Another unidirectional small loop: the Coplanar Twin Loop

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Andrew (grayhat)
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Location: JN63pn

Re: Another unidirectional small loop: the Coplanar Twin Loop

Post by Andrew (grayhat) »

13dka wrote: Sat Oct 15, 2022 4:03 pm Welcome to the forum, qrp-gaijin! Hope you guys tell me when it's time to charge my little dremel tool. :mrgreen:

~20dB more gain / less losses at little more than 1/2 the size of the SULA sounds pretty intriguing, even though the overall coverage seems to be pretty small (7-21Mhz?).
Andrew (grayhat) wrote: Sat Oct 15, 2022 10:28 am...check if it may be possible to add a passive "reflector" element to the SULA, that is, either a vertical dipole or a V shaped (with the vertex and sides parallel to the SULA back) element... as soon as I'll finish here (that is, most probably next monday) I want to check it on NEC
And if that works, why not adding a director too? (I need a new car then.)

Hi Ollie, long time no "bark" :) ! Hope things are running ok ! As for our new member, I've been following him for quite some time now, and I hope he'll pick a stable seat here ! Regarding the SULA that's just a (maybe stupid) idea, will need to give it a spin, but if it works as I think, you won't need a new car at all ;) !
qrp-gaijin
Posts: 14
Joined: Thu Oct 13, 2022 7:49 am

Re: Another unidirectional small loop: the Coplanar Twin Loop

Post by qrp-gaijin »

Andrew (grayhat) wrote: Sat Oct 15, 2022 10:28 am at the moment I'm busy helping some relatives picking olives from trees
Very nice! I had the good fortune to visit Italy (Umbria) some years ago and enjoyed taking countryside walks and seeing the local residents harvesting olives.
Andrew (grayhat) wrote: Sat Oct 15, 2022 10:28 am I just had a probably crazy idea, inspired by your loop+dipoles
I should point out that the dipoles in my tests were only present as excitational sources, and were not intended to influence the antenna pattern.
Andrew (grayhat) wrote: Sat Oct 15, 2022 10:28 am that is, check if it may be possible to add a passive "reflector" element to the SULA, that is, either a vertical dipole or a V shaped (with the vertex and sides parallel to the SULA back) element... as soon as I'll finish here (that is, most probably next monday) I want to check it on NEC
This sounds very interesting, but I wonder about how effective any small reflector will be at HF frequencies. The CTL's approach (using a secondary outer loop to cause field cancellation and generate the magnetic shadow) is quite interesting in this regard, in that it can achieve directivity (even possibly for transmitting, as we saw earlier) in a quite small antenna. Looking forward to hearing your results.
Andrew (grayhat) wrote: Sat Oct 15, 2022 4:00 pm forgot (and OT) do you know what happened to Jim's ham forum (the radio board) ? I know you were a member there, and there were a lot of cool folks, but then, it went suddenly "poof" <sigh>
The short story: it's gone.

The long story: Jim, the board owner, seems to have had some serious health problems requiring hospitalization, which made it impossible for him to administer the board anymore. The board was thus kind of running on autopilot, and apparently at some point it was the victim of a hacking attack (reportedly, some kind of phishing attack). The web company hosting the board saw that the site was hacked and the web company themselves decided to shut down the site. Apparently, Jim and Dave (the previous board owner) were notified by the web company about what needed to be done in order to restore the site. Probably (and I am guessing here), this would have required upgrading the site from HTTP to HTTPS, backing up all of the old data (which I think was said to be 50 GB or more), re-installing the web forum software from scratch, and re-importing all 50 GB of data from the backups. This would be a non-trivial, painstaking task even for a full-time IT professional, and neither Jim, nor Dave, nor anyone else had the time, energy, and resources required to get the board back up again. The domain name theradioboard.com was not renewed, and therefore the web hosting company has in all likelihood completely deleted all of the site data. I had set up a discussion forum to discuss what to do about the demise of TRB. If you're interested in reading some of the historical chatter about what happened, you can see the old discussions at https://groups.io/g/TheRadioBoard/ . This group is not active anymore and was just an emergency gathering place right after theradioboard.com disappeared.

The good news: Many of the old members have now regrouped into two web forums, which are administered by different people and run on different software platforms. One forum is located at https://www.theradioboard.org/ . I tend to hang out there. This forum uses the WiX forum software, which is fairly easy to use, but has its quirks also. Another forum is at https://groups.io/g/TheNewRadioBoard . I'm also a member there. That forum is hosted on groups.io, which is a little more difficult to use from the web, because it's more focused on being an email-based message exchange service. At theradioboard.org, people seem to discuss more complicated projects like superhets or Wadley-Loop receivers, whereas on the groups.io TheNewRadioBoard people seem to discuss more classic vintage circuits like crystal radios or regenerative receivers. Anyway, have a look and jump in if something catches your interest!
qrp-gaijin
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Joined: Thu Oct 13, 2022 7:49 am

Re: Another unidirectional small loop: the Coplanar Twin Loop

Post by qrp-gaijin »

qrp-gaijin wrote: Fri Oct 14, 2022 3:31 pm 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.
There seems to be a very small magnetic shadow effect visible in the SULA.

