Boost Garage Door Transmitter

Hi

I am trying to extend the range of a garage door remote transmitter. It is one of the older styles with the dip switch on the back to key in the code, 300 MHz analog. I don’t know enough about RF so I came for help.

**broken link removed**

So far after checking out the circuit I added an external antenna off the single coil. This has helped quite a bit but still not where I want it. I am getting about 20 feet where before I almost had to hold it to the door.

I believe the biggest problem is that it’s for a metal building, I am sure this is shielding a lot of the signal. I am hoping I could overcome this with more power :twisted: which probably isn’t the way to fix it.

Can I build some sort of amplifier to boost the power? Or perhaps build a stronger transmitter circuit.

Any ideas would be helpful

Thanks!!

Well I am not that great at RF either but I will see if maybe I can help.

So RF signal transmission is based on the propogration of the signal and the receiver being able to collect and analyze that signal. The transmission and propagation is based on the power (how much RF is radiated), the medium of transmission (antenna), and the directivity. When an RF transmitter sends a signal that signal goes through various phenomena such as power loss from passing through objects, scattering, and reflecting back. So to get a longer range you would need to improve on those 3 parameters if possible while still not hurting the communication. The metal building doesn't help though, since that has various properties that doesn't allow for RF through it very well.

On the other end, you need to be able to receive the data. This could be done by improving the antenna structure or by amplifying the weaker signal that is coming in. For the metal building, it would be better to mount the antenna on the outside.

That is a quick and dirty run down.

without a doubt a better aerial is the way to go. The problem is the low frequency, 300 MHZ, means that the wave length will be one meter, so you need a pair of wires, pointing in opposite directions (exactly) each .25 meters long, from a centre point. The maximum sensitivity will be in a direction at right angles to the aerial line. Connect the outer of a decent bit of TV coax to one wire at the centre and the inner to the other. The problem now is to connect your piece of coax to the actual receiver in the garage. The coax braid is easy, just connect it to an earth on the printed circuit board. I would locate a springy coil which will be your original aerial, one end of it will not be connected to anything. Disconnect the other end and connect the centre of your coax to this point. Its all a bit dodgy, but it can work, I have done it to a 433 MHZ from door bell receiver to know when the front door bell is ringing when I am in my shed at the end of the garden.
Frank

Thanks for the replies!

I finally had some time to come back to this.

I don’t quite have a clear picture of increasing the antenna on the receiver end. Currently the original receiver antenna is just a wire soldered to the pcb.

Would connecting on to the end of the original wire antenna be at all effective?
What are the coax connections? pcb - braid to ground center to antenna , other end- center to antenna braid to ??
Am I right in thinking the antenna would make a “T”. Could I use one long wire .5m and solder the coax to the middle to form .25m on each end?
What would be a good material to give some rigidity in making an antenna? I am guessing Something non conductive, pvc tube?

Thanks

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Also when I connected an external antenna on the transmitter I just soldered the wire of one end of the coil. I have been looking at other transmitter circuits and see most have a 10pf? cap before the antenna. What is the purpose of this? Should I add one?

Location, location, location. Door opener probably has
the receiver physically integrated with the drive control
/ motor head assembly. On the wrong side of the sheet
metal. But for the cost of a little power cabling you could
debundle the receiver from the motor package, put the
receiver (say) on the inside of a handy glass window that
faces wherever you want good range and there is your
near-free-space pickup range.

I've seen very old openers that used as low as 27MHz
citizens band frequencies; there, you have a wide
choice of linear amps and matched transmit antenna
options. And a frowny face from your friends at the
FCC.

Lets start off by using the right terms, the gadget thats in the car is the transmitter, it transmits a coded signal that the RECEIVER in the garage picks up to operate the door.
It would be best to connect your coax centre conductor in place of the existing aerial wire. Your current aerial is likely to be too short so the receiver is being fed from a small capacitor, using the new aerial assembley, it would be fed from 75 ohms, which might not be a good match, but as you will be getting , say, five times more signal, it will not matter if you lloose a bit.
Yes the aerial forms a T, but it is imperative that the wire bit is in two parts, else you will have a short across the coax and hence across the aerial input to your receiver.
As for support, I would use drawing pins ("thumb tacks?") and tack the aerial along the horizontal wooden frame above the door. If the ceiling joists go in the correct direction these would do, else string it from bottom of joist to bottom of joist.
The most difficult bit is attaching the coax to the PCB, I found some miniature RF coax and used that with a BNC to BNC adapter to get to the standard 6mm TV coax used for the aerial. What I actually did was to strip the PVC from the coax for the .25m, then fiddle the centre conductor and insulator out of the side of the copper braid "tube" and drawing pin the ends of the aerial bit to my shed ceiling, with extra pins to support the coax.
Frank