IntroductionAs part of an upcoming project, I needed a quick and easy way to amplify the reception of a 428 MHz signal coming from a specific location. Tradeoff - it has lower gain than 2 ele yagi b.August 26th, 2019 (Last Updated: August 26th, 2019) 01. It has a broad front lobe. Loop with a 4 el Yagi on 20 m.A lot of the things people rave about on the Moxon are merely design tradeoffs.428 MHz is a pretty specific and non-standard wavelength for a directional antenna (i.e. I needed it to be highly directional because I only care about the 428 MHz signal coming from my signal source and not any other random 428 MHz signals that happen to be where I’m pointing my antenna. I needed it to be high gain because I’m not going to be directly next to the signal source and probably won’t have uninterrupted line of sight, and a high gain antenna allows me to be some combination of “far away” and “there are a lot of walls between me and the signal”.They modify the radiation pattern of the radio waves that hit them, re-radiating them with a different phase. DirectorsThese are the parasitic elements of the antenna. I used PVC pipe from Home Depot, so my boom was non-conductive. It can be conductive or non-conductive, its conductivity just affects the length of the other antenna parts. If you’ve ever seen a spindly antenna on top of someone’s house, it’s probably a Yagi-Uda!Its ease of construction and the cheapness of materials combined with the amount of gain provided makes it an attractive option for DIY projects and prototypes.If you’re new to antenna building, there will be a lot of new terminology! Here’s a diagram:This is the part of the antenna to which all the other elements are fastened. Lucky day! I get to make a Yagi-Uda antenna!A Yagi-Uda antenna is a very easy to make high-gain highly-directional antenna.This is why adding more directors also adds more gain to your antenna. The length of each director is such that it re-radiates waves that are at different phase offsets, aligning them all together for as much constructive interference as possible. We can see that after the red wave hits the director and is re-radiated, the two waves are now in phase and the resulting signal is stronger. Let’s say this director’s length is tuned to modify the phase of the red wave. These waves are offset from each other.
![]() As pictured above this is just a length of wire, but I decided to make a folded dipole, which looks like this:This is the longest piece of the antenna and sits opposite (“behind”) the driven element from the directors. It has the directors on one side and the reflector on the other. For me it’ll be receiving, and this is what the coax cable will be connected to. Driven Element ("Dipole")This is what radiates or receives the radio waves for the antenna. 2 Elements Yagi Antenna Calculator How To Interpret TheI didn’t take pictures of the build process for this prototype, so some process shots will be staged and look slightly different from the final result.A three-element Yagi-Uda is kind of an edge case so I used a calculator designed with that constraint in mind ( ). This initial prototype only had one director, a folded dipole, and a reflector, making it a three-element Yagi-Uda. I’m going to walk through my build process, the calculators I used, and how to interpret the output of those calculators.It’d been a few years since I built a Yagi-Uda antenna so I decided to start small and test that my principles were sound before starting on a bigger build. These calculators usually assume you’ve already built this kind of antenna in the past and spit out a list of measurements, which can be kind of intimidating for someone who doesn’t know what they’re doing. These reflectors can be more complex, like pictured on the TV antennas above, but for my purposes a single length of wire was fine.If you don’t have a lot of experience using antenna modeling software, you’re probably just going to go to the Internet and look for some calculators (and even if you do, you’ll probably still go to the Internet and look for some calculators). This requires that you precisely measure where each element is positioned tolerances are usually less than 2mm. SecondFor positioning the elements in the prototype I decided to do what I’ve seen a lot of other Yagi-Uda antenna builders do, which is to drill holes where the elements are supposed to go. It’s also nice to have some wiggle room in case something goes wrong during fabrication. Measuring tape capable of measuring millimetersCut the boom material to the length specified (280mm) + buffer (I used a buffer of 20mm for a total of 300mm).I added 20mm here because you’ll notice that the reflector position is at 0mm and the director position is at 280mm so cutting a boom length of exactly 280mm wouldn’t be great. We won’t be using the dipole length because we’re going to make our own folded dipole. Sagemcom router setup for spectrumMake sure as you’re cutting your elements you keep them in order! They only differ in length by a few millimeters.Normally the reason that you’d choose to use a folded dipole instead of a half-wave dipole when constructing a Yagi-Uda antenna is to more easily match the impedance of the coax you’re attaching to it. Drilling too small a hole is okay because you can always make it bigger, but too big a hole will make your elements wobbly and it’s a harder fix.For the reflector and director I used the measurements from the calculator above. Make sure the drill bit you use matches the diameter of your copper wire. Using the aluminum and vise gave me a good guarantee for #2, and using my shop’s drill press in combination with the aluminum/vise assembly instead of a hand drill gave me a good guarantee for #1. ThirdIt is important that all of the elements are:To these ends I zip-tied the boom to a spare piece of right-angle aluminum I had in the shop and clamped that in a vise. Measuring from the end of the boom with a measuring tape is way easier and more consistent than trying to keep the end of your measuring tape hovering above a Sharpie mark on a PVC pipe. ![]() ![]()
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