiners. Make sure you have a blow torch or propane torch to solder the joints as you’ll need more heat than a soldering iron can supply. Whichever way you decide to build the main loop make sure that all joints are soldered or clamped as securely as possible, you want the lowest resistance possible to avoid your output power turning into heat. Other materials can be used for the main loop such as aluminium or low loss coax but copper pipe is easy to work, has low resistivity and available from just about every hardware store.
To construct the frame of the antenna you can use PVC pipe. It is a cheap and relatively sturdy building material and is available in a range of thicknesses, just about any hardware store will stock a wide selection of fittings. It insulates well and can be glued once you are sure your project is in its final form.
Once the main loop is constructed you’ll need to connect your capacitor to the two ends of the pipe at the top of the loop. Depending on the capacitor you may want to solder tags to the ends of the loop so they will be easier to attach. Copper pipe is a great conductor of heat and takes a lot to heat up and solder while it is not advisable to apply the same amount of heat to your capacitor.
It is also a good idea to attach the capacitor to a solid support so that the connections are not under strain.
The main loop and the capacitor forms the resonant circuit of the magnetic loop antenna.
To couple the main loop to your transceiver and match the expected 50 Ohms impedance you can use one of two methods. Probably the easiest is to use is a loop of insulated wire 1/5 the circumference of the main loop. The smaller loop is placed at the bottom of the main loop and can be shifted around to provide the best match. If you have an antenna analyzer you’ll be able to set it to the desired frequency, tune the variable capacitor for resonance and then move the small matching loop around till you have achieved close to 1:1 SWR. If you don’t have an antenna analyzer you can tune the capacitor for the greatest received noise and then on low power tweak the capacitor and move the coupling loop around for best SWR. Do NOT touch the loop while it is transmitting, use a wood or plastic rod to make adjustments as there are high voltages and intense RF fields near the loop.
An alternative to the coupling loop is the gamma match. The shield of the coax feed cable is connected to the base of the main loop while the inner conductor is connected to a point approximately 1/5 of the circumference around the loop. Its a good idea to use stiff wire (large gauge) for the gamma match as it can be critical of the position and orientation and once you have it in the right position you won’t want to move it again.
It would be preferable to have the ability to remotely tune the loop. A motor with a reduction gear could be used to move the variable capacitor but because the point of resonance is very narrow there should be a way of slowing the motor down. A simple control circuit using variable pulse width modulation could be used to slow the motor down while still retaining enough torque to move the capacitor. Whatever method is used to move the capacitor it should be well insulated from the other components of the antenna. Several thousand volts are generated on the MLA and care should be taken to ensure they don’t find their way onto control leads and back into the shack. Control leads should also be wrapped around toriod inductors as they leave the near field of the antenna to reduce the possibility of RF travelling along them.
With a SWR bridge and microcontroller you could build a fully automatic tuner that swept through the range of the tuning capacitor when the SWR rose above a defined limit indicating that the transmit frequency had changed.
With a little creativity and knowledge you could have an impressive MLA the equal of multi-thousand dollar military style units.
Hopefully this has given you some ideas for constructing your own loop antenna. Regardless of if you go top-of-the-line and buy a vacuum variable or build for economy and QRP you’ll have a compact, useful and unique antenna.