Ham Radio - D loop directional half loop transmitting antenna. (experimental)
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- Опубликовано: 18 сен 2024
- While reading about a military NVIS antenna, I had the idea to rotate the design vertically and make a directional magnetic loop. It works surprisingly well for such a small antenna.
Below is a link to the video where I customized the trimmer cap used later in this video.
• Ham Radio - I create a...
And here is a link to my home made field strength meter I mentioned.
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Kevin, finished the project, your D Loop. Made a couple changes, one the backing, replaced the aluminum foil with a thin sheet of copper glued to the board,made the width of the backing to 10 inches. Will be changing the width to a little more to see what will happen, did much the same for the rest of your design. Every thing checked out as far as SWR. Got a feeling this is going to be my go to antenna. I am going to try it on the air tomorrow.
Great experiment and it looks like you’ve got a winner.
There is always the debate about how big a ground plane do we need. Some folks say a vertical 1/4 wave needs 128 long ground radials to work, but lots of folks seem to get along with a small fraction of that and shorter radials too.
Thanks for the great content and great ideas. I've been thinking of putting together an NVIS antenna to be able to communicate with home while I'm out hunting and camping, as we have some pretty steep topography out here in Oregon. This seems like a very portable solution that could work out very well. I've watched a lot of your videos recently trying to increase my understanding of antennas, and I really appreciate the catalog of information you've shared with the world.
I’ve just subscribed to your channel Kevin, you are detailing all the practical experiments that I would like to achieve myself yet never have the time! Thank you, a great channel!
Very innovative, Kevin! I've been toying with the idea of a larger (~6 ft) diameter) mag loop with a wide range vacuum variable
(5 - 350 pf), to try for the holy grail of 80 - 10 m. However, seeing your video clip is intriguing. I'd love to see some quantitative results on F/B ratio, and if possible, forward gain. One idea that hit me was to simply split a mag loop in half, and insert the ground plane between halves, thus creating two back-to-back D loops. Hopefully the ground plane would act as a Faraday shield between halves, so that, with a simple coax switch, one could drop backside QRM "into the void," and possibly grab a little forward gain, in the process. Extending that idea, with two such "Double D" loops (four D loops) mounted crisscross, it's conceivable to create a "poor man's beam" by utilizing the high off-axis loss of a loop, plus the F/B of the D-loop, with a four-way coax switch, switching by quadrant. Such an antenna could well fit on a decent-sized balcony, or optimally, in a small yard, without incurring the wrath of an HOA. Have you done any quantitative measurements, and I'd certainly appreciate your thoughts about the wisdom of "siamese D loops." Thanks! -- KD2JAU
Well done... I think you have a nice winner here.
I heard you on the repeater this morning talking about this antenna. Very interesting. Thanks for posting. KB9BNI
Hi John, thanks for the interest. It surprised me with how well it works.
Diggin that 30.00 sticker on the MFJ.. I never get deals like that .. Interesting video overall.. Thanks from KK5VN
Cool. I'm surprised that there is almost no information on this concept on the web - 6 years later....
Great vids. Thinking outside the box again, keeps it interesting.
The coax at the tuningcapacitor ads a lot of capacity to the tuning circuit . Put the capacitor as close as possible to the loop . You can tune the loop to 10m. then. 73 John
Kevin, I like this. Suggestion: convert the back plane from a flat board to a curved one as if sectioned from a parabolic dish thereby possibly sharpening directivity/gain.
Hi Kevin ...this really nice antenna
I want to make the same for 3-30MHz band with comparatively larger dimensions. Is it possible to match the same with two lumped Capacitors instead of gamma match??
Kevin, I watched this video with great interest and then discovered it was over three years old! Have you done any work on the D-loop since? Would it work with a wider plate gap on 100 watts? Have you optimized the shape of the backplane? Thanks for a great idea!
now im thinkin, i have a part 15 transmitter for 1600-1700khz x band, my house has all aluminum siding and is well grounded with lightling rods and thick straps. so im wondering if theres a way to adapt this to be compliant by mounting the tx right up behind the feed point. now the ol brain is tickin
This is not a half loop. It is a D shaped magnetic loop. Your "ground plane is less than 4/1000 of a wavelength in its longest direction. This would be equivalent to a "ground plane" of under 1/2" at 144MHz.
