What is HSMM Microwave Radio?

John Champa, K8OCL

What is this HSMM stuff we hear about? What kind of microwave radio do I need to use that? Is this something cool and fun that I might enjoy? Is it expensive? What microwave frequencies are used? Will I understand how it works? What can it do?

First, HSMM stands for High Speed Multimedia radio. It is not a specific operating mode, but more of a direction or driving force within amateur radio.

Second, HSMM, although digital radio, it is NOT primarily keyboard radio communication, as in packet radio. Amongst the capabilities of HSMM radio most used are digital voice (DV) and digital video (ADV). Yes, you can type keyboard messages back and forth (chat mode) as in PSK31. You can also do file transfers as in RTTY, but at significantly higher speeds! Plus, if there is a server on the radio network, you can do e-mailing and maybe even surf the Internet by sharing a high-speed Internet connection with another amateur. That is why it is called multimedia radio.

NOW WHAT? So you may ask, how can we use this? Imagine being at an emergency scene and being able to send live video images of what is happening back to everybody on the HSMM radio network, back to the EOC, and without using expensive ATV equipment. All that is often needed is just your laptop computer, a special PCMCIA card, your digital camera, and a small antenna. Except for the inexpensive special card and the antenna, you may already have all this gear!

So, now that you know what it is, how about a little history to show how it evolved?


A survey conducted by the ARRL Technology Task Force, of League members and other amateurs revealed that the number one amateur radio interest in new technologies was in high-speed digital radio networks. Some suggestions were:

High-speed radio data links up to 20 mega bits per second (M bit/s)
Ethernet at 2 mbps on 10 GHz
Encourage development of a high-speed amateur digital radio network
High-speed digital audio/video radio

In January 2001, the ARRL Board of Directors voted unanimously that the ARRL should develop high-speed radio digital networks for the amateur radio service. The ARRL President appointed a group of individuals knowledgeable in the field from the international amateur community and industry. The group would report to the Technology Task Force, and its chairman is the author of this article.


The working group’s first focus is on creating the microwave skills within the amateur radio community that are necessary to build and support portable and fixed high-speed radio based local networking.  That's where amateurs interested in HSMM microwave radio can offer the most value to local RACES and ARES organizations, as well as to homeland security and other emergency communications efforts at the present time.

During Field Days and simulated emergency tests (SET) we encourage amateurs to hone their skills in doing rapid site-surveys and deploying broadband HSMM microwave radio networks in the field. Some clubs put all their Field Day stations with their laptop-based log books onto one radio local area network (RLAN) so, for example, if you are working 20M you can see that the 15 meter band has just opened up because that tent’s logging score is going through the roof (HI). During the recent Midwest VHF-UHF Society (MVUS) Picnic 2003, an RLAN was used to link antenna test measurement results back to a printer. You get the idea. Use your imagination. The possibilities are almost without limit.

In this process of everyday use of HSMM microwave radio, we are trying to understand how to enhance the reliability of our mainstream amateur radio network connections. Through various emergency communications training programs we will be trying to incorporate information to help local hams to be the people that deploy these high-speed microwave radio networks on demand.

One way you can become involved today is by adapting off the shelf IEEE 802.11® gear to operating within amateur radio regulations. This equipment is also known as WiFi equipment, and is commonly available at computer equipment retailers. As sold, the equipment operates in the 2.4 GHz ISM bands under part 15 rules. The 802.11b standard was developed about 6 years ago for the purpose of providing a wireless alternative for office LAN installations. This wireless capability was to allow office LANs to be deployed without the expense and nuisance of running CAT5 cable to each computer. Due to the increase of homes with multiple computers as well as rapidly falling price points for WiFi equipment, WiFi hardware has had a significant penetration into the home market place. In fact, in a recent CQ survey (What You Have Told Us, CQ, September 2003, p.40) 8% of the respondents reported already using some kind of wireless networking, so there is already a growing understanding of the technology within the amateur ranks. A number of Livingston County amateurs already use WiFi in their homes.

The equipment as purchased has significant operating limits. Due to the Part 15 operating rules, power is severely curtailed. Remember, just like cordless phones, this hardware has to allow uncoordinated operation of many unlicensed devices with minimal interference. In addition, many users of this technology adopted it because it allowed unencumbered connectivity for a laptop computer. By nature of the fact that a laptop is battery powered, most client cards offer only a small fraction of the power authorized under Part 15 rules, in order to maximize battery life.