When illuminated from the front (non-null) side by a small vertical dipole located at x=-100, the magnetic near field inside the loop has a moderate field intensity, indicated by the light blue color.
sula-illumination-front.png
sula-illumination-front.png (53.25 KiB) Viewed 208 times
When illuminated from the rear (null) side by a small vertical dipole located at x=100, the magnetic near field inside the loop has a weaker field intensity, indicated by the darker blue color. This is the magnetic shadow effect.
sula-illumination-rear.png
sula-illumination-rear.png (53.74 KiB) Viewed 208 times
Next question: what happens if we place an interior resonant loop inside the SULA, similar to the interior resonant loop of the CTL? Could we get a stronger shadow, and a stronger extracted signal thanks to resonance? I don't know. In this case, the untuned SULA loop -- with its pure resistive loading, but no inductive loading, and no resonating capacitor -- would need to serve the same role as the CTL's outer loop, but with a much reduced Q and much reduced current flow in the outer loop. Because the SULA is not resonant and is low Q, it's not clear to me if it will be able to serve the same role as the CTL's outer loop. I don't know if the non-resonant SULA loop will interact strongly enough with the interior resonant loop to provide good signal transfer and to enhance the magnetic shadow effect. For the original CTL design, Villard mentioned that the loading coils for the outer loop should be high-Q, implying that the entire outer loop should be high-Q, probably to preserve the Q of the interior loop, in order to extract the most signal from the resonant interior loop.

Many questions, not many answers... I'll try to put a resonant loop inside the SULA in 4nec2 and see what happens.

My intuition tells me that a non-resonant, low-Q outer loop (the SULA) cannot function as well as the tuned, high-Q outer loop in the CTL in producing the magnetic field shadow and in producing the unidirectional effect. I would think that there has to be some signal loss compared to the original CTL design. If the non-resonant SULA + resonant interior loop combination shows the same performance as the original CTL design, then this would mean that the CTL's outer resonant loop is needlessly complicated. Given the designer's credentials, I find that hard to believe, and suspect that Villard specifically used a high-Q, resonant outer loop because it was necessary for improved performance.
--
BEGIN EDIT

Updated information: I confirmed that a low-Q, non-resonant, resistively-loaded outer loop does not work as a replacement for the high-Q, resonant outer loop of the CTL. To test this, I used my working CTL model, and removed the loading inductance and resonating capacitance from the outer loop, leaving only the load resistance in the outer loop, similar to the SULA. I placed the excitation source on the outer loop opposite of the load resistance (just as in the SULA NEC model) and optimized the resistance value for best F/B ratio. I could observe a cardioid pattern when the outer loop was excited, just as in the SULA. Then I relocated the excitation source onto the inner, resonant loop. No null appeared in the far-field data; the far field data instead appeared to be simply that of a normal small transmitting loop (a mostly symmetrical "blob"), and only a very small asymmetrical distortion of the far-field pattern occurred. In other words, the low-Q, non-resonant, resistively-loaded outer loop cannot cause the inner tuned loop to exhibit a cardioid pattern in the far field. I didn't bother with trying the near-field illumination test, because I'm pretty sure nothing would happen -- the non-resonant outer loop is simply not coupling very strongly to the resonant inner loop, and so has very small influence on the inner loop's near field and far field.

END EDIT
--

Here's the NEC file used to generate the above plots. I simply added a 0.1 meter long vertical dipole at x=-100 or x=100, moved the excitation source onto the dipole instead of the loop, and plotted the magnetic near field data around the loop.

Code: Select all

CM ----------------------------------
CM File: SULA.nec
CM ----------------------------------
CM 
CM Small Unidirectional Loop antenna
CM 
CM feed using a 9:1 balun transformer
CM keep the bottom corner at 3mt from 
CM ground to avoid pattern distortion,
CM use a non conductive pole to raise
CM the antenna, optionally add a 20dB
CM or more preamp to raise the gain
CM 
CM  symbols definition
CM SY wire=0.00635            
CM  wires geometry
CM   ID seg    x0     y0   z0         x1     y1     z1          wire rad
CM  coax feeder "simulation" (may be changed to TL)
CM  ground parameters
CM  wires loading
CM  enable extended kernel for calc
CM  feedpoint
CM  initial test frequency
CM  end
CM 
CM 
CE
SY freq=7.100	'test frequency
SY hght=3	'height of bottom corner from ground
SY side=0.762	'length of one side
SY diag=(sqr(2)*(side/2))	'half diagonal
SY wire=0.00125	'wire radius
SY vres=530	'loading resistor value
SY segm=13	'number of segment in wires
SY segs=5	'short wires segments
SY wfed=1	'feedpoint wire
SY sfed=segm	'feedpoint segment
SY wres=4	'wire hosting the resistor
SY sres=1	'segment hosting the resistor
SY drop=hght-diag	'coax drop section length
SY coax=0.00250	'simulated coax radius
SY spac=(coax*3)	'spacing for coax routing
SY dist=diag+spac	'spacing for coax feedpoint
SY signal_antenna_x=-100
SY signal_antenna_len=0.1
GW	1	segm	0	0	hght	-diag	0	hght+diag	wire
GW	2	segm	0	0	hght	diag	0	hght+diag	wire
GW	3	segm	-diag	0	hght+diag	0	0	hght+(diag*2)	wire
GW	4	segm	diag	0	hght+diag	0	0	hght+(diag*2)	wire
GW	20	segs	-dist	0	hght+diag	-dist	0	drop	coax
GW	21	segs	-dist	0	drop	0	0	drop	coax
GW	22	segs	0	0	drop	0	0	spac	coax
GW	100	1	signal_antenna_x	0	hght+(side/sqr(2))-0.5*signal_antenna_len	signal_antenna_x	0	hght+(side/sqr(2))+0.5*signal_antenna_len	wire
GE	1
LD	7	0	0	0	2.1	wire	'insulation
LD	5	0	0	0	58000000	'copper wire
LD	1	wres	sres	sres	vres	0	'resistor
GN	2	0	0	0	13	0.005
EK
EX	0	100	1	0	1	0	0
FR	0	0	0	0	freq	0
EN
User avatar
Andrew (grayhat)
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Re: Another unidirectional small loop: the Coplanar Twin Loop

