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Dipole Versus Wideband Magnetic Loop Performance

dipole versus wideband magnetic loop

Dipole versus wideband magnetic loop performance simulations show that a good active loop will stack up fairly well on the HF ham bands. 

I have always wondered how well small loop antennas compare against traditional antennas as a signal transducer. Time to find out.

An antenna is a transducer that converts electromagnetic wave energy into regular electric energy, i.e. voltages and currents. You can read more about the theory here.

My idea for an experiment was to feed a 1 μV/m radio wave field strength into both a dipole and an active loop and see how much voltage arrived at the output terminals. For the traditional antenna, I chose a dipole in free space. While this is a theoretical antenna, the math is much simpler than with a physical dipole.  (Gain of a physical dipole is slightly higher because of height and reflections.)

For the loop antenna, my choice was the LZ1AQ amplifier connected to my 1 meter diameter aluminum loop. Loop performance was modeled in LTSpice, while the free space dipole was analyzed using an Excel spreadsheet.

Dipole Versus Wideband Magnetic Loop Performance Findings

You can see the results in the above graphic. As expected, I found that the dipole provided much more output voltage than the loop antenna, but the difference was less than anticipated. I took measurements in each of the ham bands between 1.8 MHz and 28 MHz.

We all know that traditional antennas produce less output as frequency increases, which is why you need low noise amplifiers at high HF and beyond. As you can see above, with the dipole, a 1 μV/m radio wave produces antenna output of 26 μV at 1.8 MHz, but less than 2 μV at 28 MHz.

On the other hand, a well designed active loop is a flat transducer. My LZ1AQ model converted the 1 μV/m field strength to around 1-2 μV across the full range of frequencies.

So the difference in efficiency between dipole and active loop transducers is pronounced at lower frequencies, but only nominal at higher HF. You will find that the advantage provided by active loops on lower HF bands comes from improved SNR, not signal conversion.

Lastly, the difference between dipole versus wideband magnetic loop performance is huge at medium waves and below. But who has the space for a dipole at those frequencies?

 

2 comments

  1. Frank K6FOD says:

    Very interesting post. For what it’s worth, I recently tested two dipoles against two commercial magnetic loop antennas, feeding them a pair at a time into two SDR’s that had been checked against each other and then examining strength of FT8 signals as seen in WSJT-X on the 40m, 30m and 20m bands. In general the dipoles resulted in significantly more decodes, and significantly higher reported signal strength (typically by 5 dB or more). Occasionally, however, the mag loops would decode signals that were undetected by the dipoles, which I suspect was probably due to antenna orientation. It’s likely that the mag loop performance could be improved by experimenting more with their placement.

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