What you have is nothing more than an asymmetrical magnetic loop., with lots of losses because of the numerous mechanical connections.
In order for a parabola to act as a focusing mechanism, it must be in the neighborhood of 10 wavelengths in diameter. At 20M this would be 10 X 20= 200M or about 700 feet. Then there is the issue of focal point. Parabolas only funcition when the source is at the foical point. Let's say you develop a deep parabola (focal length/diameter=0.3), then the focal point is 210' away from your asymmetrical loop or vertex in this case. A shallow parabola (say focal length/diameter= 0.4) would move the foical length out to 280' from your loop.
The simplest way to see the falacy of this is- what would prevent the aluminum foil from radiating? It carries the same current as the other half, albeit, current density is less because of the greater area- not a concern in the far field.
Dale
Dale, how is a "half loop" different in design from what Kevin is attempting here?
Dale, you raise some interesting comments, may I ask your qualifications in such matters?
Interesting idea. But from the charts I have seen of the half loop antennas yes they have a nvis factor but also appears to have a near perfect spherical pattern as well. Which tells me it could be useful for both dx and nvis
Mounting it vertically might have good fb ratio but I would guess the side rejection isn't going to be very good
Edit done not sphere. But still my guess is still that the side rejection will still be poor. Maybe about the same performance as a two element yagi or slightly less directivity??? I am still interested in the half loop for mobile hf. Am I wrong in thinking it would do equally well for both NVIS and DX?
Dome not done auto incorrect
I like it! great for a balcony where you have less than a 180 degree open area....this would keep you from transmitting through your house like a bi directional loop or omni. I'm a big loop fan for portable operation.....I'll have to try one of these......wonder how it would work on a car roof in mountains? roof would make a great gp.
great work Kevin. it reminds me of of the DDDR antenna, which sounds similar to the military antenna you referred to. For this antenna, do you have any far-field measurements (e.g. JT65 or WSPR) of the dB difference between forward and back, and to the side? Thanks Giovanni ZL2GX
Hi Giovanni,
I haven't collected data on the db reports in WSPR, I've only looked at the footprint as I showed in the vid. I do know that stations could not be copied when I turned the antenna away from them 180 degrees.
If I have some time tomorrow, I might do some more tests and see if I can get any numbers.
Excellent idea!
Thanks. After reading about the military version, I was surprised to not find anyone doing something like this. And even more surprised with how well it worked.
I might have to work on a more clean and solid version and maybe do an article on it.
I'm just thinking on putting it back on the horizontal and putting it on the roof of my pickup if it would actually work better than mobile antenna's that are commercially made, wondering if the radiation pattern would warrant such an experiment.
my next project is a 1/2 wave over a 1/4. using 300ohm ribbon wire. and braided wire over that. but take that to rhe next level. youve seen a cushcraft ringo ranger with the hiar pin like network sticking out the side of the vertical. well I'll do that to it. offering some real gain in a vertical antenna.
How much power do you think this would reliably take? Does it come down to the capacitor used?
Yeah, depends on the capacitor. Being a resonant circuit, the RF voltage across the cap can be quite high. Hundreds of volts at only 5 watts.
@@loughkb so much in the same way as a mag loop. How did you come up with these dimensions to try it with? Take a loop and half it? Thanks for your first response, I wasn't sure if you'd want to update some stranger on a 5-1/2 year old project haha
@@knightsofnee8626 Yeah, the dimensions were pretty much like you said, half a normal loop.
It's been that long? Time flies.
@@loughkb it definitely does! Well knowing this it's time to build one! Thanks for getting back so fast, and I hope your day and summer travels go well! Your videos are always great whenever you're able to upload them! 73's
Thanks for posting, I love antenna experimenting ! Have you thought about a curved reflector plate to concentrate the reflected signal even more ?
Yes. I've also realized that the reflector needs to be wider. Ideally, I think it should be as wide as the distance that the radiator extends forward. I'm not sure of the effect making it parabolic would have though. Unless it was a half cylinder, but I bet that would throw off it's resonance a bunch. Just my guy feeling.
*gut feeling.
