The Art and Science of Radio Direction Finding (RDF)
Provided by Joe Moell, ham radio callsign KØOV. (That's K-zero-O-V)
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Results and photos of the on-foot transmitter hunting session at Bonelli Regional Park on Saturday, October 30, 2021 are now in this site. The next session will take place after the year-end holidays. These sessions are open to anyone at any skill level, with or without an Amateur Radio license. Usually they include a session for building two-meter measuring-tape yagis and offset type attenuators. For earliest notification of these events, you can join the southern California ARDF mailing list. If you live elsewhere, there is a calendar of upcoming ARDF sessions all over the USA.
The November 2021 issue of CQ Amateur Radio Magazine is now online and being mailed to print subscribers. In it, my Homing In column tells why it takes a committee of the International Amateur Radio Union to create and revise the rules for multi-nation radio-orienteering competitions. There are also stories of the 2021 CQ Worldwide Foxhunting Weekend. Some ham radio stores may also have the August 2021 issue, in which my column is a review of a compact and inexpensive ARDF receiver-antenna set for the 80-meter band. My column on radio direction finding appears in CQ Magazine at least four times per year. CQ Magazine is available by subscription and printed issues are sold in Amateur Radio stores. CQ is also available by subscription in digital form, viewable on PC, Mac, iPad, iPhone and Android. I welcome your input for future articles and columns, so please continue to send me your news of mobile and on-foot transmitter hunt activities.
The twentieth USA ARDF Championships, postponed from last year, were relocated to the forests of North Carolina. The formal championship contests took place October 14 - 17, 2021, with four days of informal practice and training beforehand. Sponsoring organization was Backwoods Orienteering Klub. There were classic five-fox courses on two meters and eighty meters plus sprints and foxoring events. USA's Championships are open to anyone at any skill level, with or without a ham radio license. Medal winners may be selected for positions on USA's team to future ARDF World Championships. Visiting competitors from outside USA are welcome. Gold medal winners in the sprint competition on October 14, the 80-meter classic on October 15 and the two-meter classic on October 17 included Bill Wright WB6CMD from southern California. Get the latest news here.
Scouting's annual Jamboree-On-The-Air (JOTA) was October 16, 2021. JOTA is an ideal time to introduce young people to all aspects of ham radio, especially hidden transmitter hunting. Only the transmitter hider has to have a ham license! If your club included COVID-safe transmitter hunting in this year's JOTA, please e-mail me for possible use in an upcoming article. For more information, see the Foxhunting for Scouts page at this site.
For the last twenty years, world ARDF championships have taken place in even-numbered years. Each of the three IARU regions have held multi-nation championships in odd-numbered years. But COVID is causing major changes in the scheduling of these multi-nation competitions. The 23rd IARU Region 1 ARDF Championships have taken place in Bulgaria and the First World Radio Orienteering Championships in Turkey are still on schedule for 2021. The next two IARU World ARDF Championships are being planned for 2022 (Serbia) and 2023 (Czech Republic). Beginning in 2024, regional championships will be in odd-numbered years and world championships in even-numbered years. Get more details in this site.
Quick Links to Information for Beginners
Learn about international-rules on-foot hunting (ARDF, radio-orienteering) and how the sport is organized
What Do You Want to Do?
Obtain RDF equipment for two-meter hunting on foot
Attend a southern California on-foot transmitter hunt
Participate in national and world championship hunts
Learn about mobile transmitter hunting (T-Hunting) and the equipment that hunters use
Attend a southern California two-meter mobile T-hunt
Buy or build a two-meter "fox" transmitter
Learn about 80-meter transmitter hunting
Top-Hit Transmitter Hunting Topics
More and more hams and orienteers are discovering the fun of tracking down hidden transmitters out in the woods. It's a worldwide sport with opportunities for friendly competition at home and abroad. A ham radio license isn't a requirement, but it makes it lots more fun. Learn the basics and get the international hunt rules at the "International Style Transmitter Hunting Comes to the Americas" page of this site.
