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I have two buildings. Building A has wifi (2.4GHz) infrastructure, Building B doesn't. One side of building B can pick up the wifi signal from building A. I want to passively extend the signal through to the rest of building B, using two antennas and a short piece of coax.

Supposing I put a decent directional antenna (24dBi) in building B, pointing at a WAP in building A, then joined that to an omnidirectional antenna with a short (50cm) piece of coax. In this scenaria, for the omni antenna, would it be better to have a large (e.g. 12-15 dBi) antenna, or a small one (~7 dBi) (assuming the smaller one can still cover the range of the building)?

I am aware an active repeater would be better, but I'd like to get some thoughts on this scenario.

Background reading: https://web.archive.org/web/20141026222347/http://www.netscum.com:80/~clapp/wireless.html

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    $\begingroup$ I am aware an active repeater would be better. Never ever use simple repeaters for WiFi unless you absolutely have to. They are a thing of the past and have been made obsolete by high availability of access points. I know that this is not what you're asking, so I'm not posting an answer, but in this case, I'd just place an access point connected to a directional antenna to receive signal from building A and connect that AP using Ethernet to another in building B. This way, you'd get wireless bridge and much better service for users in building B. +1 for interesting question though. $\endgroup$ – AndrejaKo Sep 15 '14 at 16:48
  • $\begingroup$ Never say never. There are some cases where a Passive Repeater (PR) works great. A good example is where you have a major obstruction causing huge signal losses such as 50dB. Another case would be if you do not want to drill any holes such as in an apartment. When my brother and I lived at the same apartment complex, his AP was 400 ft away and I was able to put a 2 beam PR out on my balcony and steer the signal into my apt thru the sliding glass door without having to drill any holes. It worked well as the PR was like a mirror, "bending" the signal 90 degrees. Without it I could not conn. $\endgroup$ – David Oct 16 '14 at 19:48
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Trying to extend coverage in this fashion isn't worth doing. Even with highly directional antennas, most of the power transmitted doesn't end up in the receiver. Thus, your "repeater", which is really two antennas joined by coax, has very little power available to transmit.

We can do some math.

Let's assume that your buildings are 40m apart, and your transmit power is 20 dBm (the legal maximum on 2.4 GHz in the US). Let's further assume that you have 24 dBi antennas on the Wi-Fi AP and your repeater. We can use the Friis transmission equation to calculate how much of that power is received by the repeater:

$$ 20 \:\mathrm{dBm}\ + 24 \:\mathrm{dBi} \ + 24 \:\mathrm{dBi} \ + 147.6 \ - 20 \log_{10}(40\:\mathrm m \cdot 2.4 \:\mathrm{GHz}) \ = -4 \:\mathrm{dBm} $$

So, assuming no losses in your repeater, it makes the 20 dBm transmitter look like a -4 dBm transmitter. Or put another way, the repeater introduces 24 dB of loss. By reciprocity, this loss works in the other direction as well: how ever much power is received by the repeater from the clients, the AP will see it as 24 dB less.

Besides that loss, which is substantial but maybe not impossible, you have a new problem. While the AP might hear the nodes in the external building, other nodes won't. This is called the hidden node problem, and will result in transmit collisions which seriously degrade the performance of your network.

To solve your problem, best is to run Ethernet to the building, and install an AP. If Ethernet is not possible, then use a cross-band Wi-Fi repeater. They are available for $100 at any big-box electronics store, where they are usually called a "range extender". You are going to spend at least that on antennas and coax for your passive repeater solution, which will not work as well.


Regarding which omni-directional antenna is better, it's impossible to say, generally. An isotropic antenna has exactly 0 dBi gain, by definition. However, such an isotropic antenna can not be physically realized, the closest we can come is a dipole, which is 2.15 dBi in free space. Of course, the presence of the Earth or anything else conductive around the antenna changes that.

