|N5OOM's HSMM Projects - Rootenna Project|
The Need for Flexible HSMM RadiosOver the past few years, members of the North Texas Microwave Society - HSMM Working Group have experimented with installing 802.11b equipment modified for ham use. Most of our work has been to support public service events. To do this, we have had to develop equipment that is weatherproof, reliable, flexible, easy to install and easy to tear down. Any given installation may require a radio to be used indoors or out, on battery power or AC, used as an access point, router or bridge.
The system documented here is one result of those experiences. This system is not ideal. It is physically large and requires a lot of additional equipment that has to be carried externally. However, it can be used as a bridge, router or switch without opening the case, can be powered from far away, has a built in antenna but can be used with an external antenna. This allows a ham to put the system in place and configure it very quickly in many different ways.
The Linksys WRT54GThis system is based on the Linksys WRT54G router. The WRT54G is an inexpensive 802.11g product, available locally for around $40 - $60. This router has obtained a high level of popularity for hobbiest applications because it has used the Linux operating system. Some users have figured out how to built custom linux loads and upload them to the router, providing substantially more functionality than is provided by the factory.
For more information on 3rd party Linksys firmwares, see: http://www.linksysinfo.org/.
The Sveasoft development crew is very active and provides a lot of support for their firmware. Released firmware is available for free. Access to pre-release firmware and to their extensive support forum requires an annual $20 subscription. For more information on Sveasoft firmwares, including Alchemy & Talisman, see:http://www.sveasoft.com/ To download free firmwares, visit http://www.linksysinfo.com/
One of the most useful additions to the WRT54G firmware is WDS bridging. WDS, or Wireless Distribution System, is a method of bridging access points together wirelessly. There are many methods of bridging 802.11b radios together, but virtually all of the others are hard to implement while staying legal against Part 97 rules and regulations.
As with all amateur radios, HSMM radios operating in automatic control mode are required to identify themselves every 10 minutes or less. Regular access points beacon a network ID (called the SSID) many times a second. This beacon is used to allow clients to synchronize their transmissions to the access point. If the SSID is used with a call sign, the station easily satisfies Part 97 ID requirements. Unfortunately, radios operating in bridge mode use a master - slave configuration, and do not transmit anything that could be used as a callsign that could be received by anything other than expensive test equipment. This makes proper identification more problematic.
WDS is is one solution to this problem. WDS provides a method for access points to bridge together while remaining in access point mode. Each radio continues to beacon a SSID, which can be a call sign. WDS works by alternating the usual beacon packet with a special MAC-based data frame. WDS can bridge multiple access points together by using one of these frames for each bridge partner.
WDS isn't a perfect solution - by alternating packets between the beacon frame and the bridge frames, the data rate is reduced each time a new bridge partner is introduced. However, since the 802.11b/g data rate is much higher than most ham needs, the loss of bandwidth is usually not significant.
Power over EthernetThe 802.11b and 802.11g specifications both operate in the 2.4GHz band, at fairly low powers. At frequencies this high, transmission line loss is fairly high (6.8dB per 100 ft for LMR400). The WRT54G is configured for 28mw from the factory, which is low by ham standards, and has a poor receive threshold. Adding long runs of transmission line adds loss to a system that is already running on limited resources.
So, how to reduce this loss? The simple way is to put the access point up at the antenna and connect the antenna and radio with a very short piece of transmission line. The connection from the radio back indoors to the computer network is made over category 5 unshielded twisted pair wire. The ethernet specification for cat-5 cable calls for runs no more than 100 meters (about 320 feet). We can remote the radio a much greater distance if we put it outside and run CAT-5, than if we leave the radio indoors and run a long coax cable.
So if we put the radio outside, how to we power it? Easy! A Cat-5 cable has 4 twisted pairs of wires. The ethernet signal only uses two pairs, the other two are unused. We can use the other two to transmit power to the radio.
WARNING! If you implement this, you are responsible for determining if your installation is safe.
New York PoE
The "New York" PoE standard puts +v on wires 4&5 and ground on wires 7&8. Doing this is very simple using modular RJ-45 jacks that are now available at many home improvement stores. It is possible to build this up for as little as $10. However, there is no regulation using this method and you must take into account the voltage drop across the CAT-5 cable. For more information on the New York standard, see: http://www.nycwireless.net/poe/. To calculate the losses over CAT-5, visit: http://www.demarctech.com/techsupport/poecalculate.htm
The idea of power over ethernet has become very popular in the industry for powering voip phones and for remotely powering other equipment that may not be near an AC outlet. The 802.3af spec has been created to standardize the implementation of a safe powered interface. 802.3af calls for 48v to be passed over CAT-5, with detection circuitry to determine when it is safe to power the line. For more information on 802.3af, see: http://www.poweroverethernet.com/.
PoE for the WRT54G
There have been several models of WRT54G with differing power requirements, but most are rated for 1a at 12v. personal measurements have been 500ma at 12v for the v2 and v2.2 models. An inspection of those models reveals a DC to DC converter on the circuit board and a set of filter caps rated for 18v. A bit of research shows that the DC to DC converter is outputting 3.3v, with a dropout of around 5v. This gives us a lot of voltage drop to play with. Experimentation has shown that these models can operate properly over PoE runs of 325ft when powered from a 13.8v gel cell. If you want to try this, be sure to ensure that your WRT54G will operate safely at the voltages you intend to supply.
The Pacific Wireless RootennaSometime in the recent past, Pacific Wireless began to sell their Rootenna product - a small radio housing with a built in 14dB antenna. More recently, a larger housing with a built in 19dB panel antenna was made available. Both housings come with a nipple that allows a CAT-5 cable to pass through while providing a weatherproof seal around the cable.
My observation is that for ham use, a higher gain, narrower beamwidth antenna is better for bridge radios. This provides the highest signal strength, and the narrower beamwidth reduces interference from outside sources. The 19dB antenna is a bit large, but the gain and the flexibility of internal equipment options makes it my antenna of choice.
The 19dB Rootenna has an internal mounting plate, with #8 threaded holes on 1" centers. This makes it easy to attach radios to the inside plate. The antenna is mounted to the plastic faceplate, with an SMA connector in the center. A short pigtail is used to connect the antenna to whatever radio you have mounted to the faceplate. Generally, you will need to specify what type of pigtail connector you will need when you order your Rootenna.
The Rootenna comes with an L shaped bracket that attaches to 3 of four threaded studs that protrude from the back of the antenna. The bracket may be rotated to any 3 of the studs, allowing the antenna to be rotated to any needed orientation. The bracket comes with two U-bolts, which fit into slots in the mounts which allow the bracket to be tilted from vertical.
Other StuffD-Link DSS-5+
The D-Link DSS-5+ is an inexpensive 5 port, 10/100mbs ethernet switch. The units that I have use a 7.5v power supply; however, a peek inside revealed a DC-DC converter and feed capacitors that would permit its' use up to 13.8v. If you use one, be sure to check yours before running it at 13.8v.
Putting It All TogetherFitting the Connectors
For this project, I used RJ-LNxx weatherproof ethernet connectors. For cost efficiency, two of the standard PCB Board receptacles for the rootenna case and soldered on shielded ethernet jacks available from a local electronics store. "IP67 Receptacle Closure Caps" were used to cover the receptacles when not in use. Also, I made some long cables with the RJLynxx "Field-Attachable Connectors" on some 200 ft outdoor rated CAT-5 cables that I keep handy for use with the system.
In addition, I installed an male RP-TNC to female N bulkhead pigtail to provide external access to one of the WRT54G antenna connections, and mounted a weatherproof cap to cover the connector when not in use.
The Rootenna comes with only one hole for the supplied cable gland. It is necessary to drill holes and cut the sheet metal to fit all three connectors. You get to figure out how to do this with the equipment available to you. However, I will say that the RJLynxx connectors fit neatly through the hole left by a Greenlee 3/4" conduit punch.
Building the PoE
Since I soldered the internal RJ-45 connector onto the pc board part of the the RJ-LNxx jack, it was a simple matter to solder a power connector to the proper pins. Since I am putting two RJ-LNxx connectors on the unit, and either might be required to act as the power supply to the unit, both had to be connected together. This is slightly risky, as both jacks will be powered all the time, and power not taken off the CAT-5 going to the WRT54G. However, the conductors used for power are not connected on my WRT, and I'm not planning on using anything with the other jack that wouldn't be powered also.
Mounting the Radio
Next, we have to take the WRT54G out its' case and mount it on the Rootenna mounting plate. Taking a WRT54G apart is easy, but voids your warranty. Be really really sure you want to do this before preceding. The WRT54G case is a three piece affair. A description and pictures of how to do this is found at http://voidmain.is-a-geek.org:8100/redhat/wrt54g_revival.html. Simply, squeeze the grey upper and lower part of the case and pull the blue faceplate off. The router board is secured to the bottom case component by one screw. Remove that screw and the board slides off.
None of the holes on the circuit board coincides with the pre-drilled holes on the Rootenna mounting plate. Being relatively lazy, I didn't want to drill additional holes, so I attached some 1/2" standoffs to the mounting plate such that they would fit under the circuit board. A bit of electrical tape was stuck on the standoffs to keep the circuit board from rubbing against the nylon standoffs. Four cable tie bases were screwed to the mounting plate, outside the circuit board footprint, but next to the slide lock holes on the board. Then I sat the circuit board on the nylon standoffs and secured the circuit board to the cable tie mounts with small cable ties.
I secured the circuit board so that one antenna connector was near, but not directly under the center of the case so that there'd be minimum bend in the antenna pigtail connector. Hardware Upgrades
Because this unit was intended to be used for short periods where rapid installation is desirable, the nuts that came with the mast mount were replaced with wingnuts.
In OperationSo far, members of the North Texas Microwave Society - HSMM working group have used this rootenna design for the 2005 Wild Ride bicycle rally in Richardson, TX. The antenna is surprisingly hard to aim, so a temporary aiming aid is beneficial. In addition, four were built for the Irving Amateur Radio Club, for use in their events.
Follow-on WorkThe Rootenna is large enough to hold two WRT54Gs. This would let me use one box to for a bridge and an access point, or two bridge radios. If I were to do this, I'd probably put an additional metal box around each radio to minimize cross interference.
Parts SuppliersRadios, Cables, Jumpers, PoE