I am using this LoRa Radio shield and want to connect a simple ¼ wave wire antenna to it. There is a hole for soldering an antenna on close to the SMA jack, but then I would be skipping the 50 ohms of resistance the SMA connector provides. Would that be bad? I am very new to radio technology, so I am sorry if its quite a basic question...
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3 Answers
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I would be skipping the 50 ohms of resistance the SMA connector provides
There is no 50 ohms in that SMA connector, instead it has a 50 ohm characteristic impedance and that is something different.
You can just leave out the SMA connector and solder your antenna wire onto the board. A SMA connector with a proper antenna might give better performance but your wire solution will also work.
answered Apr 7, 2017 at 9:58
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Since you're a beginner, I won't go into details too much. Basically, the 50 ohms is not the resistance of the connector, it's its characteristic impedance. When dealing with "radio stuff", in order to maximize power transfer, you want to have all the parts with same characteristic impedance.
So you'd match the radio chip on the PCB to 50 ohms, use the 50 ohm connector, optionally use 50 ohm coaxial cable with 50 ohm connectors on it for the antenna, and finally, use a 50 ohm antenna, with appropriate 50 ohm connector matching the connector on the cable, to radiate out your signal.
Now the thing is that we don't know what type of antenna you want to use. You should keep in mind that the impedance of the antenna is frequency-dependent, so antenna for one frequency might not have 50 ohms on another frequency band. That's why you first need to figure out for which band is your board configured and then get an appropriate antenna.
Now, soldering a bit of wire to the via on the PCB might work as an antenna, but, since you're a beginner, you probably don't know how to cut it to appropriate length so that it works OK and you probably don't have the test equipment needed to figure out if it's working OK or not. Usually, it's not enough to just cut it to 1/4 wavelength, since the PCB and everything around it will also impact the impedance of the antenna.
So my advice would be to use the SMA connector and buy a factory-made antenna for your band of interest.
answered Apr 7, 2017 at 9:58
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\$\begingroup\$ Ok thanks! But a factory made 433 MHz antenna would also not be much more than a 17.4 cm cable? So I guess a 17.4 mm wire would work too right? Or have I got something wrong? \$\endgroup\$ Commented Apr 7, 2017 at 10:17
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\$\begingroup\$ @user5227744 Well, the radiating element of the factory antenna is most likely going to be either a straight piece of wire or a helicoidal coil. The main difference is that the factory antenna will (unless you get something really horrible!) have some matching elements in its base which, if they're working properly, will make sure that the transmitter is going to be happy. \$\endgroup\$ Commented Apr 7, 2017 at 10:50
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\$\begingroup\$ The thing is, the 1/4 wave formula only works correctly in free space. As soon as you have something close to the antenna, like your Arduino for example or your hand or whatever, it's going to affect the antenna and the length need will to be changed. Now, don't get me wrong, even a stray piece of wire will most likely sort-of work, but since those modules are low-power anyway, you'll probably want to have them running as best as they can. That's why I'm advising a factory made antenna. \$\endgroup\$ Commented Apr 7, 2017 at 10:53
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\$\begingroup\$ Of course, it's always a trade-off. If you can't, or don't know how, or if its economically unjustifiable to obtain an antenna, then you can try with a piece of wire, but if you're in situation to get an $15 antenna, then I would recommend that you do so. \$\endgroup\$ Commented Apr 7, 2017 at 10:56
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\$\begingroup\$ Ok. I think I'll first try with a wire, and if it works out - thats fine, if not, I'll just buy one. \$\endgroup\$ Commented Apr 7, 2017 at 11:46
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The reason for using a connector is so that you can connect and disconnect the antenna easily. Unless you're absolutely certain that you'll never want to change the antenna, use a connector. If you don't, then replacing the antenna means unsoldering the old one and soldering in the new one. That's not going to work very well; it's a lot of work, and you'll eventually damage the board. Imagine that in your house, instead of having light bulbs that screw in, you have them soldered in place: whenever a bulb burns out you have to shut off the power, unsolder the old bulb, solder in a new one, and turn the power back on.
answered Apr 7, 2017 at 12:55