Building a Budget 16dB 5GHz Horn Antenna with a 3D Printer and Copper Tape

Introduction

Recently, I’ve been getting more into high frequency RF design. This comes at the cost of typically expensive ICs, money on tools, more money on prototypes, and a lot of reading. I don’t really mind the reading too much, but, when I set out to do a project that would require a 5-6GHz antenna and saw the price enter the $300+ range, I thought almost about scrapping the project.

Expensive!

Luckily, I recently watched a good saveitforparts video on 3D printed satellite antenna, so I knew it was maybe possible? It’s for the 1.7GHz L-band, so we only need a few more GHz to get to 5-6GHz. This led me to eventually find the holy grail: an entire post from an antenna testing lab company on 3D printing antennas. I love this stuff, I really wish it were more popular to post stuff like this; sucks that it feels like a lot of the RF industry is pretty tight lipped. Anyway, they essentially recommend conductive spray paint and acetone smoothing. Just one problem, the conductive spray is 1, kind of hard to get in Canada (??) and 2, expensive; ugh.

Guh!

Hunt For Conductivity

Well, the spray is a bit outside of my budget, I believe I might have an alternative? The goal is that we just need something that has high conductivity which will reflect the RF signal. This is when I remembered this copper tape I had bought to make a cavity filter; looks like it worked great for that use case, so why not a horn antenna?

Dirt cheap! My style!!!

Some simple testing showed that the resistance over a foot couldn’t be measured with my cheap tools, so it looks like it might work perfectly for my case? Only way to find out is to test it.

Designing The Antenna

I recently started learning CST Studio while I started this project as I was hoping it would help me validate PCB designs before production, but, it turns out it is really good for antennas too. I have just enough knowledge to model a horn antenna and perform some simulations. Even though it sounds easy, when you don’t know what you are doing it takes a lot of effort. After about three days of googling just about every waveguide calculator and horn antenna design formula, I finally scrapped everything and ended up just running the built-in solver to brute force a solution over the course of about a day.

16dB Horn Antenna Farfield & Directivity

Looks pretty good in the farfield and directivity; seems like we should be getting about 30 degrees in front of the antenna which is plenty. The S11 chart below shows us what frequencies this operates the best at which should be around 5.5GHz.

Sharp dip at 5.586GHz

The dip is a bit much, but as long as it is roughly below -15dB over the 5-6GHz range we should expect around 90% of the energy sent to the antenna to not be reflected back. This doesn’t mean 90% of the energy is being radiated, though, that is a whole different problem to look at later.

Validating The Design

Since everything looked good in CST, I simply sent it right to Fusion 360 and modified it slightly to be easier to print and then sent it right to my printer. I decided to use ABS for these, not sure why, but I think it expands and contracts less from temperature fluctuations (I should have looked this up) meaning that any changes in temperature or weather shouldn’t affect the overall performance.

Forgot to make the antenna hole bigger, whoops

It came out pretty rough, I did zero optimizations and literally just exported the model and printed it. Thankfully, this is still enough to make sure it all works. I spent a good chunk of time using a soldering iron expanding the hole where the SMA port was supposed to go and then tried to tune it using a LiteVNA64.

First revision of the antenna!

The S11 looks awful, BUT, it does still radiate, meaning this idea works. I decided to jump a bit more on this and make it easier to print in a new revision since I think most of the problems came from the poor tape job.

Really poor S11 (not a great antenna), but, it still works!

The Second Revision

The first major change in this version was that I broke it up into multiple parts. I believe the biggest issue was not having a flat, smooth, conductive surface, which the original 3D printed antenna blog post talks about. On top of that, I also made the SMA port actually let the connector sit in it so I wasn’t melting the hole to fit it in again.

Second revision of the antenna! Much Cleaner...

Once more, I hooked up my VNA and trimmed the little antenna wire until I got a good S11 value at around 5.5GHz. First impressions are that this was a HUGE improvement over the last revision. We have increased our S11 from roughly -10dB to -15dB and now have a nice dip of -23dB at around 5.766GHz. Definitely on to something, but, I think I can do a bit better.

Looking much better

The Final Revision

The second version performed really well, but, I think there might have been some issues with the conductivity between the components of the device. The only real difference between this one revision and the last one is that I added more screw holes to really keep it tight. The second thing I did was apply more copper tape between the components to ensure there was as much contact as possible.

Lots of copper tape

Applying the copper tape between the components and ensuring it was as smooth as possible were the main goals of this revision, and it looks like all the work paid off. We are finally at -20dB with a very clean dip over 5.5GHz to about 6GHz with it reaching almost -30dB at one point.

Wow!

Well, I think I did it? I’ve put some more tape around some of the layers where one part connects to another and it did improve it slightly, but, other than that, it looks like it works very well!

Testing Directivity

One other thing I’ve always wanted to try was to map out the directivity of an antenna - any antenna honestly, but, this project pushed me to just do it. Typically, this kind of testing is done in an anechoic chamber which is able to absorb the signals so that it measures only energy coming directly from the antenna. Sadly, these don’t really exist where I live (or are in my budget), but, I do have something that can prevent RF signals from reflecting; an open field!

Anechoic chamber, designed to absorb all the stray energy (https://blog.solidsignal.com/tutorials/what-is-an-anechoic-chamber/)

An anechoic chamber is kind of like an open, endless field for radio waves so I don’t see why this wouldn’t work. I even happen to have a wide-band cheap directional antenna kind of like the one in the photo above, although, mine is definitely not designed for this. That didn’t stop me, though, so I went right to CAD and designed holders for the receiver and transmitter to attach onto some tripods I had around.

Antenna mounts

With everything fit, I moved it all outside into a big field and went to town. The setup was to have port 1 connected to the horn antennas and port 2 connected to the receiver. From there, I used my phone to move the horn in 5 degree increments and recorded the S11/S21/S12/S22 values.

Test configuration

Now I have a lot of data of the relative strength of the antenna from about 3-4m away at various angles. Using Python, I wrote a script that took each angle, normalized the strength, and then plotted it on a polar plot. Observing below, we can most definitely tell it is very directional. The beam width looks like it’s about 26-27 degrees, smaller than the simulated roughly 30, but that isn’t bad.

Directivity plots

Conclusion

Overall, looks like the antenna works great. I did a few more tests with a wifi card and as expected the range did improve. I don’t have numbers for it as I didn’t write them down so will just have to take my word for that. Eventually, I do want to do more tests with this antenna, like calculating the exact efficiency. I assume the efficiency is probably around 50-80%, good enough for my stuff, but, I’d like to make some concrete numbers. I also have no confirmation on if we are getting 16dB at the output, it sure looks like it, though.

Another thing is this antenna can most definitely still be improved. I did print this at a smaller layer height (0.12mm vs 0.2mm), but, there are still a lot of ripples. According to that original blog on 3D printed antennas, they noticed quite a big change after smoothing it. So, for the final antennas I end up using for my project, I’ll give that a shot. For now, these work perfectly and as expected.

If you want to print this your self, checkout the Makerworld or Printables link.