Minimizing antenna lead loss

Hi,

I am setting up a couple of antennas to connect 2 buildings about 300 feet apart. I have discussed this with several people and a subject that came up was antenna lead loss. One person said it was a big issue, and another says it is nothing to worry about. Can anyone give me the straight scoop? BTW, my antennas will be on roof tops, so I need enough lead length to get inside to the access points.

Thanks,

Jerry

Reply to
JerryK
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Real Big Losses per foot what cable are you using - somebody will tell you loss per ft quickly

I ordered a Cisco 15db Yaggi and part of the package included pre-made 50' belden RG8u which takes away 10db right off the bat not counting connectors ;) surely wont do much good. Good thing I got a deal otherwise I would have been upset if I knowingly paid for that cable.

Reply to
bumtracks

It's a BIG issue. At 300ft, you can probably afford quite a bit of loss. However, it's considered good form to calculate rather than guess. See:

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300/5280 = 0.057 for the range in miles. Minimum fade margin is about 10dB. For an 802.11g link, running at 25Mbits/sec, you should aim for 30dB fade margin. Use 1.0dB for every connector pair.

For cable loss, see:

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'll probably be using LMR-400 which eats 0.067dB/ft.

If the numbers look bad, then I strongly suggest at least one end of the link uses a roof top radio, with no coax cable.

If you want me to do the numbers for you, kindly disclose some hardware details (radios, coax type, coax length, antennas, antenna gain, range, 802.11a/b/g, etc).

Reply to
Jeff Liebermann

And dont forget to add in the lightning arrestors!

Reply to
Airhead

Jeff,

What do you mean by a "Roof top radio with no coax"?

On both sides, I am planning on hooking directly from the Access Point (ex. Dlink DWL-G700AP) to a directional panel antenna (claimed 16db gain). The cable length should be under 20 feet.

jerry

Reply to
JerryK

That's where the access point is mounted in a waterproof box or is imbedded in the antenna. Power is supplied with either an independent cable run, or through the ethernet cable using PoE (power over ethernet). The big advantage is that it eliminates the coax cable loss problem and allows substantial flexibility in installation, that isn't possible with a big fat lossy coax cable run.

It's fairly messy to repackage an indoor router/AP but possible.

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I didn't realize that you already owned the hardware.

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panel (patch) antennas are just fine. Probably too much gain, but with the added benifit of not hearing junk to the sides of the antenna pattern. You should have more than enough fade margin. If you're only using 20ft of unspecified coax cable, you should have enough fade margin for getting maximum speed from your bridge.

I can work out the numbers, but you may have a bigger problem. I don't think the DWL-G700AP will act as a transparent bridge (also known as a "wireless bridge" or "bridge mode"). I'm looking through the data sheets and find nothing about bridging. This is strictly an access point used to connect from individual wireless clients and cannot be used to glue two networks together.

Is there anything else you didn't mention? (type of coax, line of sight, fresnel zone clearance, interference potential, number of MAC addresses bridged (32 max for the cheapo bridges), traffic expected, etc.)

Reply to
Jeff Liebermann

An outdoor access point. Something like

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where the feedline loss is approximately zero. You can use something like
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if you want an external antenna, and then you can keep your feedline short (though you've still got connector losses). N-Type connectors (and feedline and antennas) will keep your losses low...

Reply to
William P.N. Smith

Consider something like a Senao outdoor bridge CB-3 in an outdoor enclosure such as the rootenna which has an integral panel antenna.

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$170. {If you look real hard these can be had for about $150}. Some of the wireless mesh operators in NE Texas and Louisiana are using these for the final connection of community wireless to individual homes or business. The Senao device provides for power over Ethernet so all you need is an Ethernet cable to the box with power injected onto the Ethernet cable.

The Senao card is a high powered 200 mW output with a +14 dBi panel antenna.

802.11b only
Reply to
Bob Alston

Loss IS a big deal. You want to keep coax runs as short as possible. I'm sure someone on this group (Mr Leibermann?) can advise on figures. If you cannot get your AP close enough to the antenna, you should consider getting an AP that supports power over ethernet and put everything in a weatherproof box and mount it outside. Peter

Reply to
Sandy Baby

Hi Jeff,

I own the antennas, but not access points. I am open to suggestions on brands and models. I like the idea of the roof top antenna with no coax, but the cost may be prohibited. I am not doing a "commercial" installation. A friend and I are just trying to share a connection between two hangars at an airport. My hangar has access to a wired DSL connection. His hangar is

300 feet away and we want to hook him to my connection. We are mounting the antennas on the roofs and plan on placing the access points inside the hangars. We are trying to do this as cheaply as possible.

