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Passively extending WiFi antenna. Possible? - RF Cafe Forums

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 Post subject: Passively extending WiFi antenna. Possible?
Posted: Fri Apr 20, 2012 1:50 am 

Joined: Fri Apr 20, 2012 1:36 am
Posts: 2
Hi to you RF experts. I'm hoping you can help me solve a design/engineering problem: an effective and cheap way to distribute WiFi to a 500 m radius in the jungle.

Here's the situation. I am visiting a friend who has built a home in the Yucatán. There is no phone line or TV cable out to his place. Telcel 3G is perpetually flaky. So we just spent two weeks and a several false starts welding, setting, and cabling a 9 m tower attached to the third story concrete structural pillar. That height was needed to clear the forest canopy. The experts then installed a microwave dish that brings in internet from a tower 20 km away. We now have WiFi from a little d-link router in the main house.

We want to bring this WiFi to outbuildings 150 - 500 meters away. Here I should say I have little practical engineering knowledge, but do understand physics, radio, and math pretty well, theoretically that is. Improvising in the jungle however has a way of forcing theory into practice.

Taking my laptop down the road, almost line of sight, gives a range of about 75 m from the puny 50 mw router. Shooting through the trees near ground level is hopeless: 20 m or so.

My idea is to put an RF amplifier on the router, and send the signal to a large omni-directional antenna mounted on the tower. That places the signal is above the forest canopy. I see 802.11 b/g 3w boosters available for around $100. I think I can muddle my way through successfully installing the amp and antenna.

So the problem then becomes getting that signal from the roof of the outlying buildings down to the ground floor where people will be using their laptops. I understand that we could use wireless repeaters to catch and rebroadcast the signal locally. The issue is both the expense of multiple repeaters, and providing power to the repeaters in palapas that have no AC.

Therefore I am wondering if is is possible to passively bring the RF from the rooftop to the ground level. For example, point a directional antenna to the main house, carry the signal downstairs in an RF cable, and then passively emit & receive it from an indoor antenna that the laptops can connect to via WiFi. I'm guessing that every component would have to be tuned in some way to 2.4 GHz for this to have a chance of working.

Can anyone advise me on the feasibility of this plan, and especially how to build the receiving side? I'm happy to educate myself from any references and tutorials you show me.

Thanks in advance for your help!


 Post subject: Re: Passively extending WiFi antenna. Possible?
Posted: Tue May 15, 2012 2:30 pm 

Joined: Tue May 15, 2012 2:17 pm
Posts: 1
Yes you can use a directional antenna above the canopy of trees and run a good quality cable to a omnidirectional antenna under the canopy of trees. Best to use antennas with some gain.

Don't expect wonders of it.


 Post subject: Re: Passively extending WiFi antenna. Possible?
Posted: Mon May 21, 2012 2:54 am 

Joined: Mon May 21, 2012 2:03 am
Posts: 1
My suggestion would be to not use coaxial cable to bring the RF from the
tower-mounted antenna.
FIRST, if at all possible, locate the wireless router/extender that
connects to the antenna, at the antenna, using as short a coax as possible.

Enclose the top-of-the-tower extender/router in a good weather-tight box to
protect it from the weather, and bring only the Ethernet signal down to the residence.
Use a good quality shielded Cat5 cable, but you must also provide
lightening protection for the Ethernet cable, and power cable to the
wireless device that is at the antenna.
(You may be able to power the
top-of-the-tower router/extender using power-over-ethernet [POE])
Minimizing the coax length will significantly reduce RF losses in your system,
and also improve resistance to signal fading in the long link.

Second, install another wireless router device (connected to the top-of-the-tower
device by Ethernet cable) in the residence for wired/wireless connections to the
computer(s) on the ground (inside and outside the house).

I'm not familiar with the radio rules there, but here in the US, I'd recommend running the
router link to the distant end on channel 1, and the downstairs wireless router on channel 11,
(or vice-versa) to reduce RF interference between the two routers.
Alternately, use 5GHz for one router, and 2.4 GHz for the other.

I've only provided the basics, but hopefully that it will be enough for you to get on the right
path. Perhaps I could draw a sketch for you tomorrow; that is, if you need one.
I'll follow this thread to monitor for replies.

Good luck with your project!


 Post subject: Re: Passively extending WiFi antenna. Possible?
Posted: Tue May 22, 2012 4:08 pm 

Joined: Fri Apr 20, 2012 1:36 am
Posts: 2
Dear Sycorax and Nicole,

Thanks so much for your replies.

We've decided to downgrade our grand plan and take it one step at a time.

The first step is simply to bring a signal to a single residence about 150 m distant, which also has reliable power. For that I have bought a 2 w "Wifi booster" and 25 dBi Yagi antenna. The plan is to try sending the signal directly through the understory, at 3 m above ground. You can barely see their lights at night, so maybe radio will be directly transceivable by a laptop through the trunks and leaves.

If that fails, we will move the antenna above the canopy and aim down on the roof of the building. This would make 90% of the distance line-of-sight. The we will see whether a laptop in the outbuilding can use the signal directly, or whether we need a repeater or passive antenna on the roof.

As for the passive antenna idea, I now think it is practical only for a building where there's no power. Thanks to China, the cost of two antennas and a long cable is now more than that of a cheap repeater. Besides, the antenna is more work to install and likely less reliable.

Speaking of China, I just received the 2 w "Broadband Amplifiers 802.11b/g WiFi Signal Booster" from an ebay store. It is DOA. No manual, no company website. There's no signal boost and the red LED stays lit continuously even without input connected. There's also no explanation of the LEDs, but if the red one means "transmit", something is surely wrong.

Any comments? Except please go easy on my stupidity in buying a no-name, no-company, no-support RF amplifier.


 Post subject: Re: Passively extending WiFi antenna. Possible?
Posted: Tue Jul 03, 2012 8:04 am 

Joined: Wed Apr 08, 2009 11:07 am
Posts: 34
yes, a passive antenna will work. You want as much directional gain as you can get for the roof top one, low loss cable, and then something like an omnidirectional dipole or monopole set up for vertical polarization near where you want to use it.

If you get farther away from that inside antenna, you might have interference from your indoor antenna and a direct line of sight leak thru, so a Mimo type of wifi would work best...it can separate the two signals best.

Maguffin Microwave Consulting

Posted  11/12/2012
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