On Thu, Oct 13, 2022 at 12:59 AM, Martin wrote:
The Low Z loop works as a standard H field 'magnetic' loop, and the second larger diameter Hi Z loop is more like an E field 'whip' antenna. When the outputs are combined (physical spacing) the resultant pattern is likely to be cardioid.
After thinking about the results of some old 4nec2 simulations, I think I have finally figured it out.
The outer, larger-diameter high-impedance loop is resonant and is resistively loaded. As was already pointed out before, this resistive loading creates a unidirectional response. So the outer, high-impedance loop by itself produces a unidirectional response. I have confirmed this in 4nec2.
What, then, is the purpose of the inner loop?
As was mentioned in post #16631 (/g/loopantennas/message/16631), on Sun, Aug 21, 2022 at 03:16 PM, Alan wrote:
When the resistor is added the original two nulls are filled in, at 90 degrees to the notches a new minima appears as the magnetic and electric fields partially cancel, increasing the resistor leads to a deep null when the two components cancel.? At this R value the cardioid ( heart shaped) pattern is formed [...]
The resistively loaded loop IS e-field sensitive!! so the advantage of e-field noise suppression is lost.?
We could take the unidirectional antenna signal off of the outer loop directly with a preamp or a toroidal transformer. But no, the CTL does something creative. The creator, Dr. Villard, seems to have realized that the interior region of the resistively-loaded outer loop exhibits a "magnetic shadow" effect -- the magnetic field for incoming signals from the null direction excites almost no magnetic field in the interior region of the loop (the "shadow area"), whereas incoming signals from the non-null direction excites a significant magnetic field in the same shadow area. So, there is a unidirectional "magnetic shadow" in the interior region of the loop, which I also confirmed in 4nec2 by plotting the magnetic near field data and exciting the loop with a distantly-located vertical dipole either on the null-side or on the non-null-side of the loop.
So: inside the magnetic shadow region, only magnetic signals from the null direction are suppressed, while magnetic signals from other directions are allowed to pass. All that remains to be done is to sense this local and shadowed magnetic field from the interior region of the large loop -- and this is the function of the interior, low-impedance loop, which is then connected directly to the receiver.
The same sensing of the local and shadowed magnetic field could probably be done with a simple and non-resonant inductive pick-up loop, but this would likely lead to a large loss of signal due to the lack of resonance. Making the interior loop resonant likely boosts the signal -- Dr. Villard (the CTL's creator) said that the sensitivity of the very small (table-top-sized) CTL was comparable to the sensitivity of the internal whip antenna of a portable receiver, indicating no drastic loss of sensitivity, even though no active antenna amplification is used.
Because the interior sensing loop responds primarily to the local magnetic field (thanks to its low impedance), it retains the advantage of suppression of local E-field noise -- which would not be the case if we attempted to extract the signal directly from the outer, resistively-loaded loop.
So Dr. Villard's main contribution seems to have been the recognition of the spatial existence of a magnetic shadow region in the interior of a unidirectional, resistively-loaded loop (which happens to be resonant, in the case of the CTL), and the use of a second, resonant, low-impedance loop (sensitive primarily to the magnetic field) to sense the shadowed (unidirectional) magnetic field.
I've been considering if it is possible to add regeneration to the CTL, and I think that adding regeneration to the interior loop only will boost performance while preserving the unidirectional property.
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As an interesting aside, Dr. Villard also designed an antenna to exploit an electric-field "shadow region", although such an antenna, being E-field sensitive, is more prone to detuning by nearby objects and persons. This antenna uses two whip antennas (similar to the two loop antennas in the CTL) and is described at the top of this page: . Another detailed description, with a picture of the antenna, is here: . This two-whip-antenna is also briefly described here: -- see the section titled "Antennas Generating Nulls in the Ambient Electric Field".