Regarding your questions: "(type of coax, line of

Whatever coax I can get that will work is fine, but price is a consideration. I would love to be able to run 40 feet on each end, since that would mounting the access points much easier.

There should be no interference. The roofs are approximately 25 feet and there are no interviening buildings.

He will only have 2 or 3 system on his end, so we can go with the cheapo bridge.

Traffic. On my end I will only use the connection once or twice a day. On his end maybe 5 or 6 times a day. The typical activity will be web browsing and email. No media players, streaming video, etc. So the load should be pretty light.

Other factors. All systems will be PCs. I may also have a wireless router in hangar for my tablet.

Thanks for your help,

jerry

Reply to
JerryK

Ok. We use the antennas. The existing Dlink DWL-G700AP radios will be used for local connections inside the hangars. You'll still need a pair of radios that can do bridging and a router.

Well, this is what you should have purchased:

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$950 from MoonBlink for the pair. This is actually overkill as the specified range is 2 miles. There might be a cheaper incantation.

You're in an airport. The airport manager will launder your shorts in prop wash if you do anything less than a professional installation on his hangar. You can use inferior cheap junk for the radios and non-permanently attached boxes, but if you install a pair of coffee cans or similar abomination, you will be probably be asked to clean it up or replace it. I've been there in two different airports.

Before you continue with the wireless road to connectivity, please make an effort to check out any existing phone wiring or conduit between hangars. I setup a network between hangars at an airport long ago (using Arcnet and coax). Hangars tend to have quite a bit of conduit inside which eventually terminate at the telco MPOE (demarc). That's because telephone poles and dangling wires are not appreciated in the vicinity of things that fly through the air.

If there's conduit, see if you can add some CAT5 or coax (RG-58a/u). You can go much farther than the official 100 meters if you observe some precautions and tolerate some limitations. I've posted comments on how it's done in alt.internet.wireless in the past. The longest I've done is slightly less than 1000ft using 10baseT.

If you can't add cable, but there is existing twisted telco pair, determine who owns the wire and ask them if you can "borrow" two pairs between hangars. Although not officially supported, you can run

10baseT over telco wire. You won't go as far as CAT5 or coax, but it will usually work.

If wired is not an option, then I guess you gotta do wireless. However, before you embark on this adventure, do a site survey. Install Netstumbler on your Tablet PC. Drag it onto the roof and see if you can "hear" anything. Avoid using any channels that are already in use (1, 6, or 11). If there's a local comm shop, and they have a clue, ask them to fire up their spectrum analyzer on the 2400-2500Mhz band. There's quite a bit of junk around the typical airport that doesn't show up on Netstumbler. For example, Proxim/WMux backhauls used by Nextel and Cingular. Remote radar backhaul.

I usually ask my customers how much they want too spend on such things. Once established, I can better determine what can be done with their money, rather than throw together that absolute cheapest piece of junk, and spend my time fixing it later.

Well, you lose. My suggestions will involve minimizing the coax cable length, mostly because of the loss, but also because working with coax is a pain. See below.

Assumption, the mother of all screwups. One of my friends just called asking what to do about interference. It seems he parked one end of his bridge in the path of a 10 watt TV station STL link. He's toast unless he can convince the radio station to move to a slightly different channel.

Does that mean you have line of sight AND reasonable Fresnel Zone clearance? At 300ft, the midpoint Fresnel zone is only a few inches wide. Therefore, you need the width of the antenna, plus a few inches, clearance from the center line. If your shooting across any knife edges in the path, you will have a problem.

OK. 32 MAC addresses will do fine. That what most of the cheap bridges will do.

Yeah, right. I made the mistake of showing everyone on my neighborhood WLAN how to get continuous background music from Shoutcast using Winamp. Now I've got a bunch of people pulling

128Kbits/sec continuously. It only takes about 10 of those streams and my bandwidth is gone.

Ok. We'll use your existing wireles access point for inside your hangar. Instead of multiple suggestions, I'll just supply the one I think will work. You can add or subtract as necessary. I just don't have the time to present all the options, configurations, and mutations.

First you'll need a pair of radios that will do transparent bridging. I suggest Linksys WAP54G running in bridge mode. About $70/ea. Here's the part you won't like. The radio goes inside the hangar, but up near the ceiling. Punch a hole in the SIDE of the hangar and install a proper PVC rams head entry. The coax comes down from the roof, through the entry pipe, and directly to the radio. I'm guessing about 6ft of coax. At that length, you can use just about any type of coax. However, I still suggest using LMR-400. Instead of the usual pigtail or adapter plus N coax extension, we use a piece of LMR-400 with an N-connector on one end and a R-TNC on the other. See:

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$30 for 10ft.

