Cantenna implementation: just how tricky is it? - Page 1
Forum section: electronics / rf-microwave

author: MarkMLl
author_rank: Frequent Contributor
timestamp: June 10, 2023, 09:08:03 pm
content: A few days ago I needed to check for the presence of a WiFi AP (possibly not actively advertising its availability). I knocked together a cantenna in a hurry from a somewhat-undersized drinking chocolate can, its off-axis rejection was good but its rear-lobe was disappointingly strong: I suspect because the can was too short so the probe was too close to the centre.I've come acrosshttps://web.archive.org/web/20210512030616/https://www.wifi-antennas.com/topic/52-dual-band-cantenna/which is interesting, but it appears to be an old discussion which went quiet.* Can anybody comment on the computed dimensions?* Does anybody recognise the software used for the gain simulations? Is there a half-decent free alternative?* If having to use "rules of thumb", how does one allow for the horn which various people suggest adds substantial gain?* If fabricating from "found" components (aluminium ducting, stainless cans etc.) how paranoid does one have to be about good contact? Should everything be TIG welded to penetrate passivation layers?* How disruptive are internal steps or ridges of a couple of mm at joins likely to be?Hope that lot makes sense and sparks somebody's interest. There seems to be an enormous disparity between the way people say "this stuff is non-critical" vs the amount of care that goes into professionally-manufactured waveguides.

author: MarkMLl
author_rank: Frequent Contributor
timestamp: June 10, 2023, 10:32:12 pm
content: I was in a bit of a hurry... can I used was plated steel (not stainless since it appears to be magnetic), about 84mm diameter 125mm long with 30.6mm probe on an SMA about 56mm from back. NanoVNA suggests it's broadband centred on the the 2.43 GHz region with an SWR of 1.4 and doesn't care whether the plastic lid is fitted (to add a bit of strength).Definitely better than a WiFi whip (I tried both on location since nobody was actually shooting at me). However now that I've got this far I'm interested in how to "do things properly" without having to learn thin-wall TIG welding etc. :-)

author: Marsupilami
author_rank: Frequent Contributor
timestamp: June 10, 2023, 11:23:32 pm
content: Software is CST. Not cheap.OpenEMS might be a solution for you but it has a learning curve with scripting and stuff.There were threads here too in the past about affordable/ free and open source em sim software.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 11, 2023, 12:07:42 am
content: Quote from: MarkMLl on June 10, 2023, 10:32:12 pmI very much doubt that the magnetic properties of stainless or plated steel have any effect at 2.4 GHz.Certainly, UHF Yagis are quite commonly made of stainless steel for durability.Interestingly, the term "Cantenna" has a quite different meaning, as an RF test load consisting of a large tin can full of transformer oil with a biggish 50resistor suspended in it

author: MarkMLl
author_rank: Frequent Contributor
timestamp: June 11, 2023, 06:41:47 am
content: Quote from: Marsupilami on June 10, 2023, 11:23:32 pmThanks for that. I spent a fair while yesterday Googling images etc. and noted OpenEMS: obviously my RF is rusty but learning curves don't generally bother me... I am, after all, a FreeCAD user :-)

author: MarkMLl
author_rank: Frequent Contributor
timestamp: June 11, 2023, 06:58:11 am
content: Quote from: vk6zgo on June 11, 2023, 12:07:42 amSorry, that was a "too much detail" moment: I'd originally written stainless but checked and changed it.Even if the material doesn't have a bearing on the RF properties, it does on how easy the stuff is to fabricate. Copper, of just about any size, can be intimately joined by soft-soldering, while "found" items made of stainless or aluminium are far more tricky: as alluded to in my OP.QuoteYes, as well as various Unenlightened who try to use the term for absolutely any antenna made out of a canhttps://hackaday.com/2018/01/23/an-ads-b-antenna-built-from-actual-garbage/Unfortunately, we live in a world in which vocabulary is in increasingly short supply, with competing specialties in vicious territorial disputes in their attempts to find pronounceable neologisms. However even the disruption of "peak vocabulary" doesn't absolve people of some minimal duty of care when recycling: one really is supposed to check that nobody's using stuff before appropriating it.

