Introduction: Antenna to Extend Gate Opener Range
When the snow gets really deep on Mt. Hood, it's a lot of fun skiing, sledding, building snow forts, and throwing kids off the deck into deep powder. But the slick stuff is not so fun when we try to get back to the highway and open the gate to get out. The problem is the gate is at the top of an incline that is about 100 ft long. It's not a problem getting in because gravity helps but it's been a problem getting out because the gate can only be opened when you get to within about 40 ft of the opener antenna, which puts you right on the incline. Even 4 wheel drive is not that great when you have to stop to wait for the gate and then try to get started on packed ice.
I tried to find a commercial solution, but the best one was a simple monopole antenna and we already had that one.
The solution was to build a 3 element Yagi antenna tuned to the gate opener frequency and combine it with the existing antenna to extend the range. Shockingly, we can now open the gate from 170 ft away, giving us plenty of room to maintain our momentum up the ramp!
About 2 ft of .125" brass rod
About 4 ft of .125" aluminum rod
2 ft of 3/4" non-conductive tubing for the antenna beam
Non-conductive tubing for the mast to mount the antenna (can be the same as the beam)
RG6 cable and a crimp connector (depending on your system)
A 3D printer to print PETG, ABS or something else that won't melt in the sun (don't use PLA!)
Soldering iron, solder, 4 screws, silicone sealant.
Step 1: Find the Frequency of Your Remote
You first need to find what frequency your remote works on. I used an RTL-SDR dongle and SDRSharp to determine the frequency of the remote control. The manufacturer often lists the frequencies they use, but it's hard to know which one your control is transmitting. I looked at both 315MHz and 390MHz and found the signal at 390MHz. Interestingly, by recording the signal audio in SDRSharp, I was able to show it on Audacity and see the exact pattern from the DIP switches inside the remote. The exact frequency of the remote changes slightly every time, but that's part of the security of the system.
Step 2: Design the Yagi
I used YagiCad, a software simulator developed by Paul McMahon (VK3DIP). You can set the target frequency, and in my case I wanted a total of 3 elements so also defined a reflector and a director. The reflector sits behind the driven element in the yagi design and the director sits in front of it. Varying the length and spacing of these two elements gives a directional pattern in the direction of the director. In my case the simulated gain was about 8dB.
Step 3: Build the First Iteration
Any antenna design software only gives approximations of the correct behavior of the actual antenna. In my case, I used a 3D printer to print holders for the driven element and the reflector and director. You can see these at https://www.thingiverse.com/thing:3974796. The RG6 cable is separated into the core conductor and the shield and these are attached to either side of the driven element. Beware, however, that the shield on most RG6 coax is aluminum and you won't be able to solder to it. It will require a crimp that also connects it to a copper wire so that it and the core can be soldered. Clean the brass with sandpaper before attempting to solder, and make sure you use a lot of flux and a reasonably high wattage soldering iron. In my case 400 degrees with a 60W iron worked great.
Mount the two driven elements, zip tie them and then cut and place the reflector and director.
Step 4: Verify the Design Frequency
You need to verify that the antenna is resonant at your design frequency. To do this I used a Network Vector Analyser. This is a powerful unit that gives a ton of information about antennas. I purchased mine through Amazon and there are lots of these now available spanning different frequencies. The pictures at the top of this article show the graphical and statistical results of my antenna after tuning. The target was 390MHz with as low of SWR (Standing Wave Ratio) as possible and as close to 50 ohms impedance as possible. Resonance is also when the reactance (X) is closest to zero.
So, I trimmed the length of the driven elements until I found good resonance, then plugged the actual length back into Yagicad and optimized again for the frequency it simulated for resonance. This gave me actual lengths and spacings for the reflector and director.
Step 5: Make It Waterproof!
Since this antenna is going to be outside, it needs to be waterproof. I liberally squeezed clear silicone sealant all around the solder connections, then clamped the lid on. I filled the bolt holes with sealant and screwed them down. I was happy to see some sealant squeeze out around the edges. Then I crimped on the connector on the other end of the 3 foot coax.
Step 6: Mount and Test
I used a closet shelf support to mount the antenna horizontally to our gate post, inserted a combiner for this and the original antenna and tested it. You can simply replace the original antenna with the new one if you only have an approach from one side. In our case the existing opener range on entry was fine and didn't change with the combiner.
The results were great! Being able to open the gate from 4 times the distance makes it worth the time to do this.
I'll keep track of how it's holding up in the weather and deep snow, but overall I'm very optimistic that this solves the problem and won't require one of the other very expensive solutions the gate manufacturer was proposing.