Upcoming Project: Magnetic Loop Antenna

CVARC’s next project build is scheduled for Saturday, November 10.  Adrian Jarrett-K6KY will assist CVARC members in building a Magnetic Loop Antenna.  The cost of the materials will be $110 or $175, depending on the capacitor chosen by the builder.

The details:

The Magnetic Loop Antenna is a form of compact HF antenna; a 3 feet diameter mag loop can typically be adjusted to work between 10MHz to 30MHz.  It is a form of shortened dipole, bought back to resonance by adding a variable end capacitance, thus the normal half wavelength dipole can be reduced to a quarter wave circumference or less.  A variable capacitor adjusts the operating frequency over the range and the loop must be adjusted for best match at the operating frequency.  The efficiency of the antenna suffers due the shortened element, especially at lower frequencies where the loop is much shorter than a quarter wavelength, however QSOs can still be made.  A drawback of the mag loop is that the bandwidth is small, a few kilohertz, so tuning up and down the band needs frequent adjustments of the loop tuning capacitor.  The FT8 digital mode is high sensitivity and all signals are crammed into about 3kHz of each HF band.  Both these properties make the mag loop and FT8 ideal companions.  Other modes of course can be used, but if you QSY by more than a few kilohertz you will have to retune.  This can become a bit tedious if you want to tune up and down the band, or take part in band hopping.

The build is planned for Saturday, November 10, 2018, probably at the K6KY QTH (details to follow).  It will probably start about 9:00 AM and last into the afternoon, maybe 4:00 PM or so.  I must insist that the participants do the work!  This worked well I think for the previous 6m halo project… well the folks that took part still talk to me, and so we will do it again.  The project will consist of activities such as:

Cutting the coax radiating element and fitting PL259 connectors.

Drilling and building the capacitor box, soldering the SO259 connectors to the capacitor.

Fitting the motor and drive in the box and connecting to the capacitor.

Cutting and assembling the schedule 40 support structure. Some joints will not be glued to make the loop suitable for /P operation.

Making the coupling loop, with PL259 connector.

Drilling and building the switch box.

Putting the whole lot together and testing.

You may not be able to participate in all activities, as we will have to work in parallel to get all the tasks done.  If there is an activity you are particularly interested in, such as learning how to fit a PL259 connector, let me know.

I am still finalizing the details. The most important component is the capacitor and we will use either the MFJ 282-2113 rated at 800V or the 282-2017 rated at 1.5kV.  Both will have 2 sections in series increasing the voltage handling.  The builder can choose which option.  The total cost will be $110 or $175, depending on the capacitor chosen.  The prototype used an equivalent of the 800V capacitor and it was OK at 20 Watts.  I actually pushed it to 50W, and it looked like the capacitor arced at just above that power level.  The 1.5kV should do 4 times that power, but I cannot not guarantee the power handling level of either design.  I had intended to build a manually tuned mag loop to reduce the amount of labor, but building the prototype and spending a day jumping up and down to retune, and then finally the knob falling off, my thoughts turned to remote tuning.  I am still waiting for the geared motor to arrive on the slow boat to confirm it will work satisfactorily.

If you’d like to participate, please sign up at the September CVARC meeting, and payment will be due at that time so I can order the parts.  If you wish to participate, but are unable to attend the September meeting, please email Christian Ylagan to sign up, and send your payment, payable to CVARC, to PO Box 2093, Thousand Oaks, CA 91358.  The deadline to sign-up is September 30.  If for some reason the parts do not arrive on time, or it rains, we will have to reschedule the build date.

We will be using hot and sharp things, and you participate and use the mag loop at your own risk.  I am not aware of a mag loop damaging a transmitter but there is always a risk that may happen.  The exposed metal parts will have high RF voltages on them and must not be touched during operation; keep out of the way of children, pets and people that may not know enough to keep themselves out of the way.

The mag loop antenna can make a stealthy antenna and a handy portable antenna.  Higher performance may be obtained by using a soldered copper element rather than 0.4 inch coax.  I dismissed this as they take more time to assemble and the opportunity for being burnt is higher.  I was also interested in making the element interchangeable so that I can experiment with the loop operating on more bands.  Search the internet for ‘magnetic loop antenna’ and see if this project is for you.



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