author: MarkMLl
author_rank: Frequent Contributor
timestamp: June 12, 2023, 07:36:39 pm
content: I've just soldered a second (tinned mild-steel?) can onto the first, and am now getting somewhere around 15dB (+-5) off-axis rejection. Centre still at 2.430GHz, SWR looks like it's gone up to about 1.75 (subject to my cheap gear and undoubtedly my poor technique and understanding :-)For something that's likely to take abuse it's not very robust, but I'd certainly call it an at-least adequate proof of concept.MarkMLl

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 01, 2023, 06:50:21 am
content: I made a cantenna with about 14dBi gain. I used 4 inch dryer pipe and a 4 to 6 inch duct reducer. (Try to get a thinner pipe because I'm pretty sure 2.5GHz is above the cutoff for another mode in a 4 inch circular waveguide)It's important to make the length of the dryer tube 3/4 + 1/2 * N wavelengths. I'm pretty sure 3/4 wavelength would work the best but I made my tube longer and it didn't really affect things (though I'm pretty sure it made my bandwidth worse). Mine was 4.25 wavelengths because I didn't have the tools necessary to cut the tube to size, and I ended up trimming a little from the top. Measure from the end of the pipe covered with metal to the base of the slanted part for the duct reducer. The only thing that I found to be very critical was the length of the feed element. It took multiple attempts to get the dip in S11 centered at 2.45GHz. My construction was pretty bad (everything was held together with copper tape, and I just copper taped one end to make the reflector) but it seems to work acceptably well.Of course my application (which is making an antenna for a radar project) is probably different from yours so you might need to spend some more effort figuring it out. But I did get a higher signal strength when I tested it out with a wifi dongle.

author: MarkMLl
author_rank: Frequent Contributor
timestamp: July 02, 2023, 07:27:23 pm
content: My 84mm is a bit below your 4", and of course mine are straight, but it's interesting that the results appear close. I'm eyeing up some stainless storage containers (one straight, one conical) in case I decide to tidy things up a bit: a cantenna prominently labelled "Tesco Drinking Chocolate" takes "nothing to see here, keep moving please" a bit far :-) The interesting thing there being whether I could solder stainless- using the correct flux- as easily as I soldered mild steel... plus of course how careful one has to be about a small step at the join etc.Assuming that there's a bit if strength in the tube one could probably drill a spiral or zigzag so that the axial position could be adjusted in (say) 3mm (1/8") steps, and the probe could be made out of something like a threaded brass standoff to make it adjustable.I've thought about using things like copper tape for joining before. However the thing that concerns me there is that there's going to be an adhesive layer between anything that isn't also mechanically fitted, and that's effectively going to be somewhere between a capacitor and an open circuit.MarkMLl

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 03, 2023, 03:02:17 am
content: Here's a picture of my antenna (as well as the rest of my project). You can see that the S11 is under -13dB over 2.4 to 2.5GHz. I could not solder onto the steel so I ended up just wrapping copper tape around everything, and then soldering to that. As long as you apply pressure to the copper tape while sticking it down, it makes a good enough electrical connection. I assume the adhesive layer is thin and easily penetrable.I measured the gain without an anechoic chamber, so take the 14dB number with a grain of salt. I went outside and pointed the antenna at another antenna with known gain, then used the Friis transmission formula to determine the gain of the antenna.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 03, 2023, 03:59:03 am
content: If you guys post the dimensions I'll throw it in the simulator.The geometry is simple enough for a lunchbreak run and I'm always interested to see comparisons with real life results.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 03, 2023, 05:13:18 am
content: My cantenna is more than half a wavelength wide and 3/4 of a wavelength long. However, all the other dimensions are the same.Can you simulate my design with 0.75 wavelength instead of 4.25 wavelengths for the width? I'm pretty sure it would increase the bandwidth of the antenna without losing gain, but I'm not sure about that statement. Would be nice if you could verify that.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 04, 2023, 02:30:45 am
content: It certainly is doing some wavy stuff.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 04, 2023, 10:46:45 pm
content: Looks reasonable.You can improve the matching by messing with the length of the feed. I think it took me multiple attempts to get the exact length right.Also I'm wondering if making the tube longer has any effect. Could you try increasing the length of the tube by a few wavelengths?

