What is the optimal design for loop antenna made of copper stiff wire to transmit at 81.39–90.00 kHz?

Thread Starter


Joined Sep 2, 2023
The research aims to devise an optimal copper loop antenna design specifically for testing magnetic field intensity within the frequency range of 81.39–90.00 kHz. The focus will be on selecting appropriate dimensions, materials, and configurations to ensure accurate and reliable measurement of magnetic fields, particularly relevant for wireless charging scenarios compliant with the SAE J2954 standard.

Thread Starter


Joined Sep 2, 2023
Thank you for pointing out the wavelength associated with a 90 kHz signal. I'm aware that the wavelength at these frequencies is quite large compared to typical antenna sizes. The focus of my research, however, is to create an optimized loop antenna specifically designed for testing magnetic field intensity in the given frequency range. The goal is to develop a design that allows for accurate and reliable measurements in particular use-cases, like wireless charging scenarios compliant with the SAE J2954 standard. While the large wavelength poses an interesting challenge, loop antennas can be quite effective for near-field measurements and can be scaled down in size while still being functional for the intended purpose. I appreciate your input and welcome any further suggestions or observations you may have.


Joined Nov 6, 2012
You are not dealing with "Antennas",
You are dealing with "Air-Core-Transformers".

Power is transferred from Coil to Coil.
The Coils must be physically matched to each other.
The distance between the Coils is absolutely critical.


Joined Aug 21, 2008
188 kHz 1 watt transmitter and antenna-for reference.

I made a couple of these and used them in experiments near the dawn of this century. You might find some of the comments here to be useful, for example, don't worry about wavelength when using a loop antenna - just get the frequency right.

The antenna was made with a frame of 1/2" PVC pipe and running a length of 0.5 mm square stranded copper wire (1 conductor of zip cord) through it. 90 Degree elbows are at three corners and a "T" is at the fourth corner (lower right-hand corner in this photograph), providing an exit for the 50 cm wire leads.

The output stage less the impedance converstion can be recognized as basically the same as published by Murry Greeman, Lyle Kehler, and Bill Ashlock (possibly among many others).

Raising the Antenna's Impedance so it can be driven

The reactance of the antenna at 190 kHz is = 2 x Pi x 190 kHz x 6.2 uH = 7.4 Ohms. Counting the DC resistance of less than 0.2 Ohms and the skin effect about doubles the resistance. Since the resistive losses only increases the total impedance by 0.2% (Z = Sqrt(L^2+R^2), so I will keep it simple by ignoring it. The resistive component largely determines antenna efficency as it is very large with respect to the antenna's radiation efficiency.

The the transistors in the output stage are only capable of driving up to a few hundred millliamps into the antenna, and with a load of 7.4 Ohms, saturation losses in the output stage would eat up most of the 1 watt input power permitted under FCC rules. The challenge became one of how to transform the antenna's impedance to one high enough for the output stage to drive. This challenge is the main reason for this experiment.

Note that there is no need to "match" the antenna to any particular impedance, only to raise its impedance to that which the small output transistors can drive comfortably. The overall objectives are to get as many amps of RF flowing in the antenna as possible given the constraint of 1 watt maximum into the output stage.

Finally, I decided to try a capacitive impedance transformation circuit as shown in the schematic. While I would like to say that I solved a system of equations and then ordered capacitors with the needed values,. In reality, I had to make do with parts in my junk box, and this driver produces square waves, not sine waves, so the formulae I worked out for only useful for the fundamental, and would completely miss the effects of the higher harmonics. For example, during the switching transitions, the output stage attempt to change the capacitance of the matching network instantaneously, which would result in very high charging current. That's what the 10 Ohm resistor is for, but the way, to limit current on those edges.

The method was to pick a pair of capacitances which, when put in parallel resonates with the antenna's inducance, resonates at 188 kHz. The larger the ratio of lower capacitor's value to that of the upper capacitor, the larger the ratio of amplifier current to the, and then try each combination out with SPICE until I saw something I liked.

I finally settled on the combination of mylar capacitors shown in the schematic - .092 uf directly across the antenna and .0165 uf and a 10 Ohm resistor in series with the driver. The .092 uf capacitance was made by placing two .047 uf capacitors in parallel and the .0165 uf capacitance was made by putting two .033 uf capacitors in series.

My thanks to Dr. Carlo Infante for his help in getting the calculations correct.


Joined Aug 21, 2017
First: the practicality of so damn low frequency is just brilliant. Because it is capable to go via rather deep water (even salt-water) layer. So, the underwater drones and submarines really need it.

Secondly, the large wavelength means the antenna in every way will be far less in size than lambda quarter - means the antenna impedance will be microscopic. Have no an experience in fingertips do it will be miliohms, nanoohms or picoohms, but how much it will be with that You have to live with - no way to fool the Law of Nature.

Thus all the science is reduced onto - how to down-convert impedance thousand or million fold. And how to measure. About the last I dare to suggest the Nano-VNA is capable to work so low as 50 kHz as the limit. Just what You need, some 20-50 USD and You have a mighty tool. My advice is to not squeeze the money but buy the latest version/modification and largest possible screen, and even evaluate probably boxed version is more practical. That cheapest are "nude" thus easy injurable, have damn unpleasantly small screen, and highest frequency may turn out be handy in another project, why to buy 2.7 GHz if available is up to 12 GHz. Yet 50 kHz as lowest freq is, guess, for all models. Sad news - the said that it is possible to link Nano with computer is only partially true. We tried and had a tears full eyes woe success. Probably the core cause was our insufficient "Pure Chinese" knowledge, but instruction simply not worked well in the linking process.

When You`ll stuck at the point where impedances are passed, the signal is laid into transmitter antenna, but there is will to evaluate how large is the signal amplitude in receiver place. For such task probably more metrologic correct will be apply the GQ390. It have ability to measure even 50 Hz while the upper limit is 6 GHz. You see the V/m, A/m, W/m2 and may identify the frequency by the in-built spectrometer. And this wonder of technologies cost slight above 100 USD.
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