Tamara Wilhite is a technical writer, industrial engineer, mother of two, and published sci-fi and horror author.
In this article, we’ll look at several designs for cheap free space yagis for receiving signals on the 2450 MHz or 2.4 GHz band. This frequency range is most common used for wi-fi and ham radio.
This article was written with input from Kent Britain, WA5VJB, of Kent Electronics. He is an industrial engineer with decades of RF design experience, and he's the designer of the antenna above that is the design the DIY instructions is trying to emulate.
Free Space Versus PCB Yagi Antennas
A free space yagi antenna contains the same elements as a printed circuit board yagi, though it may be proportionately smaller or larger. The main differences between printed circuit board yagis and free space yagis are that free space yagis don’t come made on a printed circuit board and can be made by hand. The benefits of the PCB yagi antennas include buying it off the shelf ready to use and getting an antenna that can’t be knocked out of alignment when carried or dropped.
The picture here is of a Kent Electronics 2.4 GHz printed circuit board yagi antenna. The antenna shown here and the one this article discusses cover the 83 MHz portion of the ISM band.
The PCB yagi here is about 25% the size of the free space yagi being built. The size doesn't matter as much with antennas as the spacing of elements and various ratios relative to the frequency range you want to receive.
To make either free space yagi antenna design, you will need 1/16” material for the elements. While silicon bronze welding rods are recommended, 14 gauge copper wire can be used. Don’t use insulated #12 wire because the insulation will move the antenna down the frequency range about 5%.
You also want a ¼” wide wooden rod or dowel onto which you will attach the wire elements.
You will want to use superglue to hold the antenna elements in place.
One wires or elements will be shaped into a J shape, turned to run parallel to half of the element. The bend of the J should be 0.2” to 0.25” wide, just so it goes back into the boom. The radius of the bend and the distance between the tip and the body of the J are not critical.
You should use a flat file or expose it to a belt sander for a literal second to remove jagged ends from the elements so this doesn’t affect performance.
Superglue can be used to hold the elements to a wooden rod. A quarter inch square wood piece from the hardware store is sufficient. Drill holes through the wood to insert the elements into. Why is wood recommended instead of a metal rod? Wood still conducts electricity due to the cellulose and moisture, but the impact on the antennas performance is less than if you connected the antenna elements to a metal rod. For every inch of wood you use, about one tenth of an inch will need to come off the length of the antenna. This won’t affect performance of the yagi on the 2 meter ham band, but it means the antenna’s performance is affected at 400 MHz or more if you use a very thick boom. The dielectric effect on the antenna is why PCB yagis use neutral fiberglass.
The shield of the 50 ohm coax cable goes into the center of the driven element. The center of the coax goes to the inter tip of the J element.
Spacing for 6 Element Antenna
|Element||Length (Inches)||Spacing to Next Element (Inches)|
Spacing for 11 Element Antenna
|Element||Length (Inches)||Spacing (Inches)|
Gain for this antenna ranged from 12 to 13 dBi for the 11 element antenna to 9.5 to 10.5 dBi for the 6 element version. If you need to adjust the return loss or the standing wave ratio (SWR) at 2.4 GHz, the free tip of the J element can be trimmed.
Its reception will be best around 2450 MHz frequency or the 83 MHz ISM band.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.