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 05, 2023, 05:21:33 am
content: The feed length change didn't help much, but I realized that my ratios were way off. The tube length and the feed position should use the guide wavelength while the diameter and the feed length is based on the free space wavelength.I got much better match, but it needs to be adjusted to bring it lower.I just have to redo my model to be able to properly parametrize it in the sim software for sweeping.

author: MarkMLl
author_rank: Frequent Contributor
timestamp: July 05, 2023, 07:05:46 am
content: In my case the frequency was quite a lot down, about 2.3Ghz rather than the intended 2.45ish. However it's definitely directional and reasonable amount of gain at WiFi frequencies, which is a win for something knocked together in a hurry for a specific job. Sorry about the crappy photo, I really don't have time to mess around with a tripod etc. and for some reason a flash didn't seem to improve things... ;-)Must organise some screen capture software, which a bit of Googling suggests should be an easy job.To my surprise, I found soldering the two cans that I used (as a rush job) surprisingly easy. I sanded off the paint on the outside of one and the lacquer on the inside of the other, put in a small kink so I could nest them, applied SMD-style flux and soldered with 60/40 (lower MP than unleaded) and a PCB rework hot-air gun.The key point, I found as I was working, was to start at the kink where there's a bit more gap: from that point onwards I was able to go round the joint fairly fast as the solder wicked into it.@Marsupilamido you have any thoughts on the dual-band cantenna athttps://web.archive.org/web/20210512030616/https://www.wifi-antennas.com/topic/52-dual-band-cantenna/which I posted at the start of the thread? The discussion there appeared to go quiet, and it would be interesting to know how well his probe/loop arrangement might work.MarkMLl

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 05, 2023, 02:51:58 pm
content: Quote from: MarkMLl[member=767140I'll run it when I have time to finish the parametrized model.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 06, 2023, 04:04:46 am
content: Quote from: MarkMLl on July 05, 2023, 07:05:46 amDo you care about the 5GHz loop feed thing or the combination of both?Also there are two sets of dimensions, red and black, is it explained which one is what? Which one did you build?

author: MarkMLl
author_rank: Frequent Contributor
timestamp: July 06, 2023, 07:24:22 am
content: Quote from: Marsupilami on July 06, 2023, 04:04:46 amThat's not one I've built, I'm just interested in what somebody who obviously knows what he's doing thinks about that design since discussion stopped abruptly. I think a dual-band cantenna is of general interest if trying to sniff WiFi leakage, even if it were suboptimal for a fixed link. I think- from the discussion- that the black numbers in the drawing are the original (2.4GHz) dimensions and that he's penned in amended red ones after adding the 5GHz loop.I was digging around, and found that somebody has stainless spaghetti containers which (from elsewhere) appear to be sized approximately 9.5cm diameter and 29cm high, and also has conical biscuit tins of a similar diameterhttps://www.ebay.co.uk/itm/262943791596Obviously I'm fully aware of the difficulty of joining stainless without using TIG, but /if/ it were possible to do so the combination might be interesting.I experimentally snipped 1/2mm off the probe in my existing cantenna yesterday (since I don't immediately need it) which moved the frequency up by a few MHz at the cost of a slightly higher SWR, then snipped another mm off it... with interesting results which might or might not be usable. From that it's clear that while the overall design might be fairly forgiving, getting it optimal takes the same sort of careful design needed for any other antenna type.Must learn OpenEMS...MarkMLl