Some notes on the RF parts. Do NOT use any right angle N connectors. I'll spare you the reasons why. Also, no sharp bends in the coax. Make sure you have a drip loop for water at the rams head. Wrap the connectors first in 1" wide teflon tape, and then embalm that in decent electrical tape. The TFE keeps the tape from making a sticky mess and will prevent capillary action. If you're really cheap, cellophane wrap will work.

So, now you ask how does one get power up there? The WAP54G has a really nifty switching power supply inside. The unit will run on 5VDC (actually 3.7VDC) so wiring loss is not much of a concern. Build an extension cable for the power connector on the WAP54G to the 12VDC wall wart supplied with the unit. Don't butcher the wall wart cable by chopping off the connector. Go to Radio Shock, buy the parts, and make an extension. Zip cord (AC power cord) will work. Watch the polarity so you don't blow it up.

How you mount the WAP54G near the ceiling is your problem. Just make sure you support the cables so their weight doesn't extract them from the WAP54G.

You'll also need a piece of CAT5 cable between the ethernet port on the WAP54G and the rest of your LAN (i.e. your router). You can crimp your own or pay for pre-made cables. Up to this point, both ends of the puzzle are essentially identical. Effectively, you've build an ethernet extension cord.

Wiring is like this:

[Linksys]==< >==[Linksys] [ WAP54G] [WAP54G ] | | | | | | phone | | line | | | | | [ADSL] |====/ ==[5 port ] |==[PC1] [modem]==[router]=|==[PC1] [ethernet]=|==[PC2] |==[PC2] [ switch ] |==[PC3] |==[PC3] | [ Dlink ]=====| |==[Dlink ] [DWL-G700AP] [DWL-G700AP]

The added router can be just about any type of router. I tend to use Dlink DI-604 because they're cheap and the later units (Rev E1) seem to work well for me. The back of the DI-604 includes a 4 port ethernet switch which is where your PC's will plug in. Therefore, it should be located near the PC's.

The WAP54G does not have a built in switch so you'll need to add one at the remote end. It's better this way or you would have ended up running wires up to the ceiling.

Selection of channels and SSID is fairly simple. The Dlink radios should NOT be on the same channel as the wireless bridge or you'll end up interfereing with yourself. The SSID and WEP/WPA encryption keys on the two Dlink access points can be the same to make roaming easier.

Reply to
Jeff Liebermann

Don't listen to all this bullsh*t, I have a setup with Dlink 800 access point and Dlink 810 bridge, both cost about 75.00 us each, they are connected to external antenna with 13dbi. Access point is mounted in a box outdoors with cat 5 cable back to the router and 5' of lmr

195 coax to the antenna, the other end has 20" of lmr400 coax between the bridge and the antenna with about 50' of cat 5 cable to my pc. The distance I am bridging is well over a mile. My internet speeds are stable and downloading at about 1 megabit. I also have a group of trees between my antenna and the other antenna, doesn't seem to pose too much of a problem. At 300 ft. you may be able to mount your antenna's internaly, especialy if you have a window that faces the other hanger. Glass doesn't block the signal to any great degree. If you would like any other info for this setup, email me at snipped-for-privacy@NOSPAMuniserve.com
Reply to
Paul James

I've had good luck with the D-Link 800/810 set as well, but I'm sure the Linksys would work well, too. One nice thing about the D-Link 800 is that it can be found cheap (around $25) on the surplus/refurb market, and you can download the 810 firmware into the 800 and make it into an 810. And, the 800/810 pair will talk at 22 Mbs instead of 11 Mbs if the signal is strong enough.

I also agree with keeping the antenna length as short as practical. Even the LMR400 cable loses a significant fraction of the signal if you go very far at all. I mount the AP or bridge near an outside wall, then run a short coax out to the antenna on the outside of the building. I've used 10

- 25 feet of LMR400 and had good results. But, of course, every situation is unique, and a couple/few db loss on a coax run could make a big difference in your ultimate throughput in any given situation.

Look at

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under the heading "LMR Coaxial Cable". They list the db loss of various types per 100 feet. If you really have to have a long run of coax and can't accept much loss, they probably have a solution for you if you want to spend enough and if you deal with thick, stiff coax. They do have one type (LMR-900) that has less than 3 db loss per hundred feet, and it's about $3 per foot. And they carry two other coax that are more expensive than that one, but they don't list the loss on their website.

LMR-400 loses over 6db per hundred feet, so even a 50 foot length of it will lose over half the signal. You have to look at your total gain & loss. You can offset coax loss with a higher gain antenna to some extent.

snipped-for-privacy@uniserve.com (Paul James) wrote in news:94cef63c.0410302350.15701ec7 @posting.google.com:

Reply to
shay

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