DIY 2100MHz / 3G Yagi Antenna

Part of my family lives in a very remote location. There’s only AM and FM radio, i need to boost DVB-T signals from a repeater 40km away, and the phone landline fails around a week a month (trees falling and stuff).

So i had the idea to build a 3G/Wifi gateway. I didn’t wanted to spend 30€ on a comercial antenna to check if i had the required signal so i built my own.

It’s important to state that this was a trial: this build was never intended as a final product since this yagi wouldn’t stand a chance to strong winds.

Finished v1.0 build.

Intro

Design is based on a 16.5dBi from Audionuts (available here) and i spent around 5€ in parts – not counting tools and time, both freely available.

 

Construction notes

Construction is based on 6mm aluminium rods and some kind of non conductive boom. My local hardware store had these PVC mounts that are sturdy enough (around 1.8mm thick) and were fit for the purpose.

Everything is pretty much straightforward:

  1. Carefully mark the boom making sure that the markings are even on both sides
  2. Drill one side
  3. Drill the other side
  4. Cut the rods
  5. Insert and align the rods
  6. Build and install the feedpoint and boom support

It’s a good time as any other to note that the Yagi-Uda (that’s what this is officially called) is a very sensible design. On 2.1GHz precision should be well under 1mm. It’s also important to note that this design is – allegedly – 50 Ohm at the feed point. But this is only true if we are 100% precise on every step of this build. What happens if you screw up the alignment/length of the elements? The overall impedance changes. Then you’ll get high VSWR, and high VSWR means very low performance. So your 16.5 dBi Yagi might perform worse than a 5dBi Yagi….

This proved challenging, and we have the following Lessons Learned:

  1. Measure 3 times, drill once
    1. Remember that we are using 6mm rods. Drill should be 6mm.
    2. If the hole is two wide the rods will fall apart or become misaligned
    3. The holes need to be REALLY accurate. If one of the holes is 1mm off, the rod will not be parallel to the other elements
  2. Measure 3 times, cut the rods 3 times
    1. Yeah, you’ll fuck up the first time for sure, so it’s better to cut wide first and adjust the length one step at a time
    2. I used an hand saw. If you cut on a specific spot, the blade will “eat” at least 1.5mm – check image:

      The blade will eat much more aluminium than you’re thinking
  3. Install all the rods
  4. Build the feedpoint
  5. Install a mount.

Feedpoint construction

On 4., i initially considered screwing the coax feedpoint to the feed element as you see above via a couple of screws. This proved impossible since i used RG8/U – a very unforgiving , stiff cable – and screwing that is not for the faint of heart. Lesson learned: you can’t solder aluminium… You can, but not with your usual 60/40 solder mix… Plan B: heavy gauge earth cable, lots of solder to make it tough. Worked great!

Sure this won’t make it 6mm in diameter. So that will screw up element spacing, so we won’t get 50Ohm at the feedpoint. You get the picture…. but off we go!

Further info on feedpoint balancing here, and here.

Finished product:

Glorious finished product

Performance and final notes

I had around -102dBm with integrated antenna, and -91 to -95dBm with this DIY antenna with speeds floating between 0.5Mbps to 3Mbps. So, it does bring some performance that didn’t existed before but it’s not consistent.

Objective achieved: time to invest some money on a comercial available antenna.

This was hooked up to my Huawei E3131 which i had to mod to fit a small pigtail.

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