Post by Andrew (grayhat) »

as for the CTL, did you consider changing the size of the sides ? I mean, for example, making the vertical size 1/2 of the horizontal (or the other way around) while keeping the same total perimeter
qrp-gaijin
Posts: 14
Joined: Thu Oct 13, 2022 7:49 am

Re: Another unidirectional small loop: the Coplanar Twin Loop

Post by qrp-gaijin »

Andrew (grayhat) wrote: Thu Oct 13, 2022 1:41 pm Regenerating the loop may be interesting but I think isn't an easy thing, see, if we're using the loop as the "L" part of the regen circuit, we'll need to tune both the antenna and the regen and that will be quite "touchy" and probably the frequency range will be limited (assuming the loop size remains the same), so I'm not sure it may be practical
As the singer Billy Joel once said, "you may be right... I may be crazy". Now, I think regeneration will not work with the CTL. My idea was to leave the outer CTL loop as-is, and to apply regeneration only to the inner CTL loop. I thought this would simply reduce the losses in the inner loop, improving reception. But it's not so simple. We're "removing the losses" from the inner loop by regeneratively pumping energy back into the system at the reception frequency f -- introducing "negative resistance" into the system. But the outer loop is also resonant at frequency f. So some -- or a lot -- of the regenerative energy pumped into the inner loop will couple back into the outer loop. This in turn will reduce the effective resistance of the outer loop. But the outer loop needs a specific value of resistance to achieve the unidirectional null. So I expect that as we increase regeneration, the null will start to disappear because of the effectively reduced resistance of the outer loop. Then if we try to compensate by increasing the real resistance of the outer loop, this increased loss will couple back into the inner loop, pulling the regeneration back down again!

Another possibility to use regeneration might be to omit the load resistance completely from the outer loop. Villard's CTL article stated that the unidirectional effect could also be achieved not by adjusting the outer loop's load resistance, but instead by adjusting the axial spacing between the outer (larger) and inner (smaller) loop. If that is true, then maybe it would still work to regenerate the inner (smaller) loop, and adjust its axial spacing from the outer loop until a unidirectional null is attained. But I guess this will be very difficult to build and operate in practice. A regenerated inner loop, on the edge of oscillation, will be very sensitive to its surroundings, and another resonant circuit -- like the outer loop -- will surely have a very large effect on the tuning and regeneration level of the regenerated inner loop. So adjusting the spacing between the loops to find a null would be very difficult -- as the distance is adjusted, the coupling changes, changing the tuning, and requiring a new regeneration level. Even if a null could be found, tiny changes in loop spacing or position would likely upset the system and require readjustment.

It probably makes more sense just to add a normal, non-regenerative pre-amp to the inner loop if signal levels need to be boosted.

--

[edit]
Still, there's a small chance that regeneration may work -- my loose chain of logic above was concerned about the "negative resistance" from regeneration fighting with the required value of "real resistance" in the outer loop, but the reality may be more complex, since the real resistance is a physical lumped component, but the "negative resistance" is not, and even with regeneration we may get lucky and the actual current flow in the antenna may still be able to generate the unidirectional null. Anyway, It is quite easy to make a small 2-transistor differential pair with a variable emitter resistor to regenerate the inner or the outer loop and to test whether it works or not. First thing I need to do is to actually build the CTL antenna! :D
[/edit]
Last edited by qrp-gaijin on Mon Oct 17, 2022 10:12 am, edited 1 time in total.
qrp-gaijin
Posts: 14
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Re: Another unidirectional small loop: the Coplanar Twin Loop

Post by qrp-gaijin »

Andrew (grayhat) wrote: Sun Oct 16, 2022 8:30 am as for the CTL, did you consider changing the size of the sides ? I mean, for example, making the vertical size 1/2 of the horizontal (or the other way around) while keeping the same total perimeter
I haven't tried this -- what would be the idea behind such a modification? I guess this might encourage the loop to act more like a folded dipole instead of a small loop.
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