loopI’ve been interested in the magnetic loop antennas after having read how efficient they are, and in some ways are “Tesla-esque” being one of the oldest antenna forms out there behind the end-fed Zepp and the simple dipole.  Having just moved to more urban digs, it seemed like a good project.  The venerable magnetic loop antenna. which is basically a large copper coil with a smaller resonant loop inside can be incredibly efficient, but its drawbacks are that it’s also incredibly finicky – the increased Q (narrow range of resonant frequency) means that you have to tune the thing for every 2-3 khz change in frequency. And to do that, you need about 250-1300 picoFarads of capacitance to tune out the inductive reactance of what is basically a tuned LC circuit.  The proper way to do it is to mount the capacitor directly to the loop (to minimize R) and to move the capacitor stator arm back and forth with a motorized contrivance of some flavor.  Because the antenna is so sensitive to capacitance, even body capacitance throws it out of tune while you’re trying to tune it, so remote operation at the very least is desired. In my case, I’m an Arduino buff and you can build the whole thing for under $20 USD with an Arduino Nano and a MAX232 and ULN2003A bipolar stepper driver (darlington array). This gives you an RS232 controllable motor controller with memory that is perfect to control a capacitor remotely. Moreover, you can write those positions to memory and recall specific positions for specific frequency recall. You could then, say send a frequency command in ASCII over an RS232 connection and make the antenna tune for resonance as this guy did. Instead, I would like to measure for VSWR and thus be able to tune by performance, like an automatic antenna tuner.

The first part for testing was to find a capacitor with sufficient voltage handling. Even at small “QRP” power levels, the voltage potential can be several hundreds of volts, so the preferred capacitor is either a vacuum variable or a Butterfly capacitor. I was working cheap though, and vacuum variable caps start at about $120 USD, so I found an air variable capacitor that was fairly cheap ($20USD) and robust on eBay with a simple shaft connection and now in the process of assembling the loop. Stay tuned…In the meantime, here are some embedded videos of what others are doing with remote capacitance tuning of magnetic loop antennas:

capacitor3 my “cheap” capacitor – lining up with a stepper I have in my shop to see how the shafts line up. Example of a design I am trying to mimic

loop antenna

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Remotely Controlling VY1JA Station in Yukon Territory a Win-Win

by American Radio Relay League | Ham Radio Association and Resources on May 1, 2015

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ARRL Facebook Repost of Interview with Ham in Nepal Draws Huge Response

April 30, 2015

The Weather Channel’s “AMHQ” program today featured an interview with Kathmandu Amateur Radio operator Satish Kharel, 9N1AA. ARRL has reposted the interview on its Facebook page, with the permission of The Weather Channel. In the interview segment, Kharel talked about ham radio’s role in the earthquake recovery effort now under way in Nepal.

More than 5000 people have been confirmed dead as a re…

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AMSAT to Roll Out Updated Edition of Getting Started with Amateur Satellites

April 30, 2015

The 2015 edition of Getting Started with Amateur Satellites by Gould Smith, WA4SXM, will be available from AMSAT at Dayton Hamvention. This latest edition will discuss how to get ready for the Fox-1 satellites, set to launch later this year. Additional chapters will talk about tracking software, orbital mechanics, antennas, radios, Doppler tuning, and operating techniques.

Other new chapters wil…

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URE to Sponsor ITU Anniversary Event

April 30, 2015

May 17, 2015, marks the 150th anniversary of the signing of the first International Telegraph Convention and the creation of the International Telecommunication Union (ITU). The Spanish Amateur Radio Association URE (Unión Radioaficionados Españoles), an IARU member-society, has announced an anniversary operating event, May 1-30, with awards available to all radio amateurs and SWLs.

Eleven stati…

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ARRL Rookie Roundup SSB a Big Success

April 30, 2015
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Central States VHF Society Annual Conference Set for July 23-26 in Denver

April 30, 2015

The 2015 Central States VHF Society (CSVHFS) Annual Conference will take place Thursday, July 23, through Sunday, July 26, at the Denver Marriott-Westminster. Rocky Mountain Ham Radio is the local sponsor.

The conference will feature a banquet, luncheons, and hospitality suites as well as technical programs, noise figure measurement, an antenna range, and a rover vehicle show-and-tell.

ARRL First…

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The Sinking of the Lusitania: A Ham Radio Connection

April 29, 2015
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Armed Forces Day 2015 Crossband Communications Test to Offer New Modes

April 29, 2015

The annual Armed Forces Day Crossband Communications Test set for Saturday and Sunday, May 9-10, will offer Amateur Radio operators a chance to try their hand at using more modern military communications modes, such as MIL-STD Serial PSK. Also new this year is the inclusion of a crossband Automatic Link Establishment (ALE) test. The Army, Air Force, Navy, Marine Corps, and Coast Guard cosponsor…

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