The Southern California T-Hunts for Beginners page has has information about three monthly mobile hidden transmitter hunts in the Orange County area where first-timers are especially welcomed and encouraged. On two of them, there are usually some clues to help everyone eventually find the transmitter and on one, you can compare bearings with other hunters on a separate frequency. Mount some RDF gear on your vehicle and come on out!
Easy-to-use apps for Apple and Android tablets and phones can simplify bearing plotting and triangulation over short and long distances. They are useful for both on-foot and mobile transmitter hunting. Bearings can be entered manually or with the internal device compass. Some apps allow entering bearings taken by other hunters or networing with them during the hunt. One new app has an RS-232 interface to Doppler RDF sets. Read all about Foxhunt Pro, SigTrax Plus and Map-n-Compass apps in this updated compilation of Homing In columns.
Mobile hidden transmitter hunters have regularly prowled the streets in search of the elusive sources of unusual signals for more than four decades. Equipment has evolved, but the adventure and intrigue remain the same. Read "T-Hunting Then and Now -- From Gooney Birds to GPS" in this site for stories of classic mobile T-hunts in the Los Angeles area. Some of them, but not all, could be done again today. Then to find out what it's like nowadays, and to help get your club started in this activity, read "Transmitter Hunting, Southern California Style."
When it's your turn to hide the transmitter, what will you use? It depends on the range and duration of the hunt, as well as whether or not the transmitter must be unattended and automatic. It's important to match your foxbox and its location to the level of proficiency of the hunters. There are many options, and you can read about them in the Foxboxes for Mobile and On-foot Transmitter Hunts page in this site.
For three decades, international-rules radio-orienteering competitions have had two major competition days. Each participant must compete on the two-meter band and the 80-meter band. The 2012 USA and World ARDF Championships included competitions in two new events: sprints and foxoring. The sprint is a shortened form of the five-fox 80-meter ARDF run that's intended to be a demonstration for the public. Foxoring is a combination of classic orienteering and direction-finding on 80 meters. More information about these new events can be found here.
For over twenty years, I have used a special cubical quad for mobile transmitter hunting on two meters. From inside the vehicle, I can select the signal polarization. Find out why this is important, why I like this antenna and how to make one for yourself in a classic Homing In column titled "Build a Multiple-Polarization Quad."
The Agrelo DFjr Doppler RDF set has been out of production for over ten years, but there is still a great deal of interest in it. DFjr was the first inexpensive plug-and-play Doppler set designed for interface to computer mapping systems and APRS. For those who own one or are considering buying a used one, the DFjr page on this site has a downloadable manual, my 73 Magazine review, antenna system improvements, and frequently asked questions about this product.
There are over 100 additional fact-filled pages in this site, including techniques, product information and build-it-yourself projects for radio direction finding. For the complete Table of Contents and site search, click or scroll down.
What's "Homing In?"
Homing In refers to the process of tracking down the source of a radio or other electromagnetic signal using radio direction finding (RDF) equipment.
Homing In is also the title of my regular column on RDF that ran for 15 years in 73 Amateur Radio Today magazine, then for ten years in CQ-VHF magazine and now appears quarterly in CQ-Plus digital magazine. At this Homing In site, you will find more about these columns, plus RDF articles that I have written for other publications, including Monitoring Times, CQ VHF and QST magazines. There is also information about my comprehensive book on the subject.
Radio direction finding is used to find sources of interference to any
form of wireless electronic communications, including broadcast and two-way
radio, television, and telephones. It is also used to track missing or stolen
cars and other property. Search and rescue workers use it to find persons in
distress. Emergency Locator Transmitters in downed aircraft are tracked with
Most of the information at this site pertains to RDF equipment and techniques
for Amateur Radio (ham) operators. Hams use RDF to track jamming stations and
stolen equipment, but more often, they use it just for fun. Hidden transmitter
hunting has been done by hams for about seventy years and it is a growing
activity. T-hunting refers specifically to hunts involving hams driving in
RDF-equipped vehicles. A mobile T-hunt is best described as hide-and-seek for all ages with radio gear. When you set out on a T-hunt, you never know where you'll end up, and you have no idea what you're going to find. No form of ham radio contesting is more fun! Mobile T-hunting is done in cities and towns all over the USA, and elsewhere in the world. Depending on the frequency band and the nature of the hunt, the hunters use loop, yagi, quad, doppler and time-difference-of-arrival RDF antenna systems mounted on their vehicles. Click here for for general information about mobile T-hunting or click here for beginner-level T-hunts in southern California.
