WAPR News December 2004

by Andy Nemec, KB9ALN


Hello and belated Happy New Year. I hope your holidays were fun and you're weathering winter well.

As we're in the doldrums of winter, there's not too much news to report. As you may have noticed by looking at the top of the page, there is a new mailing address for those of you who wish to contact us to join WAPR.

Al, N0GMJ has been our treasurer for a number of years now, and his schedule no longer permits any time for that activity. So, he's turned things over to Joel for the time being. Joel is currently working on the software end of bookkeeping for WAPR, and has not gotten things set up just yet. For those of you who've sent membership dues checks in, please be patient. Joel will get them deposited as soon as he can get things straightened out. Thanks to Al for his work, not only as treasurer, but as a node operator and as a radio man.

Joel reports that his BBS computer is giving him problems and is intermittently on the air. As soon as he gets the computer back up to snuff, his BBS will reappear on its normal, consistent basis.

I mentioned in the last issue that we're working on a node map of the state of Wisconsin. I haven't had too much time to devote to the project, but have gotten one really good response from Josh, AB9FT in Sheboygan. Thanks to Josh for a very comprehensive list of many, many nodes.

I am looking for more. If you are in a position to do so (meaning that you have sufficient knowledge of your local nodes and you have the time), please reply and send me information that we'll find helpful. Here's what we have been collecting:

Location - Nearest City or Town Alias of the Node or BBS Call-Sign of the Node or BBS Node type, BBS type, or other info. Such as weather BBS or Emergency Management/ARES BBS. Node versions can be obtained by issuing the "U" command). Frequency Baud Rate and if the node is a LAN, Backbone, DX or other type of node. Any notes on special operation.

When the completed list is compiled and checked, I will distribute text versions via the packet network and E-Mail, and place a graphic version here (space permitting) and on the Internet. I thank you in advance for your help.

That's our short news report for this time out.

Until next time, 73 from Andy.

Ham Implementation of 802.11Packet Radio by Andy Nemec, KB9ALN

In our last features, we had a short introduction to 802.11 technology, we talked about how a wireless ethernet network is configured and continued by exploring antennas. This time out, we'll explore feedline and connectors. These are very important parts of a permanent, long-range amateur station utilizing 802.11 devices.

A Few Feedline Facts

There are 4 basic types of feed systems used by amateurs, mostly distinguished by their physical characteristics. These are:

- Open-wire Balanced Feed
- Flexible coaxial cable
- Inflexible coaxial "Hard Line"
- Inflexible Wave Guide

Open-wire balanced feed is mostly used in HF work, but can be used with excellent results at VHF and even UHF. Think TV twin lead when you think of this type of feed, that's basically what it is. It features very low loss, but has a few disadvantages. Among them are a lack of shielding, allowing external signals to interfere, specific installation requirements and a lack of durability. Any gain experienced by reduced feedline loss is often negated by the loss in one or more balun (balanced-to-unbalanced) transformers. It is really not a consideration for use in 802.11 devices.

Flexible coaxial cable is what we most often use, and is available in a dizzying array of types and qualities. Its advantages include its ease of installation, price, shielding and increased durability (as compared to a balanced feedline).

Inflexible feedline, commonly called "Hard Line" is what is used in most commercial applications where the high quality, durability and low feedline loss are required. The fact that it is nearly inflexible makes it a little tougher to deal with in a typical Amateur installation. This type of feedline uses air for an insulator, with spacers that keep the center conductor in the proper place to maintain proper feedline impedance. It's not often used in home Amateur stations because of its expense (more on that later) and relative difficulty of installation. It is often used on well-installed FM voice Amateur repeaters and packet nodes.

A good compromise variation of inflexible feedline is semi-flexible feedline. This features a metal jacket that is wound, and therefore flexible to a certain extent. Instead of an air insulator, it uses a foam insulator. Therefore, it looses a little more RF energy than non-flexible air-insulator feedline. But it's also a lot easier to work with.

The last feed option for Amateur antenna systems is a waveguide. Waveguides are also a non-flexible. They are basically a tubular chamber of fractional-wavelength dimension that carries the RF waves. It is most often used in microwave and satellite work. While loss is extremely low, it must be very carefully installed or it won't work or continue work well for very long. It is only for the extremely brave Amateur.