Depending on your needs, as an Amateur neither of these considerations is a limit on your use of the WiFi technology. While a system as sold may have a range of only 50 to 100 feet, proper setup of a system under amateur regulations can provide coverage far in excess of that distance. In fact, one of our HSMM Working Group’s test networks, called the Hinternet, in Livingston County Michigan can easily do 5-15 miles ranges, at speeds of up to 54 M bit/s (half-duplex) using small mast mounted dish antennas in conjunction with off the shelf consumer grade hardware.


Getting operational with this equipment is a bit more complex than going to Universal Radio®, buying an HF, VHF, or UHF rig and going home and connecting a key, microphone and antenna. Because this is data radio local area network (RLAN) equipment, it expects to be communicating with a computer, or more precisely with software running on a computer. So you first must decide what interfaces you are going to need to connect to your computer. Luckily, equipment is available for all the standard computer interfaces: Ethernet, USB, and PCMCIA.

If you use a laptop in your station, get the PCMCIA card. We recommend the type with an external antenna connection.

If you have a PC, get the WLAN (Wireless LAN) adaptor type that plugs into either the USB port or the RJ45 Ethernet port. Select the one best suited to your computer and your experimentation.

This is the heart of your new microwave digital radio station. It is a computer-operated HSMM 2.4 GHz microwave radio transceiver and it will probably set you back about $60-$80. It is usually easier if you start off teaming up with another ham radio operator living nearby, and do your initial testing in the same room together. Then as you increase distances going toward your separate station locations, you can coordinate using a suitable local FM simplex frequency. The most often used HSMM microwave voice coordinating frequency is 446.00 MHz, the National Simplex FM Calling Frequency for the 70cm band

Go to the local OfficeMax ®, Radio Shack ®, ABC Warehouse ®, or other consumer electronics outlet and purchase some economical and readily available wireless local area network (WLAN) devices. We recommend that you select devices that state they comply with IEEE 802.11b and are WiFi compatible. Because these devices are made by numerous manufacturers each using different techniques to achieve the same thing, there were initial complaints about interoperability between devices of different manufacture. An industry group known as the WiFi consortium, was formed to provide testing and certification of 802.11b devices. If the equipment is WiFi certified, it will interoperate with any other WiFi certified equipment, thus easing your initial installation and troubleshooting by assuring that device compatibility is not the root cause of a start up problem. These devices operate on 2.4 GHz band using direct sequence spread spectrum (DSSS) modulation at speeds up to 2 mbps and complementary code keying (CCK) modulation for speeds of 5.5 and 11 M bit/s. Operating speed is automatically selected by the equipment based upon signal to noise and signal strength of the operating channel. These 802.11b devices are usually the least expensive, are the easiest to work with, and offer the good propagation.

If you can afford a few extra bucks, move up to the newer IEEE 802.11g devices. 802.11g is a relatively new standard that increases the speed of the channel from 11 M bit/s maximum to 54 M bit/s maximum. They also operate on the same 2.4 GHz frequencies, but use a form of modulation called orthogonal frequency division multiplexing (OFDM) to achieve the higher data rates. OFDM requires significantly more signal strength and signal to noise ratio in order to achieve 54 M bit/s throughput but appears to tolerate multipath effects caused by radio signal reflections better than CCK modulation, so it may offer better propagation characteristics in certain cases.

When purchasing your equipment there are a few things to be aware of. First make certain the supplied rubber duck antenna(s) are removable and/or there is an external antenna port. If the device does not have an external antenna connection, check Don Rotolo’s (N2IRZ) article in the February 2003 CQ for details on how to modify the device. Second, look at the radio specifications for the device. The transmit power and receive sensitivity varies widely among devices. Try to buy a device with the best (highest power and lowest receive sensitivity) specifications. The best generally available equipment has 100 mW (20 dBm) transmit power and –93 dBm receive sensitivity at 11 M bit/s, while the poorest specs are 25 mW (13 dBm) transmit power and –87 dBm receive sensitivity at 11 M bit/s.

Radio at these frequencies behaves the same as radio at any other: a 6 dB power increase will double effective range. Here we’re dealing with a 12 dB advantage of the higher performance equipment versus the lower. In a small Part 15 home WLAN the difference is probably not noticeable. For our purposes operating longer distances under amateur radio regulations, a 12 dB difference is critical, and can make the difference between successful experimentation and frustration and failure.

If the device does have an external antenna connection, then go to any issue of CQ Magazine and look up Nemal Electronics ®, CableXperts ® or other cable supply source and order an 18"-24" strain relief cable, also called a pig tail. Order the type of pigtail needed for your device. It will probably cost less than $20. If you purchased a PCMCIA card, the pigtail will have a strange looking miniature antenna connector at one end, and should have a normal N-series connector at the other.

