Radio Control Garage Door Opener
How does a radio station’s broadcast, affect electronical equipment. (E.g. Garage doors, TV’s.)?
(Hypothetical Question I would like to know the answer of)
A new FM radio station, NOVA opened.
In the suburbs of North West Sydney, especially those in Greenwhich, Lane Cove and Woolstencraft, residents found their remote control garage doors doing strange chaotic things:
The doors developed a life of their own;
Randomly opening and closing at all hours, sometimes when people where not at home;
To open their garage doors some residents had to get out of their cars, stand close to the door, in order to open the garage with the remote;
Some residents found their ABC TV reception was effected;
Some residents likened the anomaly pattern to watching a TV set with a rabbit – ears antenna.
Some residents noted that this phenomena also occurred with those in the eastern suburbs of Sydney during the 2000 Olympic games.
Your problem – explain why the above has occurred.
(If possible please explain scientifically)
I would not expect this to be real. I live very close to a number of high power digital and analogue TV transmitters and FM stations, and it does not cause problems. The transmissions are required to meet certain standards, and are monitored so that modulation, power, bandwidth occupied etc are within standards. The other devices like garage door openers, TV receivers etc, must also meet regulatory standards that they are not susceptible to interference in this way. Current door openers have a complex code that needs to be correct. It is not impossible that these issues can occur, but they are most unlikely to have occurred with current equipment.
The cause…
Let us assume the transmitter is operating correctly. A receiver with poor performance can suffer interference for a number of reasons:
Image response. There is insufficient filtering before the mixer stage of the receiver, so that signals on the wrong side of the local oscillator can be received simultaneously. This could appear on a certain TV channel, related in this way to the frequency of the new station. It depends on the relative signal strengths, so more likely if the TV channel was weak and the FM station was strong. The interference (on analogue TV) would be like “shimmering patterns” superimposed on the picture, that can be related to the sound being broadcast. In the past frequencies were allocated in an attempt to avoid this, but the situation gets more difficult as more and more channels are allocated, so it becomes more dependant on the receiver being “up to scratch”. Older designs are much more likely to have this problem.
Cross Modulation.
If there is insufficient rejection of unwanted signals before active stages in the receiver, any combination of signals can mix with each other to form products falling on top of the desired channel. This can be very complex as there are so many possibilities. It is solved by better receivers, but sometimes a single new signal that is very strong (so local) can cause the level to exceed thresholds where inter-modulation increases dramatically. Sometimes the sum of various signals may be pushed over the threshold by a new signal, like “the straw that broke the camels back”. In these cases an attenuator in the receiver antenna line may help. If there are also weak signals on the same antenna, filters may be needed to separate the weak and strong signals. This is specialised. Sometimes a separate antenna for weak signals helps, as it may reject the strong local signal in favour of the weak signal by directional response.
Other possibilities are faulty antenna and connections that lead to cross modulation on rusty connections, or the antenna is not suitable (faulty or inadequate). Faulty receivers could be the issue in a few cases, but it is more likely they are just inadequate, much older.
I expect newer analogue and digital TV is less likely to have these issues. Older garage door openers were unsophisticated, and may be more prone to issues like this.
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