Antenna cables - maximum length

I would like to add some extra antennae so as to give good coverage around and outside my house.
Is there a limit on the length of the antenna cables?

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Hi @ArthurD welcome!!

We recommend to use the LibreRouter in the outside of the house and to use the shorter possible RF cables and if possible use the RF coaxial cables provided with the LibreRouter that are excelent quality LMR200. This cable loss is 0.5dB per meter at this frequency of operation. This is a simplification but each 6dB you loose you get half the coverage in distance from the LibreRouter. So a 12m LMR200 cable will halve the reach.
The external pluggable antennas of the LibreRouter are to connect to other LibreRouters. The internal antenna is to create a wifi zone (hotspot).

Can you tell us more on how do you want to use the LibreRouters?


I thought it was 3dB = half.

Extra antennae besides the internal 2.4 GHz and the two external 5 GHz antennae? Also, if you want gain coverage with 5 GHz, like @SAn says, those are used by default for mesh. You would have to replace LibreRouterOS / LibreMesh with OpenWrt in order to use the 5 GHz radios as access points. If you did so, I wonder whether 802.11r fast transitions would make sense to manage connections. I don’t know much about it.

Yes, 3dB of something = 0.5 ‘something’. Half the distance would be -3dB distance, but that was not my point.
The free space path loss, the amount of signal power that is “lost” when you move away from a RF signal source, is 6dB each time you double the distance. This is beacause the equation has the squared distance in its denominator. The way I reason about it is with a sphere with its center in the signal source. If the signal is equally distributed in the surface of the sphere then when I increase the radious of the sphere then the signal per unit of area has to be reduced to equally cover all the sphere. And then as the area of the sphere increases with its radious squared then the signal has to be reduced in this proportion…I hope this idea helps, it asumes an ideal omnidirectional antenna that has a perfect spherical radiation pattern.

So if you have a 100m coverage and then you increase the power by 6dB then you would reach 200m instead of 100m.

ooooooooookay. Your example makes sense. But… if I use a directional antenna, then the signal is not dispersed over the surface of a sphere… but we could imagine the signal projected on a section of the surface of a sphere, and I guess it still changes over distance in a squared way. So, this means 3 dB only equals 0.5 if we talk about cables or lasers, since those things have the same surface area at both ends?

I guess I have more to study in terms of dBi used for gain, dBm used for power, and dB? used for signal intensity when we align antennas.

If I understand correctly:

  • signal intensity of -60 dBi is twice as strong as -63 dBi, and maybe this yields higher throughput in Mbps?
  • In a given installation, if Tx power of 20 dBm yields Rx signal of -60 dB at 1000 meters, then Tx power of 14 dBm (20-6=14) should yield Rx signal of -60 dB at 500 meters.

Yes, if it is not an omnidirectional antenna the same equation holds (but is not as easy to draw the picture in my head).

The dB definition depends on the magnitude referenced. If the dB refers to a power magnitude then 3dB is 2 (and -3dB is 0.5). When the dB refers to a non power (like voltage) then it is diffferent…so it is very easy to get confused :frowning: (the wikipedia article that has more context).

When aligning antennas the value shuld be always a power (received power) so it should be in dBm (and less common in dBW / dBmW, etc). If it does not say the unit and only says dB it may be because of lazyness/oversight or because the receiver is not calibrated so the reference value is unknown so it can’t say -60dBm because it does not know exactly what 1mW is (so it only serves as a relative measure).

When we are refering to power in RF we can add and substract power gains and losses. So when you have a transmitter that has 1mW of power (0dBm) and then a power amplifier that doubles it power we say that it is a gain of 3dB. An antena expressed in dBi can added to this, and power losses in cables can be substracted, then free space path loss also can be expressed in power dB losses (and almost always is).

So to know exactly the power of the signal at a receiver you can do:
rx_power [dBm] = txpower [dBm] - cable_losses [dB] + antenna 1 and 2 gain [dBi] - free space path loss [dB] - pointing loss [dB] - ...

Yes, but I would talk about power and not intensity, so -60dBm is double power than -63dBm. Also 63dBi is an antenna that in its maximum radiation point has a gain (in power) that is the double of a 60dBi antenna. And you it may yield higher throughput in Mbps (not lower but not always higher).

Yes! (but I would say Rx signal of -60dBm, so referred to 1mW, and not -60dB as a loss without reference)

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