The first thing you will need to do is install the device in your computer. If you are using a PCMCIA or USB device you will need to install drivers. If you are using a device with an RJ-45 Ethernet interface, no drivers are needed for the device, but there are drivers needed for the Ethernet port in the computer. Additionally there will be a method to communicate with this device for configuration. The included directions will explain how to accomplish this.

After you load up the software drivers on your PC, you will have 2 choices for configuring the equipment: ad-hoc and infrastructure mode. For now, set the device for "ad hoc" mode, and set it to any channel between 2 and 5 (they’re in the correct portion of the amateur band). If all is operating correctly, the two cards (yours and your buddies) should see each other and set up a communication session. Once the cards are talking, you can share files between the 2 computers in the same manner as if the computers were hardwired together on a LAN.

Once you have the cards tested and know you have a connection between them, it’s time to add the outdoor antennas and see what distances you can achieve between the 2 devices. Hook up any external commercial (e.g., Comet ®) or home-brew 2.4 GHz antenna. Shop around where the AMSAT-OSCAR 40 guys buy their Mode-S antennas for some good designs. Keep in mind, the higher the gain of the directional antenna, the smaller the main lobe will be, so aiming a high gain antenna will be more critical than aiming a low gain one. Most 802.11 equipment has a utility included that shows signal strength and signal to noise ratio. Using this utility to monitor signal strength as you aim your antenna will be of great assistance in finding the optimal aiming direction. Remember these antennas are directional in both horizontal and vertical planes, so you have to carefully aim in both azimuth and elevation to get optimum signal at the receiver.

Another thing to keep in mind is coax. Coaxial cable losses at these frequencies are enormous. Don’t even try to use RG8 cable to connect between the device and antenna. You will need to purchase the best coax you can afford in order to keep line losses minimized. In fact the antenna coaxial cable will likely be the most expensive part of the entire station, as you will want to use the lowest loss type you can handle, e.g. LMR-400, etc.

That’s all there is to it. Best of all, you may not have spent more than $100 so far, depending on what antenna hardware you have around.

Now point your antennas at each other, and fire away. At these power levels there is not much concern for RF safety, but if you are using a high-gain antenna, it is recommended you avoid standing directly in front of the business end while on the air.

Do remember that it’s your responsibility to properly identify your station during use. In the mode you are presently in, the ad hoc or direct station-to-station mode, the most common technique is to simply ID in-mode, i.e. if you transmitting voice, simply speak your call sign into the microphone. If you are transmitting video, just hold a QSL card up to the camera. Or, you can send a ping containing your call sign. Remember that as long as the RIC (radio interface card = short for a WLAN PCMCIA card used for HSMM radio) is operating, even with no traffic, the system is transmitting!

Depending on how close your ham buddy is to your location, how high and clear over the trees your antennas are (2.4 GHz doesn’t go through trees very well), the quality of the coaxial cable you are using and many other factors, you should be able to get several miles range. Remember, these HSMM microwave radio devices are truly QRP and run only about 30-100 mw of RF output, so be resourceful and experiment often with different antennas, etc.

If your signals are not covering the path between you and the nearest other HSMM microwave radio station, then open a copy of any edition of the ARRL Handbook and read the sections on antennas, transmission lines and UHF propagation. Consider putting the antennas higher, getting or building higher gain antennas, using lower-loss coaxial cable, etc. until the link is achieved.

You may also find a way to mount you gear at the antenna, and avoid the expense and loss of coaxial cable, too. This is another reason to consider devices that have Ethernet output. Standard CAT5 Ethernet cable can be run up to 300 feet with no loss. In comparison, USB can only be run 9 feet without a signal booster being installed. By using an Ethernet based device, it is easy to remotely mount the unit close to the antenna and run cheap CAT5 cable back to your computer.

Running higher power is an expensive last resort. Not just because it is a sound operating practice to run the minimum power needed to maintain the communications, but also because it is just good old-fashioned common sense. Be considerate of others who may be using the band, both amateur and non-amateur. Use only the minimum power needed for the link.


The next step up the functionality ladder is to add a "repeater" to your system. More properly called wireless hubs or access points (AP), this device will allow several amateur radio microwave stations to share the network (and all the devices and circuits connected to it). An 802.11b AP will sell for about $100 and 802.11g AP for about $140. The AP acts as a central collection point for traffic, and can be connected to a single computer or to a network. It can also be used to allow several amateurs to share one high-speed Internet connection. The AP is provided with an ESSID, which is the name it broadcasts. For our purposes, the ESSID can be set as your call sign, thus providing automatic, constant identification. To use an AP in your network the computer users have to exit ad-hoc mode and enter infrastructure mode. Infrastructure mode requires you to specify the network the device belongs to. This is what the ESSID does: identify the AP to its users, so the users can find the home system they belong to. Set your computer device to recognize the ESSID you assigned to your AP.

