Need help to improve receiver sensitivity 3

[mcwick]

Dear All

I just bought wireless home security system that works on 315 Mhz. I have a problem that the controller does not pickup signals from magnetic contacts located little away, and specially inside rooms.The controller (which is the receiver of signals) has small telescopic antenna.

can someone advise me how I can improve the reception range, probably by installing high gain antenna.

Wicky

 

[VEBill]

Most security systems would have sensors distributed in all directions. High gain antennas work by focussing in one direction (although there are omni-directional antennas that focus just along the horizontal). Long and short of it, a high gain antenna is not likely to be a practical solution for your application.

I'd recommend a refund. The system should be designed to work right out of the box. If it doesn't, forget it.

 

[Higgler]

Did you try a sensitivity test to see just how far the distance is for proper operation? Maybe you have defective hardware? It sounds odd that you have problems at very close range. Call the manufacturer.

If your walls have chicken wire and stucko, that may limit your range.

kch

 

[Comcokid]

As Higgler points out, houses with stucco frequenty have a chick-wire mesh. Tile in bathrooms may have a wire meash underneath. Some rigid insulation panels is aluminized on one side. These things can tend to shield one end of the link from operation.

Where is your controller? Is it in a closet with the hot-water heater or metal duct-work on one side? These may block the controller from receiveing signals.

Are these "magnetic contacts" transmitters mounted between the window and a later add-on strom window? What kind of external antenna (if any) do they have?

What can work suprising well is to add a passive antenna to the system. Take a length of coax. Strip off 9.5" of insulation/shield on one end (leaving only center conductor). Place this end in the attic over the window/door that has the contacts. Run the other end of the coax to the place where the controller is and remove 9.5" of shield/insulation as before. Place this near the telescoping antenna. 9.5" is approximately 1/4 wavelength at 315 MHz. Use either 50 or 75 Ohm coax. I just did something similar that greatly improved cellular reception within a production area that had become somewhat shielded due to metal siding.

 

[mcwick]

HI Friends,

Thank you all for advice. The walls are all brick and no wire mesh anywhere. Its just that receiver sensitivity is not sufficient I feel. I'll try Comcokid's advice tonight and see. The antenna of magnetic contacts ans host are both simple telescopic ones.

I also think of installing some sort of "repeater", that is back to back receiver and transmitter at an intermediate point. But what I fear is that since both are on same frequency, will it lead to oscillation. has anyone tried this??.

 

[zeitghost]

The repeater I worked on received the packets, verified them and then retransmitted a short time later.

So it didn't receive & transmit at the same time.

 

[VEBill]

If you're going to run coaxial cable(s) from the transmitter(s) to the receiver, then perhaps we should review again the advantages of a <<WIRELESS>> security system...

;-)

 

[zeitghost]

Advantages: you don't have to run cables everywhere... oh dear.

Disadvantages: changing batteries, burglars blocking the receiver with a powerful transmitter... etc.

 

[Higgler]

Regarding the antennas in the system, if you've located them very close to the walls, they may have shifted frequency and the patterns may be very ugly.
You didn't say what they were, I realize that magnetic latches on the windows trigger a signal, but the antennas on the transceivers might be bad.

This may sound wacky, but if you connect your base transmitter (safely, capacitively coupled) to the electrical power plug in your house wiring, that may provide a wired solution by only adding only one wired connection to your system. It'll probably be better than what you have now.

kch

 

[mcwick]

Dear Higgler,

That sounds a good idea. have you tried this. What the value of the capacitor you think would work?, Will a pair of twisted wires work.

Wicky

 

[VEBill]

House wiring isn't normally used for RF coupling at UHF frequencies such as 315MHz. For example, my wireless (AC-coupled) intercom system operates on 200kHz (0.2 MHz).

If you try it and it does actually help, it would be pure luck.

 

[Higgler]

Mcwick,

AC current in your wall socket is deadly. Please be careful and talk to experts/electricians if you your experience is low.

if you are safety conscious, then ;

using a capacitor, it's impedance is
Z= 1/(2* Pi* C* 315,000,000), hence trying to get say 10 ohms impedance would make the capacitor value
C= 1/2*6.14*10*315,000,000) = 25 pico farads.
So I would look for a value larger (200 to 500 picofarads), it needs a high voltage rating, 500 volts or the wall socket AC power will damage your electronics when the capacitor fails.

If you don't know much about capacitors, and you just throw any capacitor in your circuit, such as a low voltage electrolytic (which is only used for dc circuits, not wall AC power circuits), it'll blow up on you and probably destroy your transmitter.

With 3 wires in the wall socket, you'd want to connect to two of them, one probably the ground (the middle one on the bottom), and one on the top (those are the more deadly ones). Use a cut extension cord to get into the socket

VE1BLL may be right, but 315 Mhz is 3 foot wavelength and twinlead from TV antennas is essentially two wires. What may help is that in the room where your transmitter is weak, you would take another extension cord, cut it to a length of 10 inches, spread out the ground wire from the other two wires to form a dipole, tape the ends of the wires really well so no one gets electricuted, then place it in the socket to form a 315 Mhz antenna to talk to your sensor antenna.

You could try all of this very safely. I recommend running a long extension chord (just on the floor, not connected to the wall AC power) from your tranmitter to your sensor point. Set the system up essentially without deadly AC outlet voltage to see if it works. Put your transmitter into the extension chord using the capacitors and just some clips to get onto the pins of the extension chord, and make the dipole for the other end of the extension chord.

On that thought, for the real system, you could just use an extension chord from the wall socket, split it, tape it very securly, and wrap it around the antennas on each end - no capacitor needed. Energy from the antennas goes thru house wiring. It sounds safer than using a capacitor.

good luck,

kch

 

[mcwick]

Dear Kch,

Thank you for your advice. Well I am an Electrical Power Engineer by profeession so the safty issues are well taken care of.

About the capacitor, I myself thought about the concept of a twisted wire ( see my posting above) which will have adequate capacitance to give the required coupling, SO let me try this and post the results here.

Thank everyone for valuble contribution.

Merrry Christmas and happy New Year!!

 

[IRstuff]

Another option is perhaps to couple to the telephone lines. Back in junior high, we used to build crystal radios that we connected the antenna wire to the telephone dial hook (no cracks about age!). Of course, if you're running DSL, there might be some interference.



TTFN

 

[VEBill]

Another option is to have a hard-wired system professionally installed. Although fishing wires through a finished house sounds frightening, the professional installers are in and out in less than a day.