More Feedline Fodder

Feedline is at least a very important part of any Amateur operation, even one operating in the HF spectrum. The most likely problems at that range of frequencies involve poor connections, moisture contamination of the feedline, or poor shielding, perhaps resulting in a poor SWR or TVI. Rarely is feedline loss a factor at HF. With 802.11, we're operating at microwave frequencies. Feedline loss becomes critical for us.

Jump up to VHF, particularly the 2 Meter band, and that difference begins to appear. While standard RG-8 cable will not present much of a loss at HF, it becomes unsuitable for anything but short jumpers inside the shack at VHF frequencies. Consider the feedline loss per 100 feet vs frequency; at 10 MHz, it loses .5 db. At 100 MHz, it looses 1.9 db, and at 1 GHz, it loses 8 db. That means that by the time you get to 2.4 GHz, you'd be wasting about 95% of your signal in feedline loss.

Now let's take a look at what used to be called "Poor-Man's Hardline", Belden 9913 double-shielded coax. It loses 1.4 db at 100 MHz, and 4.5 db at 1 GHz. At 2.4 GHz, you'd lose about 90% of your signal. This is a little better, but still unacceptable.

Now let's take a look at real Hardline, 7/8" in diameter. It loses a mere .36 db at 100 MHz, and only loses 1.4 db at 1 GHz. At 2.4 GHz, it loses about 2 db of RF. That means that about only 40% of your signal is lost with a 100 foot feedline run. Quite a difference from flexible coaxes.

While the advantages of hard air-insulated feedline are readily apparent, it is often impractical to install it in a standard Amateur house station. After all, it is nearly rigid 7/8" pipe, doesn't bend very well, requires special hardware and connectors, and is very expensive when purchased new. For that reason, we may want to consider semi-flexible hard line.

The most popular variation of this is LMR-400. At 100 MHz, it loses 1.2 db, at 1 GHz it comes to 4.3 db. At 2.4 GHz, you can expect to lose about 6 or 7 db. Still substantial, but one sometimes has to compromise. And don't forget, most installations will require far less feedline. If you want to upgrade to larger and better semi-flexible coaxial feedline, LMR-500 and LMR-600 are also available, with expected performance gains.

There's one more feedline consideration: jumper cables. Even LMR-400 presents a strain on the hardware that it is connected to. It makes it tough to move your computer too, because it's only semi-flexible. So, you'll want some kind of flexible jumper cable to connect to your feedline. The best all-around choice for that is LMR-240. Although it loses a lot over long runs, it loses far less than other flexible cables. And we're only using it for very short distances.

One more special note: Don't, under any circumstances, use RG-58 or RG-8 Mini - even for jumpers. It is totally unsuitable even for short jumpers.

The Connector Connection

Feedline is only going to be as effective as the connection to it. Some 802.11 cards have no option for connecting an external antenna, some don't. You may have to actually dig into the wireless card and add an antenna jack. If you do, and even if you don't, you should follow these suggestions when selecting and using connectors for 802.11 products:

- Do not go cheap, you will regret it. Some cheap connectors and adaptors, particularly 90-degree adaptors, use springs to "make bend" of the center conductor. This introduces unwanted inductance in the circuit, which detunes it. As time goes on, they develop intermittent connections. Cheap connectors don't last long, either.

- Do not use any PL-type connector (such as a PL-259). They do not present the proper impedance and often come loose. They also don't have any seals to keep out moisture.
- N-type connectors are the best bet. They also have sealing rings that help keep moisture and contaminants out of the connections.
- Do not mate silver connectors to nickel connectors. This causes an electrolytic reaction which in turn causes deposits to form on them. These are semi-conductive deposits, which cause problems.
- Properly weatherproof all connectors. Covering them with Coax-seal, then pure sili-cone rubber followed by self-fusing tape, with a final covering of tightly-wound electrical tape topping it off will be the best. Do not use "caulk" type silicone rubber. The curing agents used in these products often will corrode the metal on the connectors.
- Use as few connectors, adaptors and splices as possible. Every connector introduces a certain amount of loss, fewer connectors mean less loss.

Additional Information

Here are some of the sources of information I used in preparing this feature, and where you may find additional helpful information:

http://www.hamuniverse.com (Operated by Don, N4UJW)
http://www.bawia.org (Boston Area Wireless Internet Alliance)
http://www.andrew.com (Manufacturer of semi-flexible feedline)
http://www.ERlink.com (Manufacturer of rigid feedline)

And that's where we'll leave our discussion for this time.

Until next time, 73 from Andy.


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