# miniature parabolic antenna

I have to build a 2.4ghz wifi antenna with one SMA feedpoint. The rules are that it must fit inside a 15cm cube shaped box. For testing, the antenna will be pointed at a laptop that is moved to various distances however remains in the same direction. Therefore, I want to build the most directional antenna possible. Upon a quick google search, I started looking into making a parabolic antenna. However, since most parabolic antennas I have seen are at least 1 or 2 feet in diameter, I was wondering if making such a small parabolic antenna would still be effective and better than other types of antennas. If not, what other types of antennas should I look into? Yagi was one option, but I thought parabolic might be better.

From this equation, for a passive antenna of 15 x 20 cm, you would expect a gain of under 14 dBi. $$A_e = 0.15 * 0.2 = {G\lambda^2\over{4\pi}}$$

I don't recommend trying to make a reflector antenna this small. There are two main problems:

First, you have to illuminate them properly. For highest gain, you want uniform illumination, this means the feed provides a constant power across the surface of the dish, and nothing outside that. This is difficult in practice, so there's usually a smooth taper from centre to edge, where the power density at the edge is about 6 to 10 dB less than the centre. The rest spills over the edge and is lost. This leads to practical (large) dishes having an effective area only about 60% of their physical area. This will reduce the gain of your antenna by ~ 3 dB.

Second, the feed itself is not small. At 2.4 GHz it might be a patch or a yagi - both are in the order of 5 x 5 cm at least. This is hard to fit in the box, and the feed will block some of the signal.

Finally, given the size constraints, the "dish antenna" will be almost indistinguishable from a "corner reflector" antenna.

The great thing about building wifi antennas is that you have quite good quality test equipment available, in the form of your laptop or phone.
Find a wifi signal strength app (ubiquiti wifiman is excellent). Set up your antenna on an access point, old router etc. In an open space, walk slowly away from the antenna and measure the signal strength with your phone. (rotate your phone around to get the strongest signal, and hold it that way). This will let you compare the performance of different versions of your antenna, without any special test equipment.

Start by considering a corner reflector antenna - this will fit nicely into the box. I've linked to the original 1940 patent but there are many websites about it.

You could make the reflector from aluminium foil taped to cardboard (as it doesn't have to connect to anything). The dipole could be a simple wire dipole; the absolute simplest would be to salvage a dipole from an old wifi router, as that already comes with a connector and cable...

The best gain will probably be from a 2 x 2 patch array. You'll need a PCB to split the power and feed the four patches, this would need to be microwave board to keep the losses down. The patches themselves should be air spaced, at least 5 mm above the PCB, fed with a pin or an L.

If you don't have access to microwave PCB manufacturing, but you can simulate the structure, you will probably do almost as well with a single patch, and a groundplane that is somewhat cupped. The ground can be 15 x 19 cm with a ~ 3 cm sides. The patch needs to be $$\lambda/2$$ on the active side, but can be $$3\lambda/4$$ on the other side, so say 61 x 90 mm in air. Mount it about 6 mm from the ground. Feed it in the centre of one edge, directly with the inner wire of the SMA connector.

• Nice alternatives. I'd suggest to the OP that yagi types tend to be narrow-band, requiring precise construction. Corner reflector is less fussy. Arrays are nice if you can split power evenly and maintain phase coherence. Commented Nov 25, 2022 at 19:31
• yes! Patches with wide air spacing are also super-forgiving. A yagi would definitely require a network analyser to tune, especially to fully cover 2.4-2.5. Commented Nov 25, 2022 at 19:33
• For a corner reflector, would it be worth it to make it a cone shape? (revolve the corner reflector around its axis) I have access to a 3d printer, so shape isn't a problem. Commented Nov 25, 2022 at 20:05
• Also, does the half wavelength antenna begin where it says antenna, or is that where it ends. Commented Nov 25, 2022 at 20:06
• A corner reflector is two flat plates behind a dipole antenna. I don't think there's any benefit to curving it. Look at the 3D sketches in the patent. The dipole is vertical in the figure in my answer, i.e. into the page. Commented Nov 25, 2022 at 22:17

Try a Kraus helical antenna.

The Kraus end-fire helical has wide bandwidth, circular polarization, and is very non-critical in its dimensions.

At 2.4 GHz your wavelength $$\lambda = 12.5\text{ cm}$$. Then the dimensions of your helical antenna would be $$\text{circumference} = \lambda = 12.5 \text{ cm}\\ \text{diameter } D = \lambda/\pi = 4 \text{ cm}\\ \text{turn spacing } S = \lambda/4 = 3.1\text{ cm}$$ Groundplane should be at least $$\lambda/2 = 6.25 \text{ cm}$$ across (square or circular).

Inside your $$A = 15\text{ cm}$$ box you could fit a helix with almost five turns. This would give you a gain of perhaps 10 dBi.

This is a circularly polarized antenna. For Wi-Fi you want right-hand circular polarization (RHCP). Make sure you wind the helix in the correct direction! As your right thumb points along the axis and toward the target, then your right fingers curl in the direction of the twist.

If you can measure the matching, you can lower the input impedance by tweaking the first half-turn of the helix closer to the groundplane.

In contrast to a Yagi-Uda antenna, the Kraus helical is quite forgiving in its dimensions. Here's an example improvised Wi-Fi antenna which actually works fairly well:

Another fun hack: build a LHCP version, and bounce the signal off a wall. The reflection will swap the polarization from LH to RH.

• Good idea, and easy to build. Why specifically RHCP though? Wifi antennas are generally linear so either sense will work well. Commented Nov 29, 2022 at 21:51
• If the other end of the link is linearly polarized, then yes either RH or LH would work. But there do exist circularly polarized Wi-Fi antennas, and invariably they are RHCP. So build yours RHCP to stay compatible. Commented Nov 29, 2022 at 22:05
• @SteveGolson-W1SEG there's circularly polarized Wifi – wow, didn't know that. Also feels a bit counter-intuitive, wave-wise, since Wifi's signal design (OFDM) assumes a multipath channel, and thus reflections, and reflections can (and will) have the opposite polarization! Commented Dec 4, 2022 at 12:42
• @MarcusMüller I think the CP WiFi antennas are mostly intended for highly directional point-to-point links. CP antenna on both ends gives you some nice gain. If the base station has typical ~omnidirectional linear polarization, then CP antenna on the remote station will be advantageous because you won't get as much polarization mismatch. Commented Dec 4, 2022 at 23:11
• Ah. Makes sense! Commented Dec 5, 2022 at 8:20