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In our collective building lot we have an underground parking, in which a company recently changed a functional garage door system, since then the reception is a lot worse than before. (Like it doesn't open even when you stand 5 meters away from the antenna.)

The entrance to the parking is like a rectangular tunnel. The antenna inside the parking, is a single wire antenna, is 5 meters away from the garage door, 5 centimeters away from the tunnel wall and 50 centimeters from the ceiling, and pointing alongside the road, towards the metallic garage door.

I tried to remember my physics lessons, about electromagnetic waves in rectangular guides, and it seems to me that the antenna should be pointing up.

Can someone explain to me the rationale of making the antenna pointing the way it points?

[edit] adding information : the frequency used is 868.3 Mhz. (Before it was 29.960 Mhz). I measured the emitted power of my remote 50cm away using some USRP with a +3DB log periodic antenna, and it reads a power level of about -70dB (No idea, if that's enough, but my car key on 433.92Mhz in the same setup reads at about -68dB, and works really good)

[edit] I know in open air that single wire antenna emits best when pointed upwards, but here I'm wondering whether the fact of being in an underground (probably armed concrete), would explain the alongside the road orientation, (that why I was evoking 'rectangular wave guides')

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  • $\begingroup$ Hi Darkblue. I removed the part of your question which asks for legal advice, as that is something you will have to discuss with a lawyer familiar with the laws in your particular jurisdiction. As for the rest of the question, it seems to me like it fits within the "the technology of radio" part of the site's scope, so I have left that as is. If you know, it might help answerers if you can edit your question to indicate which frequency band the garage opener works on. That should be printed on the equipment or stated in some sort of reference manual. $\endgroup$ – a CVn Sep 8 '14 at 13:32
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(short version of the below answer: direction probably doesn't matter, talk to someone at the garage)

This sounds like the antenna was installed by someone whose main concern was NOT functionality.

Much better reception (regardless of frequency) would be if it pointed vertically (either "up" or "down" is equivalent) or perpendicular to the road instead of alongside it. Radio waves propagate off the broadside of the antenna, not the ends/tips... so to get 2 antennas talking to each other optimally, their "sides" should be pointed at each other.

Regardless, the metal garage door is certainly attenuating/degrading the signal; how much DOES depend on the frequency.

Finally, the super short distances involved between you and the antenna should make all of this for naught... if you haven't already checked the battery in your remote control, then the problem will lay somewhere in 1) the antenna, 2) it's connection to the transmission line, 3) the transmission line's connection to the receiver, or 4) all of the above.

At 868 MHz, roughly 35 cm wavelength, the relative size of the tunnel won't act as a wave guide, but the walls will simply absorb the energy, or reflect it back to the antenna.

Your description of the antenna being oriented "alongside the road" seems to mean that the broadside of the antenna is parallel to the wall and the ceiling, perpendicular to the garage door. If so, that's the worst orientation possible for the person trying to activate the opener from the door. It should be realigned parallel to the garage door.

While the "legal" portion of your question was edited out before I read it, your best bet will be to have a civil... no, downright friendly... conversation with the attendant or whomever you can find at the garage to see how to fix it. I've personally found an approach along the lines of, "hey, am I doing something wrong with my remote? I can't seem to get it to work, show me what I'm doing wrong" is VERY effective. When someone else finds they can't get your remote to work, they'll tell you it's not just you and be interested to work toward fixing it.

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  • $\begingroup$ In our building lot we all had problems with remotes. That's why they already replaced the antenna once for a "more powerful" one, which only helps a little. I edited my question to specify that I'm interested by the fact that being in an underground armed concrete might have strange effects ('rectangular wave guides'), and was seeking the advice of experienced radio amateurs. $\endgroup$ – darkblue Sep 8 '14 at 16:44
  • $\begingroup$ If that was my antenna I would have reoriented it long ago, but as it's a collective one installed by an external company, I'd rather not touch it. But if some experienced radio amateurs confirms the antenna is badly oriented, we can ask them to solve the problem. $\endgroup$ – darkblue Sep 8 '14 at 16:53
  • $\begingroup$ I'm certainly not the most experienced ham around, but I do have 22+ years operating experience and have been an Amateur Extra class licensee (w4pso) for about 12 years. And I'm certainly not the end-all be-all authority on wave propagation, so if someone else cares to chime in or edit this answer I'm certainly not going to take offense... I just want to be helpful! $\endgroup$ – webmarc Sep 9 '14 at 16:36

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