In any case, any antenna with higher gain works by being more directional. Remember that the antenna's radiation is a three-dimensional function. An "omnidirectional" antenna radiates equally in all directions in one dimension (typically, azimuth), but this says nothing about how it radiates at different angles of elevation. Thus, an antenna that is still "omnidirectional" but quotes a higher gain is either:

  1. an outright lie by Chineese marketing departments, or
  2. radiating more horizontally, and less up or down.

Depending on the orientation of your antenna, and the location of your radios, this could be good or bad. What you want to do is minimize radiation in directions where you don't want coverage, which will in turn maximize radiation in directions where you do want coverage. Which antenna achieves that depends on your particular environment.

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  • $\begingroup$ I would add that your calculation is actually to optimistic. You calculated path loss for just one side of the passive repeater. For path loss for other side, signal would be much weaker. $\endgroup$ – Pedja YT9TP Nov 23 '18 at 7:27
  • $\begingroup$ @PedjaYT9TP I don't think it says that the path loss: it says that's the power received by the repeater. $\endgroup$ – Phil Frost - W8II Nov 25 '18 at 13:45
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You'll get an awful lot of signal loss from trying to do this. A wifi AP can lose something like 30 dB of power between the wired section and the air. Picking up a signal and then retransmitting it will probably lose at least that much, plus the loss in the original signal (originally sent from an omni antenna). You probably won't notice much improvement.

On the other hand, if you get a cheap active wifi repeater and attach a directional antenna, this could work quite well. Even more so if you install an identical one near the original building. (There's no reason you can't have two APs in that building.) Two repeaters will cost you not much money, but give vastly better coverage, reliability, and speed.

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My suggestion is to get an Alfa R36 wi-fi router and an Alfa wireless card. The R36 can provide wired/wireless internet in the building which has none. Set it to a channel far apart from the channel used in the building which has internet. Connect the R36 as a client with the USB Alfa card and a yagi. This is seamless, and my setup has never failed. I point the yagi directly toward the remote accesspoint (which has generic omnis). Across 35-40 metres and through a few walls I still pulled 35-40mbit/s. The nice thing about the R36 is that it is an excellent accesspoint on its own, with ethernet port so you can wire it to a switch or other accesspoints. A yagi in the building with internet, pointed at the R36 would be best. Don't use parabolic dish from inside a building to inside another building, the waves get thrown around too much.

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  • $\begingroup$ Hi, your response sounds like good advice, but it doesn't actually answer the question about how well a passive antenna-to-antenna "repeater" would work. Please consider taking the tour to see how the site works, and also have a look at how to answer. $\endgroup$ – rclocher3 Aug 18 '16 at 20:59
  • $\begingroup$ I'll refrain from anything but conclusive answers. OP could save considerable time with this, however. $\endgroup$ – user400344 Aug 18 '16 at 21:01
  • $\begingroup$ no worries, I'm not trying to read you the riot act, just point you in the right direction. Cheers! $\endgroup$ – rclocher3 Aug 18 '16 at 21:07
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a couple of years ago i saw a passive repeater array that was named after the engineer that designed it... now i can't find that article. but a google search for passive repeater array brings up a lot of entries, and it looks like it's very common above 1Ghz to use passive repeaters in many communications systems. everything from reflective (think of a metal billboard) to antenna-waveguide-antenna systems seem to be abundant. the one i was thinking about (wish i could remember the name of it) used several dipoles in a line, running through coax to another line of dipoles (like on the other side of a hill, etc..), the idea being, a single dipole-to-dipole repeater has a very small "aperture", but an array of passive repeaters increases the efficiency by the use of a larger "aperture". since 2.4Ghz is such a short wavelength, you could probably fabricate a few arrays on panels of PC board material and connect them with coax cables (you do want to use a good grade of coax, because you really don't want to have a lot of loss in your transmission lines) and make certain they are all the same length.

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Passive repeaters can and do work but require some experimentation. Since there are large losses, you may have to use several beam antennas to make it work. Why not have a separate transmitter for building A and beam a signal (on a different channel) over to building B and then redistribute Bs signal with a high gain antenna? Since Bs coverage is only in one corner of the building, why not use another beam antenna to illuminate the rest of B? An omni wouldn't make much sense to me in that case.

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