Radiating cables - are they any good?
Last Post: August 15, 2006:
I recently heard that one of our competitors implemented a wireless network using radiating cables to improve the coverage. I don't know much about radiating cables so I need to ask:?
1. Are they highly seceptible to or do they generate interference?
2. Are these cables any good is anyone using them if yes what has your experience been like?
I'm not sure exactly what you mean in question #1. If one antenna can hear another interfering signal better than the second, I guess you could say the former is more susceptible to interference!? But that would be ture with any kind of antenna - receiving OR transmitting.
#2. Yes these cables are good. We don't use them too often, but they do have their place. I have personally installed them in two locations. The best example I can think of was an outdoor semi open warehouse that had small trains move through on tracks. The site survey showed we had terrible multipath inside the building. Besides using standard omni and patch antennas, we found leaky coax gave us the best throughput along certain tracks. Wireless clients in the trains needed communications with the WLAN, and the leaky coax served their needs the best. It covered the area along the track with RF better than a dipole, patch or yagi would.
I've also heard of cell phone sites using leaky coax inside tunnels and in certain underground locations to improve their cell site coverage.
Radiax at 2.4 GHz and 5 GHz is not a good way to deploy Wireless LANs IMHO. There is too much line loss at those high frequencies without amplifiers along the way. It alone, will hurt your link budget.
The better coaxial cable to use is LMR 600/ 900 type by Times Microwave . At 100 feet, you lose roughly 4.5dB on the 600, and 3 dB on the 900.
If you can sustain to take a half power loss on the wifi signal and make up for those losses with a higher gain antenna ... say 5 to 6 dBi gain , then go for it.
See this link it mentions radiating coax and some of the challenges
I read some of the links you gave in the last post. Since I have slept a lot since that warehouse install, I could be wrong. Maybe we were using 900 Mhz instead of 2.4 Ghz? I'm fairly sure it was a 2.4 install however. Anyway, the leaky coax we used was probably 100 to 125 feet long, and the coax itself was at least a half inch in diameter. Yeah, it's lossy at the high frequencies, but it still spreads the RF exactly where you want it. In my case, the area where the tracks entered the warehouse. We used a single AP in the middle, and ran one coax to the left, the other to the right. It worked GREAT. You have to terminate the end of each leaky coax with a 50 ohm resistor.
I also wondered if the coverage area near that resistor would be less than the end near the AP. I couldn't tell any difference at all. With the terminator installed, the SWR is low, so the power along the cable is fairly constant.
Some of the in-building Wireless Distributed Antenna System (DAS) designers /engineers use it (leaky coax) religiously. Tunnels, mines are the best case for this. You have to be careful not to get the cable to close to metal objects. If you place multiple frequencies over it ...interference issues are suspect.
This article talks about ways to combat some of this.
I was going to use it at 800 MHz, for a certain application (mobile phone enhancements) and was told by a very experienced Motorola partner engineer to think very carefully about using it. He stated the EMI from machinery close by can cause some nasty harmonics if not engineered right.
Thanks for the response. It is good to see you are getting your hands on the equipment out there. There are a number of ways to deploy wireless and the leaky coax option, if engineered right could be a way to save the client some $. You have gotten it right .. it is working, and that is the GOOD thing! :)
Leaky cable systems are notorious for causing the "hidden node" problem.
I can attest to the fact that you can't run voice over these things.
OK, I'll bite. Why not?
For just basic data access, leaky coax is useful but for advanced features like location tracking, rogue AP tracking, and VoWLAN, it causes more problems than it's worth. For example, location tracking uses multipath to determine positioning of whatever is being tracked. Leaky coax, since it's one long antenna, will not provide the information you need to pinpoint tracked devices. While you may not think this is important now, it certainly will be as WLANs continue becoming more and more sophisticated. Why deploy something that will have to be yanked out when you're ready to use advanced features?
I had never even thought about all the outstanding points that Joel made.
The big reason why leaky cable causes problems with VoIP and Wi-Fi is the hidden node problem that often develops with this type of design. A re-transmission rate of 2% or less is usually required to guarantee proper delivery of VoIP packets in a timely fashion. Hidden node can cause re-transmsion rates of over 10% which simply is not accepable in a VoWiFi environment.
Leaky cable will save you $$$ in the short term but could cost you $$$ in the long term.