I am working on your design and hope to get it finished in the next week or so. I have used a metal magnetic memo board, cut and bolted together for the reflector. I plan to cover it with recycled aluminium cans. The D tube shaped tube I will solder in sections and use elbows. I don't want to send the signal up to high, so plan to make the vertical section say 18-24 inches long and use and elbow and 2 shorter length pipes to make the D. Yes curved dies seem a bit too much messing, so I may add 2 panels to widen the reflector and angle them slightly to hopefully concentrate the wave even more. Thought about a director copper tube on a wooden dowel, or fibre glass rod stand a little shorter than the main vertical tube on my D. Experiment with distance to again see if signal can be concentrated to the horizon and not lose some to sky wave or ground ? Conditions are poor right now, so I will put an effort into making your antenna. Thanks again.
I'll be very interested to hear about your results.
if it radiates stright up and is bounced back from streight up and being a hemisphere that should work beter than a dipole. because it works on takeoff angle. and that creates the skip region you signals bounce over. if you use the ionosphere like a high altitude repeater. their should be no skip region. there would be no one you cant get to, for an ultamate net control. i was thinking on making a legal limit magnetic loop, using a high voltage vacuum variable. most vacuum variables have great range of capacitance. so would the range of frequency coverage.
The military version was like a D laying on it's back. It was designed to beam straight up and bounce back down. It was intended to allow communications in mountainous regions where you couldn't send a signal through a mountain. Instead, they bounce it back down on the other side of the mountain.
I just rotated it 90 degrees so that it is sending the signal horizontally, allowing for skip.
Interesting design! I like this a lot. I am having trouble with my current space in my home. I think this will do it. What can be done to get me on 80-10 meters?
I'm still experimenting. This D is 2.5 feet tall, measured in a straight line from one end of the curved element to the other. Depending on the range of the cap, I can get it to behave very well from just below 20 meters up to 15.
If it were just a bit shorter, I'd probably get form 20 to 10.
If you want 80-40, I think you'd need to make it much bigger. Maybe 4-5 feet tall.
Presently, I'm using a cap out of an old AM tube type radio. I'm not sure of the power limit on this antenna design. I don't think it's going to be as hard on the cap as a full magnetic loop is.
Kevin Loughin a varible cap rated around a couple 100 volts will handle 10 watts. look up voltage to watts. and thats what ive found on how much it one transmit. it has to do with arching. a low voltage cap will arch. thats bad.
Dale Myers In the case of a magnetically coupled loop, 10 watts will produce well over 1KV RF voltage across the capacitor. This is due to the fact that the magnetic loop is basically a tuned L/C circuit. The reactive energy is additive during oscillation between the L and C components and produces such high voltages at that point.
Full technical information is here:
www.nonstopsystems.com/radio/pdf-ant/article-antenna-mag-loop-2.pdf
To get on 80 through 40 you will need a bigger half loop than 2 feet. More in the range of 5 or 6 feet diameter, I would guess, and a bigger value capacitor too. 73 de w8nsi jim
You seem to have a pretty good understanding of pretty advanced antenna theory. Any tips on getting a better understanding of the topic? I love building antennas, but dipoles and end feds are starting to get old.
Thanks, but I wouldn't say I'm any kind of expert. I read a lot about military antennas and other designs through history and then just think about what the antenna is doing and how it could be changed.
My advice would be, read read read and then, experiment! What happens when you take a zig zag dipole and turn it on end and wrap it around a cylinder? Does it work with the reduced space? Try it and see. etc.
Hmm, that actually might be a good idea, the zig zag thing.
I like that mindset!
so for 10 - 12 meters would the D loop have to be a little smaller?
Yes. Probably 6-9 inches shorter. You'd have to move the gamma match in or out a bit too.
so you mentioned that the ground plane should be wider than the loop, so did you mean the diameter of the radiating tube or the radius of the loop (2 1/2 feet)? in the horizontal plane I assume it would be Omni directional but the other parties might need the same antenna to capture the high angle of radiation?
Kevin as usual great video. Btw, have you ever tried, or thought of, putting a large square metalized plate being behind the full loop, or sides, trying to cause a reflection effect? N7PB Paul OldNormanBates Seattle WA USA CN87uq RememberTubes©
What's your variable cap value that you're typically using on your full Loop, plus trimmer?