Extending VEXnet 2.0 range

Doed anyone know if the range on VEXnet 2.0 canbe improved? Doesn’t matter if it involves modifications, its not for a VRC application.
I can use some old black WiFi VEXnet keys which will be a bit better but would like to increase the range of the white keys if possible. I guess a decent antenna might help.

If you want a lot of range, switching to a waveguide antenna would help a lot. Most of that would be fairly easy. The problematic part would be opening up the controller and gaining access to its internal antenna. Some of the older controllers with the external antennas are much easier since you don’t have to open them up. I haven’t opened up one of the newer controllers to see how difficult the process would be, though.

The antenna is in the VEXnet key, which can be easily pried open…

Usually both ends have an antenna, not just one. Look at some older VEX joysticks and you can see the external one quite evidently.

It would also be a really, really bad idea to put a waveguide antenna on the robot. Waveguide antennas are directional, which is why they work so much better over distance for certain purposes. This isn’t much of a problem for a controller controlling a robot at a distance because the robot is unlikely to move really far very quickly. But on the robot, you’d have to make sure you keep reorienting the antenna as the robot moves, which would generally require rotation around a couple axes.

I’m very confused now. A VEXnet 2.0 key is used on both the controller (joystick) and Cortex. In both cases, the antenna is contained within the VEXnet key.

Only the old crystal system had external antennas, but that is unrelated to VEXnet 2.0-based communication.

No, my bad. I’m confused. You’re right. I don’t know why I’d forgotten about the key there, just haven’t had a key attached in quite a while and spaced out.

So, yes, you’re right. Open up the VEXnet key. However, if it’s just an antenna connected to the USB with nothing extra inside, even better. You can order USB antennas for computers. Get one of them. Unscrew the antenna. Connect a cable from where the antenna had been to a homemade waveguide antenna. I used to have students in electronics build them. They could use their laptops on the network from a quarter mile away from and outside of the building the nearest wireless point was located in.

Go to Internal Photos in the FCC filing to see what the inside of a VEXnet key looks like. The antenna is labeled in the last picture, seemingly on an internal layer of the PCB. Doesn’t look too easy to work with.

Well, there is an alternate solution. VEXnet runs over the same frequency as wifi, 2.4 GHz. If the key itself is just to translate from USB to the 2.4 GHz signal, as it probably is, then any 2.4 GHz USB antenna should do. Has anyone tried this out? Just hook up a USB wifi antenna and see if everything works fine? You can buy USB wifi antennas with antennas that can be unscrewed, as I mentioned above.

So the first thing I would do is pull out the VEXnet key and replace it with any regular USB wifi antenna you can find. If everything works fine, then get one with an antenna you can remove. Building a decent waveguide antenna takes very little effort, and there are lots of instructions to be found online. Then you just need to get a cable with the appropriate two ends, one to fit where the antenna had been and the other to connect to the waveguide antenna. You should be able to run your robot from a significant distance by doing this, so long as the waveguide antenna is aimed at the robot.

It might work, but I highly doubt it given that VEXnet uses special firmware to transceive encrypted signals. Also, I believe the Cortex and controller are specifically programmed to only interface directly (by a USB A-A cable) or by VEXnet.

There’s no such thing as a “USB wifi Antenna”.
Any USB wifi dongle is a full network adapter, and pretty much each such adapter has a different USB protocol towards the host and needs special drivers.
VEXnet was pretty much a network adapter for the standard 802.11g network. Still, that only means the airwaves were the same as wifi, the communication over the USB side of the dongle was specific to the chipset used and the dongle firmware. You could have used any 802.11g capable device (laptop) to observe the air traffic, but neither joystick nor the Cortex would be able to use any other 802.11g USB dongle.
VEXnet2, in my understanding, went even further and used their own air protocol too.

Now to the possibility of extending the range - you can turn the VEXnet key into a directional antena - the more directional, the longer distance it can span. As @callen mentioned, you don’t want to make the robot side directional (much), but you could do so on the controller side.
It could be pretty easy - search for “can antena” (pringles works well, since it is lined with aluminum, yet easy to cut).
You would plug a short USB extension into the controller, affix the vexnet key in a good position inside the can (with the USB end sticking out through the wall) and connect the other end of the USB extension.

Now, FCC might not like it, which might be enough reason to not go that way, but let’s talk about some theory:
Besides modulation, interference and plenty other things, there is a Tx power limit associated with a license or band usage. In USA, for the ISM band (2.4GHz), I believe the limit is generally 1W to antenna and 4W EIRP. You’re not changing the device itself, so nothing happens to the power to the antenna and it is actually much lower - 30mW according to the FCC filing. So what is this EIRP?
It stands for “Effective Isotropic Radiated Power”.
If you take all the power output of the transmitter and feed it to an ideal isotropic antenna (an antenna that radiates equally in all spehical directions), you have gain of 0db (or multiplier of 1) and your EIRP matches the transmit power. But if you use a directional antenna (and the can antenna is a very directional one), you take that power and radiate it into a subset of the space. The smaller the arc, the more concentrated the radiated energy is. This is called antenna gain. If you radiate into 1/100th of the sphere around the antenna, you have 100x higher energy density, or 20dB of gain. It also means your EIRP is 100x the transmit power, since from the direction of your gain, it looks like if you were transmitting at 100x the power from an isotropic antenna.
This means that to stay within the regulations, you need to limit either the gain or the output power so that power * gain < 4W. Given that VEXnet2 key only uses 30mW, you should have quite some room for gain, I’d say.

One more compilcation is that the VEXnet2 key uses antenna diversity - 2 orthogonal meander antennas that it switches based on the link quality. But you’d only be able to place one well into the focus of the can. What would the firmware do in such a scenario, I have no idea :wink:

Thanks all, good old black keys it is then!

I know what you mean, which is why I tried to couch my statement conditionally upon how it communicates. It looks like perhaps with VEXnet 1.0 there wasn’t the same approach as with VEXnet 2.0. As for no such antenna, they’re labeled roughly that way, such as “TOMTOP 300Mbps Wireless USB WiFi Adapter With External Antenna,” which is why I said it that way.

So the next question would be if VEXnet 1.0 (with the black keys) used the standard computer protocol or not. If so, switching over to them, this could be done. If not, if the antenna can be accessed more easily inside the black ones than inside the white ones, that could still work.

I would recommend against this. The Pringles can really doesn’t work especially well, though surprisingly well considering its problems. It’s more slender than you want, having a radius/diameter smaller than the cut-off you want to avoid. Also, as it’s so slender, that means you need it to be fairly long, and if memory serves that may actually be longer than the Pringles can. It’s also not a very tough can (e.g. don’t get it wet). Separate from the Pringles can, I can’t remember right now how useful it is to have the antenna grounded to the can itself, which is normally done.