Homebrew WiFi Antenna (Proper Cable, LMR-195 GR-58?)

Unanswered Question
Jul 20th, 2007


I don't know if this is the best forum for my questions, and if not please direct me to one that better suits my needs. I was simply referred to this forum through another one. Anyhow, me and my friend are attempting to build our own wifi antenna and after searching google for a while we have come up dry on a couple things.

1), From the wifi card we wish to attach a cable that will be say 25-50 ft in length which will then attach to the card's reg 3 inch antenna, pretty much extending the antenna from the card. What is the best cable to do this. Originally we were told to use GR-58 50 Ohm Coax cable however after reading further we found that would only be good for 3-4 ft before it pretty much "spills the connection all over the place". We were then recommended to use LMR-195 or better though what type of cable is this as it does not seem to be supplied by Radio Shack. The question is what cable would you guys recomend that could hold a wi-fi signal 25-50 ft from the end of the antenna on one end back into the wifi card on the other, and where can we purchase it?

Thanks in advance and if this is not a good forum for these questions please direct me to another. As time goes on we will build our own antenna but right now we want to get the reg antenna to work over a decent length of cable.

I have this problem too.
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scottmac Fri, 07/20/2007 - 19:29

This is likely to get complicated ...

First, keep in mind that the antenna you are removing from the NIC probably has very little, if any, gain (you put one measure of "thing" in, and you get one measure of "thing" out ... minus some losses).

Next, if you used the Best Coax In The World to connect the antenna back to the NIC, there will be significant loss in the cabling (you would lose somewhere on the order of 7/8ths to 15/16ths of the signal, just in the cabling, both transmitting and receiving.

So, without adding something to the signal, either actively (with an amp, for example ... very dangerous at these frequencies) or passively (with a "gain" antenna, like a Yagi or parabolic), you are not likely to be able to make the trip between these two endpoints (unless they are within 10-20 feet of each other).

The above is why you would usually see "bridge" devices ... basically an access point with antenna connectors, so you can put the AP/bridge closer to the antennas to reduce the losses.

Most of the "Do it Yourself" antennas (like th Pringles can antenna) are usually on ~ one meter pigtail ... the gain of the antenna isn't really enough to overcome the losses of a really long cable.

If you could post up what you are trying to do, and the distances you are trying to cover (more detail is better than little or no detail) perhaps we can offer some alternatives to get you going.

Good Luck


protivakid Sat, 07/21/2007 - 04:55

Alright here is the ultimate goal of the project. My neighbors have generously offered to share their wifi for this experiment as I only have a wired network. I have this old TV antenna on the roof and a wireless card on ground level of my home. I want to modify the old roof antenna in some way that it can receive wifi signals, point it at my neighbors wifi, and then run a cable down to the wifi card. Pretty much extend the antenna. I am aware I may have to start from scratch and build a whole new antenna rather than modifying an existing one though either or the antenna will be roof mounted and I need to get the connection over some sort of cable from the roof to the wifi card. I know I can buy a bridge and just run 50ft of ethernet cable to the pc but that's not as fun.

scottmac Sat, 07/21/2007 - 09:29

OK. Understood. You have a massive challenge ahead of you. Also be warned that an external antenna that is not properly protected and grounded can be a fire and safety hazard, and that it can accumulate enough of a static charge in dry, windy environmants to blow up your system (not so much like "boom" , but static electricity is deadly to most semiconductors).

Next, you may want to study on how to make a BALUN / UNUN because the WiFI stuff is 50 Ohm impedence, and your average TV antenna is 300 Ohm (assuming it's a classic LPDA - Log Periodic Dipole Array).

An LPDA for TV will have some elements for ~50 MHz, 260 Mhz, 512MHz, and if it's also a UHF antenna, it might be good up into the 900 MHz ranges. LPDA ware pretty wide spectrum, and you'll probably luck out and hit a harmonic somewhere.

You'll need to craft a 6:1 balun, not that big of a deal actually. Shop some of the amateur radio sites (like aesham.com) for materials or to just buy a 6:1 BALUN.

The Gain of a typical ~ 6 ft LPDA could likely provide enough gain to make the system work =-->IF<--= the cabling is very high quality, the cable termination is nearly perfect, your BALUN is efficient enough, and you don't lose too much signal through the protection system (that would be the protection system against lightning and static ... includes a thingy called a "lightning arrestor" that in no way, form, or fashion could do anything to stop lightning).

By the way, you'll want to bring that TV antenna down and clean every connection on every element. Any noisy junction will kill your signal.

All-in-all, the cable shouldn't cost you more than a couple hundred dollars, the materials for the BALUN another ~US$10-20, connectors another US$20-50, protection elements, maybe another US$25-100 depending on who and what you go with.

Here's a link with some good info regarding wireless signal propagation, 802.11, and antenna stuff.


Good Luck, make it a great learning experience!



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