diy gate opener on 315mhz - problems with range and relay chatter

[chansen1963]

Hi, I have built an electric sliding gate opener based on a 4Wd remote controil winch motor and curcuit. The mechanical have been working fine for 12 months now, but the electronics remain problematic.
The gate and adjoining fence is made of colour-bond -sheet metal, and so it blocks the radio range a lot when on the opther side of the gate/fence.
I bought a 315mhz 4 channel relay unit, which has a coiled wire antenna on the Rx unit. which is about 25cm when stretched out.
I mounted the receiver atop the 6' high fence, in a weatherproof box, in order to get a good radio range.. however the main relays down at the motor seem to chatter ie switch on/off rapidly causing a slow and jerky gate movement.
I think the 4-ch relay unit is having problems with the voltage drop in getting the power and return signals from the remote unit back down the 6' cable to the main relays to switch the motor on/off..
Q. will the 315mhz antenna work best coiled or stretched out ??
Q. is there any other way to increase the range , or use a different remote servo unit that will penetrate the fence, and thus NOT neeed to be mounted on top ??
The units work fine as long as I stay within 20' of the receiver, with no metal between.. but this ispretty useless for a gate opener, as you want ot begin opening it when you are say 1/2 a block away !!
anyone got any suggestions to improve this system
or improve the method of re-locating the remote 4ch relay receiver, so that the voltage drop will be managable ?

 

[Boncuk]

Are you supplying the receiver and gate opener circuits off the same power supply?

A metal fence (or gate) is no Faraday cage - hence radio waves will pass it.

Boncuk

 

[chansen1963]

Hi boncuk,

thanks for the reply..

The receiver is powered from the same 12V car battery as the winch motor. the rx unit has 2 relays, which activate depending on which button on Tx you push. The relay then sends 12v down the wire (2 mtrs long) the big 70Am,p relays toe make the motor go foreward or reverse as desired. there is a friction clutch to low the moter to kepe going when gate reaches the end stops. user just stops pressing tx button when gate reaches the ends.
If I put the tx close to the rx, it operates flawlessly..
onyl seem to get about 20' away before the rx starts to drop out.. and gate then operates jerkily..
how can I improve the distance ??

 

[Boncuk]

Hi chansen1963,

to get rid of the relay chatter you might consider using separate power sources for the gate motor and the receiver.

Before separating power I suggest to connect a 470 to 1,000µF electrolytic cap near the receiver chip to compensate for power surges.

Increase antenna lenght to 43 or 86cm and try out if vertical or horizontal polarization increases range.

Boncuk

 

[crutschow]

A metal fence (or gate) is no Faraday cage - hence radio waves will pass it.

It is a Faraday cage for long wavelengths. The waves readily pass through only if their wavelength is less then the distance between the wires in the fence. If the wavelength is longer, then the signal will be attenuated.

A 315MHz signal will have a wavelength of 95cm so that would be significantly blocked by a standard wire fence.

You may notice this effect in a metal frame building or going through a metal girder bridge in a vehicle. The AM low frequency signals will be significantly attenuated, but the higher frequency FM signals are often little affected.

 

[Boncuk]

It is a Faraday cage for long wavelengths. The waves readily pass through only if their wavelength is less then the distance between the wires in the fence. If the wavelength is longer, then the signal will be attenuated.

A 315MHz signal will have a wavelength of 95cm so that would be significantly blocked by a standard wire fence.

You may notice this effect in a metal frame building or going through a metal girder bridge in a vehicle. The AM low frequency signals will be significantly attenuated, but the higher frequency FM signals are often little affected.

I guess the gate is not grounded - so waves can pass freely. Following your statement it should also be a matter of polarization if radio waves pass a gate or not.

We tested 243MHz emergency transmitters in a real Faraday cage (thoroughly grounded iron mesh cube cage with wire squares of 2 inches and the control tower received the test transmissions clearly over a distance of 2NM.

Metal frame buildings and metal girder bridges are normally well grounded.

Radio controlled central locking devices wouldn't work either since the receiver is located inside a car, which would block all radio waves.

Boncuk

 

[chansen1963]

hi guys, thanks for the comments.. the fence is waht we call a "colourbond" - it is made of 6foot high roofing-like panels in zincalume.. so it is basically all solid sheet metal.. not a wire or mesh fench as you have supposed..

I will try the cap across the power supply at the receiver , as this may reduce surges, and I can easily increase the antenna length.. It was a colied plastic coated wire which is about 25cm when stretched out.. what is the next optimal length if I wanted to lengthen the antenna.. ??
The battery supplying power is probably 80 amp-hour or more, so it is certainly sufficient. and has an amophorous solar panel across it (about 30w or so ) , to keep the battery charged.. although I do need to re-charge it after a few months in winter if it has had extra activity and not much sun !1

 

[crutschow]

I guess the gate is not grounded - so waves can pass freely. Following your statement it should also be a matter of polarization if radio waves pass a gate or not.

We tested 243MHz emergency transmitters in a real Faraday cage (thoroughly grounded iron mesh cube cage with wire squares of 2 inches and the control tower received the test transmissions clearly over a distance of 2NM.

I would expect that a metal fence would use metal posts and be grounded.

I don't see that polarization would have any significant effect on the shielding unless the opening through the wires is significantly unsymmetrical.

How far is 2NM? A newton-meter is a unit of torque and a nanometer is pretty short.

 

[RMMM]

Nautical Miles

 

[Boncuk]

How far is 2NM? A newton-meter is a unit of torque and a nanometer is pretty short.

Hi Carl,

an "NM" is commonly known as "nautical mile" (used for aviation and shipping)

One NM equals 1.151512848mi or 1.8553184km

where mi stands for "statute mile" and km stands for "kilometer".

Regards

Hans

P.S. A garden fence with the poles dug 3 feet into the ground is not really grounded!

Real ground goes below ground water level and consists of a large and heavy copper block connected via 24square millimeters copper cable to the object to be grounded.

Attached is a sketch of a completely unsymmetrical fence with transmitter waves hitting the fence.

 

[Boncuk]

Hi chansen 1963,

the wave length of 315MHz is 95.24cm. λ/2 is therefor 47.62cm. Using an open dipole (or two, one horzontal and one vertical) with a shortening factor of 2% the dipole length will be 46.67cm or 18.37inches.

You can also make a ground plane antenna with all elements of equal lenghts of 18.37inches.

A ground plane antenna receives omnidirectional while the dipoles are directional antennas.

Boncuk

 

[crutschow]

an "NM" is commonly known as "nautical mile" (used for aviation and shipping)

Should have thought of that. Obviously I'm not very nautical.

 

[chansen1963]

hi guys... I think the jerky motor issue is really caused by the reception at the receiver.. As I have stretched out the 25cm coiled wire antena in order to try and increase the reception, it is also not supported and the movement of the gate is transferred thru to the fence, so the receiver unit is moving a bit, which is also causing the antenna to 'bob' about .. If I move the Tx closer, the problem goes away. The type of fence does not matter, as the radio signal is going directly to the rx on top of the fence, not through it !!
I wil ltry to increase the antenna to 43 or 86 cm .. I can use an old uhf whip antennas and tape the 315mhz antenna to it.. funnily enough, the gate operates in the "close" direction much better than the "open" , for some reason... There is a separate rx driving each relay, and the "close' direction is slightly downhill, and therefore easoer for the motor ..
thanks for you help..