The AP can also be used as one end of a point-to-point network. For example, if you wanted to extend a network connection from one location to another, you could use an AP at the network end and use it to communicate to a computer at a remote location.

Using an AP allows more features and security than provided by ad-hoc mode. For example, most APs provide DHCP service, so they will automatically assign an IP address to the computers connected to the network. In addition, they provide filtering that allows only known users to access the network.

What frequency do you set on the AP? The HSMM Working Group recommends to everyone that they avoid using channel 1 (center frequency of 2412 MHz). Use of this channel may cause interference to the AMSAT-OSCAR 40 satellite downlinks. The WG also recommends that amateur radio stations avoid the use of channel 6 (center frequency of 2437 MHz). This is the most common default frequency used by the majority (80%?) of Part 15 unlicensed stations for wireless local area networks (WLAN). At present, the most popular frequency used by amateur radio microwave stations for HSMM research is 2427 MHz (channel 4).

NOTE: If you use an HSMM microwave repeater to share high speed access to the Internet, don’t forget amateur radio content restrictions, e.g. no porn, no commercial business e-mails, etc. Don’t worry about pop-up ads. Pop-up ads, although a nuisance, are no more illegal than an ATV station transmitting an outdoor scene inadvertently picking-up a billboard in the station camera.


For operating software, most amateurs are using Microsoft ® NetMeeting collaborative software, which comes free with the Microsoft Windows ® operating system. Other forms of open source groupware using Linux are also popular. Try using OpenH323 or Speak Freely. By connecting a microphone to the audio input of your soundcard, you can have digital voice QSOs. By connecting an inexpensive digital camera ($20) you can do digital video QSOs. These are not the same quality as the usual ATV contacts, but the equipment is much less expensive! (HI)


How do you keep Part 15 unlicensed traffic from accidentally using your Part 97 licensed HSMM network? A traffic separation technique that is considered acceptable involves the use of WEP (wired equivalent protection)...NOT for encryption, but for authentication.  If you use this approach under Part 97, therefore, you must publish the WEP key. We recommend that you ask that your HSMM repeater’s WEP key be published on the HSMM webpage (www.arrl.org/hsmm/), or simply use the amateur common WEP key already designated on that webpage. Again, the WEP is used to avoid the accidental mixing of Part 15 and Part 97 traffic, i.e. authentication, NOT encryption. Another approach gaining in popularity with many HSMM microwave stations is the use of 44 domain IP (Internet Protocol) addresses, which are only available to the amateur radio service.


Amateurs of all license classes are encouraged, to get on the air with inexpensive HSMM microwave radio using 802.11 off-the-shelf gear operating under amateur regulations. It is easy, low-cost spread spectrum microwave radio experimentation. For more details and for the latest developments on all these initiatives, check out the link to HSMM WG open reflector at Texas A&M University on our webpage: www.arrl.org/hsmm.



Ford, Steve – WB8IMY, "VoIP and Amateur Radio," QST. February 2003, pp. 44-47.
Mraz, Kris I – N5KM, "High Speed Multimedia Radio," QST, April 2003, pp. 28-34.
Olexa, Ron - KA3JIJ, "Wi-Fi for Hams Part 1: Part 97 or Part 15," CQ, June 2003, pp. 32-36.
Olexa, Ron – KA3JIJ, "Wi-Fi for Hams Part 2: Building a Wi-Fi Network," CQ, July 2003, pp. 34-38.
Reinhardt, Jeff – AA6JR, "Digital Hamming: A Need for Standards," CQ Magazine, January 2003, pp. 50-51.
Rinaldo, Paul L. – W4RI, and Champa, John J. – K8OCL, "On The Amateur Radio Use of IEEE 802.11b Radio Local Area Networks," CQ-VHF, Spring 2003, pp.
Rotolo, Don – N2IRZ, "A Cheap and Easy High-Speed Data Connection," CQ, February 2003, pp. 61-64.


John Champa, K8OCL, is Chairman of the American Radio Relay League (ARRL) Technology Task Force on High Speed Multimedia (HSMM) Radio Networking. He can be reached Moon Wolf Spring, 2491 Itsell Road, Howell, MI 48843. e-mail: <k8ocl@arrl.net>.

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This site last updated  Oct 22, 2003