The CTL article showed a gamma match (tapping at 2 particular locations) on the inner loop to achieve an impedance match to 50-ohm coax. The article also shows how an additional small coupling loop placed inside the inner loop can achieve the impedance transformation to 50 ohms. Both techniques are the same techniques used to match small transmitting loop antennas to 50 ohms.
Yes, the broadband nature of the SULA is interesting. I did a quick run of the 4nec2 model just to make sure it works for me and it did. I wonder how small you can make the SULA and still have acceptable performance. Would 50 cm sides work? 30 cm? The CTL may be have an advantage of smaller size due to the signal boost caused by resonance -- but as you said, the CTL is more complex to build and operate.Andrew (grayhat) wrote: ↑Fri Oct 14, 2022 2:36 pm and this adds another degree on complexity, not that it's bad, but sincerely I prefer simpler stuff in the "SULA" case, the same resistor which gives the cardioid pattern also serves to give a wideband impedance around 450 Ohms, which just using a 9:1 can be easily matched to a coax feeder or to a standard LNA
One quite interesting thing about the CTL, which is mentioned extensively in the CTL article, is the existence of a magnetic shadow region caused by the outer loop. I had confirmed the existence of the magnetic shadow with some 4nec2 near field simulations several years ago, but I couldn't find my old simulation files and I recreated them just now.
This paragraph is tediously long and can be skipped, but I am writing it just to keep a record of what I did for future reference. Basically I created a test environment with three short vertical dipoles located at x=-100, x=0, and x=100. Then I positioned a 1 meter x 1 meter resonant and resistively-loaded loop at x=0, enclosing the center dipole. The tuning capacitor is located in the vertical center of the positive side (x=0.5m) of the loop, and the null direction of the loop is on the negative side of the loop. Tuning the system requires setting the excitation location to be on the loop and optimizing the capacitance until X=0, then moving the excitation location to the x=0 dipole and optimizing the loading resistance and capacitance for best F/B ratio. Next, two excitation sources are configured: one on the x=0 dipole and one on the x=100 dipole. This produces a very strange pattern due to the dual excitations, but nevertheless in this condition it is possible to again optimize the capacitance and resistance for best F/B ratio. Next, the excitation is removed from the x=0 dipole so that only the excitation on the x=100 dipole remains, which is located on the non-null side of the loop. In this condition, I then plot the magnetic near field data around and inside the loop. Finally, I relocate the excitation to the x=-100 dipole, which is located on the null side of the loop, and again plot the magnetic near field data.
If we compare the magnetic near field data when the loop is excited from the front direction (non-null direction with excitation at x=100) with the magnetic near field data when the loop is excited from the rear direction (null direction with excitation at x=-100), then we see that the front excitation induces a higher magnetic field in the interior of the loop, whereas the rear excitation can only induce a smaller magnetic field in the interior of the loop -- in other words, the magnetic field from the rear of the antenna has been obscured by a magnetic shadow, resulting in less illumination (from the rear direction) in the interior of the loop. The magnetic shadow exists, just as Dr. Villard said -- and I suspect this is exactly the kind of numerical simulation that he ran to confirm the existence of this magnetic shadow.
Below is an image of the magnetic near field data when the loop is excited from the front, non-null direction.
Below is an image of the magnetic near field data when the loop is excited from the rear, null direction. Note that the scale of the image is identical to that of the previous image, so it is valid to compare the colors between the images. Clearly, when excited from the rear, there is less illumination (due to a magnetic shadow) in the interior of the loop.
Thanks; that looks useful. The darkness of the above shadow region may be improved with a circular loop instead of a square loop. I'd like to rerun the magnetic shadow simulation with a round loop.
I'm also curious if the SULA also exhibits a magnetic shadow region in the interior of the loop. I hope to run some simulations soon.