Experimenting with Magnetic Loop Antennas
2018-12-08 Every once in a while over the 60 years I've been into ham radio the topic of Magnetic Loop Antennas has come up. Each time I have sort of rolled my eyes and paid little attention. Not sure why, other than it all seemed somehow like magic. But I stumbled across a YouTube video by OH8STN recently which got me thinking in practical terms about trying to actually make one! So we'll see!
2018-12-10 As I've thought about the magnetic loop antenna it strikes me that it is simply a tuned-secondary RF transformer with an un-tuned primary. The one turn (usually) secondary is high impedance and couples to the "ether" while the one turn primary is roughly 50 ohms and couples to the transmitter. That is similar to the tuned-secondary matching transformer circuit I used with my end fed half wave antenna with the primary difference being that a "radiating element" was attached to the secondary in the case of the end fed half wave whereas no such radiating element is used with the magnetic loop. I guess the secondary itself IS the radiating element.
If that is even close to a valid way to think about it that also helps explain why the conductor size and RF resistance of the secondary is such a big deal as it is in essence in parallel with the impedance of the ether as seen by the secondary. And in a parallel circuit the lowest value is the dominant one and will be the biggest factor in absorbing energy.
For whatever reason around 36 inches seems to be a practical trade-off diameter for the secondary turn in many of the examples I've looked at. So I dug around in my junk box and found an almost perfect length of low loss 3/8 inch coax. When formed into a circle its inductance measures 2uh. My junk box also yielded a 16-210pf variable capacitor with fairly wide spacing between the plates. So my plan is to start my project with those. I haven't decided on what I'll use for the primary yet but am thinking of a piece of old cable TV 75 ohm coax. the references I found indicate it should be 1/5th the circumference of the secondary which is about 20". I'm planning this for use with a QRP transmitter, 5 watts.
2018-12-10a After getting most of my chores for the day done I decided to "build" the main loop. I say build with tongue in cheek as I really kind of hung it together. I wanted to see if it really would resonate near some useful frequency using the 16-210pf variable capacitor.
Rather than using the piece of coax mentioned above I tied all three wires of a 102 inch piece of 12AWG with ground electrical wire together. Then I "hung" the wire on a 40 inch piece of wood attached to a piece of OSB flooring and soldered the ends to the capacitor.
Loop, counter dipper and capacitor setup on left. Center shows the connection to the capacitor. Right shows the home brew dipper.
Finally, using my trusty home brew FET dip meter set to about 14.05 mhz as indicated on my antique frequency counter I tuned the capacitor to see if I could find a dip. I DID! In fact I could tune it all the way from 18mhz down to 6.7mhz. Not sure that is a practical range for use but it does resonate over that range. My experiment is successful! All I need to do now is add a small loop and see if I can get a useful match to a transmitter!
2018-12-10b Well, I quick threw together a loop made of 20 inches of #12 electrical wire and soldered the ends to a BNC connector. Then using tie wraps I attached it alongside the top of the large loop. A quick check indicated that the main loop resonant frequency wasn't affected much so I tuned it to 14.05mhz. Then using a three foot piece of RG-174 coax terminated with BNC connected the small loop to my Xiegu X5105 and ran a SWR scan with it. To my amazement and pleasure the SWR looked just like it should with the lowest point near 1:1 at roughly 14.05mhz! I ran SWR scans for 30 and 40 meters as well and the plots for all of them are below. The concept is proven!
Would it, will it work on the air? I guess that comes next. Then I may build a "nice one" properly! Though I am very pleased with this concept model.