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 07, 2023, 03:06:34 am
content: A S11 of -10dB is decent, and you should be able to use it without any problems.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 07, 2023, 05:47:00 am
content: Quote from: MarkMLl on July 06, 2023, 07:24:22 amWell, in that case I'm not your guy.But also I still feel there is no real point to the dual band antenna this way since it still uses two separate feeds. Like if you only had one coax to hook up to you SA then fine, but if you have to have two separate connections you might as well ducttape the two tubes together and then have two properly tuned antennas instead of one that's kinda crappy at both bands.Tuning these is not trivial. A single simulation run takes 15+ minutes on my potato computer, exponentially more if I up the accuracy and then there are 4-5 dimensions that can change to sweep over.I'm sorry, I don't mean to complain. I offered to simulate these as I am interested too, I'm just trying to explain why is it not easy to do anything else then a single, precisely dimensioned drawing. Being able to tell whether a very particular configuration is good is still somewhat tedious, but to come up with "something like that" just better is an iterative time sync even with proper tools.At least you can snip the feed wire in real life and see the result immediately on the analyzer. I wish the simulator could do that.I'll have more time over the weekend I'll play around tho.Can you post the dimensions of your build as is now? We could start from trying to get a match on the S11 plot between your VNA reading and the sim and work from there on.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 07, 2023, 05:50:14 am
content: @Eggert,Same for you, can you clarify the dimensions as you built them? The second drawing has wavelengths and stuff. I'd love to see an S11 plot from you and my sim to line up.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 07, 2023, 12:12:02 pm
content: If you like, I can provide you with the dimensions for the horn I tested along with details for the transition. The nice thing is you could start with just the coaxial to waveguide transition. Once that model is correct, you could then add on the horn. I also recently replaced the directional coupler I am using to measure S11 with one with a higher directivity (and wider BW) along with some decent home made standards. I should be able to measure S11 fairly accurately. I can also measure the radiation patterns as well.Keep in mind this was all done in the X-band. I think I ran those horns at 10GHz.***Comparison of S11 using my original coupler and the new part. The original coupler was only spec'ed to 8.3 or so GHz and performed much better than I had expected. New coupler allows measurements above 12GHz.https://www.eevblog.com/forum/rf-microwave/experimenting-with-waveguides-using-the-litevna/msg4903016/#msg4903016

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 07, 2023, 03:29:12 pm
content: A solid model is better (STEP) but if the geometry is not too complicated then an STL is also easier than redrawing the whole thing from dimensions.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 07, 2023, 05:05:51 pm
content: You may download the 120mm length 100x75mm mouth RFHorn.7z from:https://github.com/joeqsmith/MiscFilesThis is only the horn. For the transition, I guess I am not sure how we handle it. Normally you would cal it at where the horn would bolt on. Not sure if you can do this with your simulation so you are only looking at the horn.***For simulation, if you want to assume the horn is using a silver plate, I believe that it what the production horns I have use.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 08, 2023, 01:08:03 am
content: I left my design at school and I'm back at home, so I can't really measure it precisely.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 08, 2023, 02:56:13 am
content: Quote from: joeqsmith on July 07, 2023, 05:05:51 pmThis is only the horn. I can excite it with a given mode. I only did the first one (TE10), the next one only starts above 13GHz.The S11 plot is likely not accurate at this resolution, because the better it gets the harder it is. It's very good over the whole band tho so I wouldn't be worried about that.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 08, 2023, 07:03:31 am
content: I don't have a chamber at my house and sadly, the office isn't large and void of junk. Then we are using home made standards with a $120 VNA with a poor mans home made frequency extender. So I doubt things would get much worse. Still, note the dips in S11 appear to align with your simulation.For the radiation pattern, using two of the same horns with same polarization, far field, taken at steps of 2 degree azimuth and 7.5 degree elevation. Again, a lot of junk in the office and I'm limited on how far I can get the two horns apart.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: July 08, 2023, 09:58:39 am
content: Ha!Ok, this match is pretty sweet.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 08, 2023, 04:38:13 pm
content: If I point it down the length of the office instead of the ceiling much of that ripple will go away. Even adding a slight angle helps a lot.With the most recent changes to my frequency extender, I can get somewhat meaningful data above 12GHz. The coupler is spec'ed to 12.4 but the bigger problem is the mixers are limited to a 4GHz IF. About 12.2 is the limit. On the low end, the LO, image rejection filter and mixer set the limit. 8GHz is pushing it.I attempted to cal it from 8-12.5. Shown with one of the 3D printed horns attached (#8). Again, very similar to your simulation.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 01, 2023, 01:38:51 am
content: Quote from: Marsupilami on July 04, 2023, 02:30:45 amI got similar results from using HFSS, getting 11.4dB gain. It looked like making the tube longer negatively affected the gain.The correct way to squeeze more gain out of the system is to increase the size of the horn aperture. It is probably possible to manufacture a 12 inch long cone which goes from 4 inches to 12 inches diameter, and this can be attached to the 0.75 wavelength long cylinder. From my simulation results I got 17 dBi gain with this setup, which is pretty decent.In terms of manufacturing, it should be easy to cut out the nets for the cone, cylinder, and the reflector on a copper sheet. A 3D printed mold can be used to ensure that everything stays circular, and all the copper pieces can be joined with solder.