Kent Electronics, WA5VJB 2.4 Ghz Antenna

Often PLers outside the US do 2.4 Ghz WIFI links for show control. Those who do not have "Spatial Diversity" setups, where there are two antennas at each end of the link, could appreciate this antenna. Diversity setups are nice because the receiver has a voting circuit which chooses the stronger signal, compensating for multipath and fade issues in a venue.

However, not all WLAN boxes are good quality. Most are fitted with antennas that are designed to have a nice spherical pattern, and the antenna is adjusted to have just the right radiation pattern to fill a house or small area of a business. After all, if all boxes blasted away with good antennas and high RF power, one could not reuse the WIFI frequencies the way modern systems do, and every time a neighbor put in a new box, we would all have to readjust our boxes.

So what ya gonna do when you do not have enough range, or your being jammed by another license free device that shares frequency spectrum with you? Well, you can try changing the channel, but that does not always work, for reasons that I'm not going to get into. Those that work with radio professionally do not crank up the power, they crank up the antenna.

In my case, a little 50 mW FM video sender at my church had a noisy picture. 1oo feet was just too long a shot for the cheap, low gain, single element patch antenna in the video sender. So I did a little hacking. I sliced off "stripline" on the pcb board leading to the patch antenna, and shorted out the patch to ground. Shorting/removing the internal antenna is imprtant to make sure it does not interact with the RF field when you add in the new antenna.

I then used 50 ohm micro coax to connect up the WA5VJB YAGI antenna, and checked the pattern with a diode detector and a level bar on my frequency counter.
I now have a roughly 7 degree wide beam coming from the video sender, instead of a omnidirectional pattern.

SWEET!, the picture went from a P2 to a P5. The P scale is 0 to 5 for analog video, with a 5 being a perfect picture with no noise. 0 is no signal, 1 is a picture that rolls or blinks, 2 is marginal and noisy, and so forth, with P5 being perfect.

A quick test hacking a USB 2.4 GHZ WIFI produced similar results. I quit testing at 500 feet.

A really scientific test with a 2 to 4 Ghz swept signal on my RF workbench showed a VSWR of 1.2 or less at 2.4 Ghz.

You need the right low loss, very small coaxial cable, this can come off a 4$ reverse SMA connector for LAN stuff, from say, Sparkfun Electronics, and you need to find a place in your device where the impedance is 50 ohms. Most patch antennas and "F" antennas in WIFi gear have 37 Ohm impedances, so you have to locate the part of the circuit board that has a 50 Ohm trace, This is a narrow trace, that usually widens to a 37 Ohm trace.

Or if your lucky, your device just has the usual reverse SMA connector and you can just buy the cable from Sparkfun and solder it to the antenna.

For WIFI, you want the 2.4 Ghz YAGI, unless you have 5.8 Ghz wifi. For robotics, you want a "Wheel" antenna, and he sells those as well.

6 dBi of gain delivers roughly four times the Rf power at the receiver. For a bi-directional link like WIFI, you may want a YAGI at both ends.

You do do want it mounted in a plastic box, and to keep metal away from the antenna. You want to use nylon screws when mounting the antenna, you can see in my pic the metal tripod mount is spaced well away from the antenna, and the old shorted out patch is now in line with the antenna and spaced far enough back to avoid interference, as it is behind the last or "reflector" element on the yagi.

As it is "Planar" there are no sharp bits like on a traditional tubing YAGI to poke your eyes out.

Not bad for six bucks and shipping:

http://www.wa5vjb.com/products2.html

This just saved going to a 80$ bidirectional amplifier.


Note to RF folks, yes, he does make his antennas on low cost FR4 instead of expensive Teflon or Duroid. He does compensate well for the FR4's extra loss tangent at 2 Ghz. But they work well, and to specification.

Now we can beam images of the kids in the children's chapel to the main video screen in the sanctuary. This makes parents happy when they can see their kids doing what kids do.

Steve


Heres a PIC: