Hams have always managed to adapt inexpensive commercial gear for amateur radio use. In decades past we adapted military surplus and used commercial radios. Now in the digital age we can use inexpensive WiFi gear for amateur use. A complete 13cm (2.4ghz) ham station can be purchased for $100 or less (see WiFi Devices below). The station can operate under FCC part 97 on a portion of the 13cm ham band that isn’t used by unlicensed 802.11 WiFi devices. Or the station can be operated under FCC part 15 on 802.11 frequencies and inter-operate with non-ham wireless networks in your neighbourhood.
WiFi RF Spectrum:
Most consumer 802.11 devices, commonly called WiFi, operate in the 2.4ghz (13cm) band. Some WiFI manufacturers make 5.8ghz (5cm) and 900mhz (33cm) equipment. Hams share the 2.4ghz and 900mhz bands with ISM (Industrial Scientific & Medical) devices which includes WiFi devices.:
Unlicensed ISM (FCC part 15): 2400 – 2495 Mhz
Amateur (part 97) 13cm band: 2390 – 2450 Mhz
Hams are the PRIMARY users in the US on 2390mhz-2417mhz and secondary users elsewhere. The ISM band is divided into 11 overlapping 802.11 channels, each 5mhz wide. Channel 1 is centered at 2412mhz. Channel 11 is centered at 2484mhz. An 802.11 RF signal is 20mhz wide, using parts of 5 adjacent channels. So, there are only three non-overlapping channels, 1, 6, and 11. Hams may use WiFi gear under FCC part 97 if they stay within 802.11 channels 1 through 6 so the RF signals will be within the 13cm ham band. Also, some WiFi devices can be changed to use a 5mhz wide signal, at 1/4 the data rates, on frequencies outside the 802.11 channels. For example, Ubiquity (http://www.ubnt.com) outdoor WiFi devices can operate at 2397mhz with a 5mhz wide signal, as suggested by the HSMM 13cm band plan. This is within the PRIMARY portion of the 13cm ham band but not in the ISM band. This should reduce unwanted interference from part 15 devices.
Earlier 802.11b equipment used spread spectrum techniques. Later 802.11g equipment uses OFDM (orthogonal frequency division multiplexing). Most modern WiFi devices will support both 802.11b and 802.11g, automatically adapting to the method in use. WiFi devices can automatically choose different modulation techniques, depending upon signal strength, to achieve higher data rates when RF signals are strong enough. 802.11g devices can choose among BPSK, QPSK, 16-QAM and 64-QAM modulation types, used to modulate each sub-carrier of the OFDM signal. This gives data-rates betwen 6 and 54 mbps (megabits per second). 802.11b modulation gives data rates between 1 and 11mbps.
RF Power levels:
Transmitter power of home WiFi gear is typically around 50mwatt to 100mwatt. FCC part 15 limits transmitter power to 1 watt. There are some inexpensive WiFi devices with 0.4 watt to 1 watt transmitters. Part 15 limits the ERP (Effective Radiated Power) by requiring that the transmitter power be reduced by 1db for each 3db of antenna gain in excess of 6db. So, for example, a WiFi device using an 18 db gain antenna must transmit using no more than 1 watt minus 4db (i.e. 0.397watts). The 4db is calculated from (18-6)/3. Part 97 users have no such antenna restrictions, and transmitter power is limited to the usual 1500watts PEP (for 802.11g which is not spread spectrum). Very high power is a moot point for most hams, since 2.4ghz power amps are expensive. A 10 watt 2.4ghz two-way amp, which includes a receive pre-amp, costs over $800 at this time. However, using a 2.5 foot reflector or yagi antenna with 24db gain, costing about $60, and a 0.8 watt transceiver costing less than $100, hams can achieve an ERP of 200 watts. Such an antenna will have a “beam width” of about 8 degrees, which will reduce interference from other WiFi devices.
2.4ghz propagation distance:
The main disadvantage for hams use of Wifi, as with all microwave communications, is that range is effectively limited to line-of-sight distances. I observe that trees, houses, etc., reduce a 2.4ghz signal by 20db or more. Antennas mounted atop typical 50′ ham towers in my experience provide about 2 to 4 mile range. To achieve 2.4ghz line-of-sight communications over a distance of 24 miles, comparable to the range of a typical ham VHF/UHF repeater, assuming level ground with 50′ tall trees and buildings, you’d need both antennas to be 120′ above ground. This is beyond the ability of most individual hams, although ham clubs can often put antennas above 200′ on existing towers or tall buildings. A 2.4ghz repeater with antenna at 300′ would be needed to provide a 24 mile coverage radius to users with antennas only just high enough for the signal to be clear of nearby trees, buildings, etc. The required antenna height is determined by adding the height of obstructions to the height needed to achieve the desired “radio horizon” distance. Visit http://www.qsl.net/w4sat/horizon.htm and enter the two antenna heights. This calculates radio horizon distance. You must add to this antenna height the height needed to overcome any obstructions in the signal path. When considering the height of obstructions, it is common to add an additional 10′ to 15′ to keep the entire Fresnel zone free of obstructions.
You must also transmit sufficiently powerful signal to overcome the “path-loss”. The 2.4ghz free space path-loss is about 100db+20log(kilometers). For a 24 mile (40km) path, the path loss is 132db. A 24db gain receiving antenna reduces it to 108db. With a 0.8 watt transmitter and 24db gain antenna, the ERP of the transmitted signal is +50dbm (200 watts). This signal will be 35db above a typical -93dbm noise floor at the receiver. With the WiFi devices I’ve used, any signal stronger than about 20db above the noise provides a reliable connection.
An on-line calculator, provided by ve2dbe and towercoverage.com, will predict the received signal strength between any two points you select on a map. It takes into consideration the frequency, transmitter power, receiver sensitivity, antenna gain, antenna height. It uses terrain elevation map data, vegetation (trees) data, and urban (building) map data. I’ve found this calculator to predict fairly accurately the actual signal strengths I’ve observed. Received signal strength in excess of about -75dbm usually is sufficient for the WiFi equipment I’ve used. You must sign-up for a free account to use this online calculator
Ham WiFi Uses:
Because of the need for line-of-sight, one practical use for WiFi may be the wireless linking of existing analog VHF/UHF ham repeaters. Most ham repeater antennas are at fairly high locations, so maintaining line-of-sight over many miles should be achievable. Assuming level ground and 50 ft obstructions (trees, buildings, etc), two repeaters with antennas 200′ AGL have line-of-sight for 34 miles. For 150′ antennas, line-of-sight is 28 miles. The repeater audio would be changed to digital VOIP and sent between repeaters. There is free software (Asterisk, Ekiga, etc) that could be used for this. There would be plenty of bandwidth left over to use for other purposes, such as APRS, etc.
Digital Voice and Digital Amateur TV:
Digital Voice and ATV are easily accomplished with WiFi. A WiFi link provides plenty of bandwidth for digital voice communications without the need for voice data reduction schemes, as are required on D-STAR, P-25 and other narrow-band schemes. Free VOIP software such as Ekiga or older versions of NetMeeting can be used. As you speak into a microphone attached to your computer’s sound card, the software sends it as VOIP data to the receiver’s computer, where it is converted back to audio using that computer’s sound card. The same software can also be used for video links as well as audio. The only difference is that video from a camera is digitized and sent rather than audio from a microphone.
Make use of AMPR IP addresses (44.xx.xx.xx):
In the late 1970s, at a time when IP addresses were not in short supply as they are now, hams were granted a large block of about 16 million IP addresses (44.xx.xx.xx) worldwide, for use in connecting ham radio equipment to and through the internet. This IP block is known as AMPRNet (Amataur Radio Packet Network). See http://www.ampr.org for more information. Sub-blocks are given to each state and major cities. For example, Missouri has block 44.46.xx.xx. States have one or more hams who coordinate use of the IPs. WiFi gear used by hams could make good use of these IP addresses, before someone tries to take them away from hams. IP addresses are now valuable. Microsoft recently paid $7.5 million for a little over half a million IP addresses.
WiFi devices typically don’t have control knobs like a radio. Instead they contain a web-site which is used to control the device from your computer using your favorite web-browser (Internet Explorer, Firefox, Chrome, etc). You normally plug your WiFi device directly into your computer using an ethernet cable, and configure it using your favorite web-browser.
IP addresses are the “call signs” of computer networks. Hams call each other on-air using call signs. Similary, when computers want to talk to each other over a network, they call each other using “IP Addresses”. Each device on a computer network is assigned an IP address, just as each ham operator is assigned a call-sign. Hams usually give the “destination” call-sign and their own call sign at the beginning of each transmission. Each computer MUST give the “desgination” IP address and it’s own IP address (the “source” address) at the beginning of each transmission. Each computer transmission is known as a “packet”. Therefore, to talk to the WiFi device, your computer must know the “IP Address” of the device. For the WiFi device to reply to your computer, it needs to know the IP address of your computer. Unlike ham call-signs, which can only be assigned by the FCC, any computer can assign itself an IP address, provided that IP address isn’t already used by some other computer in the network. Your WiFi device user manual will tell you the factory default IP address of the device. It is typically 192.168.1.1 or 192.168.1.20 or 192.168.1.50. You normally plug your WiFi device directly into your computer using an ethernet cable, and point your web-browser at http://192.168.1.1 (or xxx.20 or xxx.50).
Normally each computer on a network doesn’t choose it’s own IP, but is assigned a unique IP address by one computer on the network acting as a “DHCP server”. Just as the FCC assigns each ham a different call sign, the DHCP server assigns a different IP address to each new computer when it is turned on and joins the network. In your local home network, the router that was provied by your ISP probably acts as the DHCP server. However, after you’ve plugged your computer directly into the WiFI device, your computer is no longer connected to the DHCP server, so it cannot automatically obtain an IP address as it usually does. So you probably need to use the “network connections” icon on your computer’s control panel to manually specify an IP address for your computer. Choose an IP address such as 192.168.1.XX, but don’t choose the same IP as your WiFi device. You’ll also need to specify a network “mask”. Specify 255.255.255.0. If required, specify a “gateway” IP address of 192.168.1.1. After you’ve done this, you should be able to enter “http://192.168.1.1″ or “http://192.168.1.20″ into your web-browser and see the login-page of the WiFi device’s website. Your WiFi user manual will tell you the login name and password to use.
Router vs. Bridge mode:
Each WiFi device can typically be configured either in “router” or “bridge” mode. Your internet service provider probably provided you with a router that your computer connects to. Each single local computer network needs only one router (although more routers are allowed). So you’ll probably want to configure your WiFi device in “bridge” mode, which is usually the default setting. Router mode and computer network routing is beyond the scope of this webpage. Hams most often use WiFi routers in a wireless MESH network (see http://hsmm-mesh.org for example).
In bridge mode, your WiFi device acts much like a ham “repeater”. The “bridge” device will re-transmit any transmission that it “hears”. The bridge records each incoming transmission and then immediately re-transmits it. Unlike your local ham repeater which has one transmiter and one receiver, the bridge has two transceivers, one attached to the ethernet cable and one attached to the antenna. Each incoming transmission on either transceiver is re-transmitted on the OTHER transceiver. So data heard on the wireless receiver are re-transmitted over the ethernet cable, and data heard on the ethernet cable are re-sent over the wireless transmitter. So a pair of wireless bridges, each of which is wired to a computer, act just like a long ethernet cable connected between the two computers.
Access Point vs. Station mode
In a ham VHF or UHF repeater, all users transmit to the repeater and receive the repeater. They don’t communicate “direct” to each other. Similarly, in a WiFi network, there is one WiFi device configured as an “Access point” which acts like the ham repeater. Every other WiFi device is configured as a “station”. Each station communicates only with the Access point.
Just as hams can communicate “direct” with each other, without using a repeater, so can WiFi stations communicate with each other. This is called “peer-to-peer” communications, or sometimes “ad-hoc” networking.
WDS (Wireless Distribution System) mode:
Hams sometimes link nearby repeaters to each other using RF links. Each repeater “forwards” incoming transmissions on to the other repeaters. This provides for a wider coverage area, allowing anyone within range of any repeater to talk to anyone else within range of any repeater. Similarly, a non-standard 802.11 extension called WDS can be used to link several nearby WiFi access points together. One of these “WDS Access Points” is usually connected to the internet. Each other WDS Access Point forwards data between wireless “stations” connected to it and the other WDS Access Points. In this manner, data from any “station” Wifi device uses multiple wireless “hops” between WDS Access Points to reach the internet connection. Each “hop” takes about the same amount of time to send one packet of data to the next hop. So, with 2 hops the effective data-rate is reduced to 1/2, with 3 hops the data-rate is reduced to 1/3, etc. Finally, since WDS is not part of the 802.11 standard, different manufacturers might implement it differently. So there’s no guarantee that WDS will work among devices from different manufacturers.
WIFi MESH networks:
One problem with using WDS is that each time you add a new WDS Access Point you must manually specify to it which other nearby WDS nodes it should forward data to. You must also reconfigure the nearby WDS nodes, adding the newly added WDS node to their list. This slows down rapid deployment of a WDS network, especially when the network must change often to accomodate changing emergency conditions. This manual configuration problem can be eliminated using the HSMM-MESH firmware from http://hsmm-mesh.org. The firmware is uploaded into Linksys WRT54G WiFi devices. When several such devices are deployed as a MESH network, the devices will automatically discover each other, and automatically configure their routing so that data can go between any two computers attached to the network, or between any computer and the internet connection. As MESH network nodes are added and removed, the firmware automatically discovers the changes and reconfigures itself.
The OpenWRT firmware is also popular among mesh network enthusiasts. OpenWRT can be loaded onto many different WiFI devices, including new models with higher powered transmitters designed to be mounted outdoors next to an antenna. OpenWRT is basically a small distribution of the Linux operating system designed to run on the CPU within WiFi devices. It provides many ways to customize your WiFi devices using various application firmware (smart phone users call them “apps”). Among these are the popular B.A.T.M.A.N., OLSR and R.O.B.IN firmware, available for free at http://opensourcemesh.org. These firmware “apps” implement mesh network routing protocols.
In metropolitan areas, there may be two or more ham repeaters on the same frequency within range of a ham with a good antenna. So, ham repeaters use CTCSS to allow each ham to select which repeater he wants to communicate with. To use any particular repeater, each ham configure his radio to use the CTCSS of that repeater. Similarly, each WiFI Access Point chooses an SSID. The SSID can be any word of 32 letters or less. Each WiFi station that wants to talk to the Access Point must use the same SSID as the Access Point.
802.11 WiFi devices may encrypt their transmissions for security reasons. There are two encryption methods to choose from: WEP and WPA (now WPA2). When using encryption, the WiFi Access Point is configured to use a secret word, called a “key”. Every station wishing to communicate with that access point must be configured to use the same key. Hams using WiFi devices under part 97 must disable encryption which is expressly forbidden by the FCC.
Deploying a WiFi device:
Hams usually want to mount a WiFi antenna outside, or at least near a window, so the $20 to $30 USB WiFi devices are not a good idea unless your computer is outside or near a window. There are many WiFi devices designed for home use, often called “Access Points”, in the $30 to $50 range, that have an ethernet connection to your computer instead of USB. These typically have 50mw to 100mw transmitters, and attached antennas about 4 inches long with a gain of about 5db. Due to the excessive losses in long runs of coax cable, hams usually mount the entire WiFi transceiver, including antenna, outside on a roof or tower. In years past I’ve enclosed indoor WiFi devices in PVC weatherproof boxes from Home Depot ($12 to $35 depending upon size). You then run an ethernet cable (Cat-5 network cable) from the WiFi device to your computer. Now days there companies like Ubiquiti and EnGenius make outdoor Wifi transceivers with 500mw transmitters, integrated antennas and “power-over-ethernet” (POE) for $65 to $100. POE eliminates the need to have electric power atop your tower or roof. At these inexpensive prices, there’s no longer any economic reason to build your own weatherproof enclosure or antenna. For example, the Ubiquiti AirGridM2 with 16db reflector antenna and POE is outdoor mountable and costs $65 new.
Some WiFi Devices I’ve Used
The WRT54G has been around for almost a decade. It costs about $35 on ebay these days. It has about 100mw transmitter and a detachable antenna with a reverse polarity TNC coax connector (RP-TNC). Over the years Linksys built 8 versions of WRT54G. The first 4 versions are still popular with hams and others wishing to create a wireless MESH network. The http://hsmm-mesh.org group provides free firmware which can be uploaded into a WRT54G device (versions 1-4 only). Such devices can automatically detect each other if within range, and automatically route data through the network.
See http://ubnt.com for Ubiquiti’s line of WiFi devices. The NanoStation LoCo (100mw) costs about $48. The NanoStation2 (400mw transmitter) costs about $80. Each have a built-in 10db directional antenna. It has a weatherproof case suitable for outdoor mounting. It has power-over-ethernet, so you don’t need electric power at your antenna. Like other Ubiquiti outdoor products, the NanoStation2 uses the Atheros wireless chipset. These chips are capable of operation on additional frequencies in the 13cm ham band that are not allowed for unlicensed WiFi use in the US. Configure the Ubiquiti firmware’s country to “Conformance Test” instead of “USA” in order to gain access to these additional frequencies. Be careful to insure you then select frequencies within the ham band. Also like other Ubiquiti products, you can configure the device to use a 5mhz wide signal (non standard) instead of a 20mhz wide signal. This should provide increased range due to 4x power spectral density, at the expense of reducing the data rate to 1/4. So data rates are between 1.5mbps and 13mbps , instead of 6mbps and 54mbps. Even 1.5mbps is plenty of bandwidth for most ham uses.
The Bullet2 (100mw transmitter) costs about $40 new, and the Bullet2-HP (800mw transmitter) costs about $80 new. The outdoor weatherproof unit is shaped like a cylinder (a bullet) with the N-series male coax connector at one end. This makes it is easy to mount directly to a high-gain antenna which often come with an N-series female coax connector. It uses power-over-ethernet, but you must purchase the power supply separately (about $14). Several 24db gain parabolic reflector antennas, about 24″x39″ in size, are available for about $60 plus shipping. So a complete 13cm ham station with 200 watts (53dbm) EIRP costs only about $154, using your existing tower or antenna mast.
The AirGridM2 (500mw transmitter) costs $65 new and comes with integrated 16db reflector antenna, POE, and antenna mast mounting clamp. Just clamp it to a pole or tower and run cat-5 cable to your ham shack’s computer and you’re on-the-air on 13cm band. A version with a 17″x24″ 20db antenna is available for about $95, providing 50 watts (47dbm) EIRP. That provides a complete 13cm station using your existing tower or antenna mast.