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Short version: Is the same antenna type used for transmitting as is used for receiving if the frequency is the same?

Long version: My garage door opener uses 433 MHz to receive signals from the remote control. The antenna it a wire, about 30 cm (one foot) long. The door opener transmits its status to a control device we have in the house. When transmitting it uses 868 Mhz. The antenna for transmitting is a wire about 8 cm (3 in) long. The control device we have in the house contains two antennas, both wires about 8 cm long. One I assume for receiving and one for transmitting.

I assume I can replace the 30 cm wire in the door opener with a 433 MHz antenna like this (http://www.ebay.co.uk/itm/New-SMA-Male-Crimp-RG174-GSM-GPRS-433MHz-Antenna-Sucker-Cable-3dbi-Magnetic-Base-/161525785843?hash=item259bafe4f3:g:fcoAAOSwuMFUkqWO) and the receiver antenna in the indoor control device (which I'm anyway going to put in a custom case) with this (http://www.ebay.co.uk/itm/868MHZ-915MHZ-GSM-3G-Sucker-Omnidirectional-High-gain-Antenna-7DBI-super-/201249960496?hash=item2edb6eb230:g:J3IAAOSwuMFUj-2c).

But what about the transmitter antennas? Are there custom transmitter antennas or can I buy a 433 MHz antenna and use that for transmitting at 433 MHz?

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Antennae are mostly symmetric in their receive and transmit behaviour, i.e. they will have the same gain and frequency response in each role.

The main place where the symmetry breaks down is power handling: an antenna designed for receiving needs to carry only a tiny quantity of power whereas a transmitting antenna, e.g. for a large radar or a long-range communication link, might be carrying kilowatts or even megawatts of instantaneous power.

Power is not an issue for garage door opener radios though ;) However, I would not necessarily expect that replacing the existing wire-antennae with these new antennas will yield any improvement. They may do, or they may be worse depending on how well their impedance is matched to radio circuits you're using them with; have a google for VSWR. The relationship between the antenna you use and any ground plane below is probably more important than changing one cheap piece of wire for another slightly-less-cheap bit.

By all means experiment with those, but don't have high hopes for longer range.

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  • \$\begingroup\$ Could you elaborate on "ground plane" and what I should think about there? \$\endgroup\$ – MathiasR Jan 20 '16 at 8:50
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    \$\begingroup\$ A ground plane is a big flat conductive surface acting as a radio reflector; for lower frequency systems, the earth itself is a very good ground plane. When you put an antenna above a ground plane, it acts similarly to there being both the antenna and its mirror image on the opposite side of the ground plane, which means you can use a smaller antenna. At 433MHz, the wavelength is about 70cm so your 30cm is pretty close to being a half-wave, which is a reasonably good design for transmitting in all directions. \$\endgroup\$ – William Brodie-Tyrrell Jan 22 '16 at 0:27
  • \$\begingroup\$ Since the antenna is attached to the motor, which sits close to the ceiling of the garage, transmitting upwards is probably not necessary. Could I then paint a part of the ceiling of the garage with conductive paint? \$\endgroup\$ – MathiasR Jan 22 '16 at 10:32
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Antennas both transmit and receive in the sense that they can do either. I'd be careful to not just "splice" the antenna in and to actually use a proper connector (which you never mention, but I assume they exist because your products have them).

The only exception being antenna that have active devices in them that are not bi-directional (linear). Those are fewer and farther between but do exist. I suggest posting photos for confirmation.

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Antennas can equally send and receive for the most part. The reason for replacing the antenna is more important. Are you trying to get more range or is something wrong?

A reason to replace an antenna is to get better range. When replacing the antenna, neither the transmitter nor the receiver is changed. For the case of the transmitter the power sent does not change. What the new antenna can do is focus the same amount of power into a more narrow beam, sending more power in a particular direction and less in another. This is called directivity. The same thing happens with the receiver as well. An antenna with a greater directivity is able to receive a weaker signal compared to its less directive counter part.

One way directivity is represented is gain. The gain is how much the signal is "amplified" so to speak, in the direction of greatest directivity if compared to a true omnidirectional antenna.

All of this was to say that the antennas that you have, lengths of wires, are called monopoles. The gain of a monopole is roughly the same. The antennas that you listed are also monopoles. By relapsing a monopole, no matter how good looking, with another monopole the change won't be noticeable.

The other problem is impedance matching. Antennas can't be just hooked up willy nilly. The impedance of an antenna must match the impedance of the transceiver. If you connect a 50 ohm antenna to a 75 ohm transceiver there will be problems. Without knowing the impedance of the transceiver is important before hooking up the antenna.

One way to easily make a directional antenna is to place a parabolic reflector around it. Cut out several cardboard sheets in the shape of the parabola. Cut an antenna sized hole at the location "F". Join all the pieces together on the corners. Cover the curved surface with a metal substance. Fine metal netting or aluminum foil should do it. Slide the antenna through the hole "F" and fix it in place directing the signal toward the house.

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  • \$\begingroup\$ The transmitter in the garage will only transmit to the console we have in the hallway in house so getting a directional antenna and pointing it towards that will probably produce better results? The receiver antenna I linked to had a couple of meters of wire before the antenna. I can then place the antenna close to the garage door which will be better than where the wire antenna is not. But can the wire reduce the effect of the antenna so I will get no net effect? \$\endgroup\$ – MathiasR Jan 20 '16 at 8:58
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    \$\begingroup\$ @MathiasR You are correct. A directional antenna pointed toward the house will give a better result. I have a different suggestion for you, I will edit it into my answer. You are also correct that a couple of meters of a feed wire will probably wipe out the effect of moving the antenna closer. \$\endgroup\$ – vini_i Jan 20 '16 at 11:43

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