Mobile T-hunting is called foxhunting in some parts of the USA, but everywhere else in the world, the terms "foxhunting" and ARDF refer to another kind of RDF contest, done completely on foot in large woods and parks. It's a map-and-compass sport similar to orienteering, with about a half-dozen "fox" transmitters to find in a period of two hours or so. Someday this sport, which is also called foxtailing, fox-teering and radio-orienteering, may become an Olympic event. Meanwhile, it's a fun-filled activity for your hamfests and Scout Jamborees. Try it, and you may find yourself at the next annual national USA ARDF Championships. You might even become a member of ARDF Team USA, which has competed in five foxhunting World Championships. Click here for for general information about radio-orienteering or click here for beginner-level ARDF events in southern California.
Keep reading---you will find lots more about foxhunting, T-hunting, and other uses
of RDF at this site.
What's at the Homing In Site?
Find your topic of interest below in the complete Table of Contents (or as some call it, the Site Map). Or you can Click here for the Site Search page.
Getting Started -- The basics
- Let's Go T-Hunting, an overview of how southern California hams organize their mobile hunts and the kinds of equipment they use. Put this article in your ham club's newsletter.
- Southern California T-Hunts for Beginners has details of two monthly mobile T-hunts in the Los Angeles and Orange County area where first-timers are especially welcomed and encouraged, plus some typical hunt photos.
- Frequently Asked Questions and answers about T-hunting and RDF from the Homing In mailbag. Topics include "fox" transmitters, dopplers, ELTs, covert and child tracking systems, "fingerprinting" and more.
- The CQ Worldwide Foxhunting Weekend is an annual event in the spring, sponsored by CQ Magazine.
- The Radio-Orienteering Video by Gary Pearce KN4AQ would be a great program about on-foot foxhunting for your local radio club. It was made at the 2006 USA ARDF Championships. See the competitors in action and learn about how their RDF gear works.
RDF Topics in Print -- Read all about it
- TRANSMITTER HUNTING---Radio Direction Finding Simplified (THRDFS) by Moell and Curlee, a comprehensive illustrated text on all aspects of RDF including theory, techniques, and projects for the HF, VHF, and UHF bands. You can get it by check or with your credit card using PayPal.
- Sneak peek at the latest Homing In columns in CQ VHF Magazine and other current KØOV articles.
- Homing In Article Index, with topics for all of the regular columns since 1988 plus other KØOV magazine features (over 250 total).
- "T-Hunting Then and Now -- From Gooney Birds to GPS" has tales of classic mobile transmitter hunts in the Los Angeles area during the 1950s and 60s.
- Sample Homing In columns from recent Amateur Radio magazines:
Home-built RDF Projects -- Inexpensive and educational
Commercial RDF Equipment -- Getting the most from it
Follow-up and Support -- for readers of THRDFS and Homing In
- Updates to THRDFS since publication, new addresses/phones, parts substitutions, etc.
- Circuit boards and kits by third-parties for the Roanoke Doppler.
- More on the KØOV Shrunken Quad, parts and construction hints for this popular THRDFS project.
- More on the KØOV NorthScope (July, August and September 1997 columns), including a circuit analysis plot and HVPS schematic, plus resources for fluxgate compasses and analog multipliers.
- More on the VE2JX/VE2EMM Montreal Fox Controller. Source and object code downloads, circuit board information and useful links for this project by VE2JX and VE2EMM in Homing In for April 1998.
Championship Radiosports -- Taking on the world
- International-Style Transmitter Hunting Comes To The Americas. The basics of international-rules on-foot foxhunting and the history of its development on our continent.
- Championship Foxhunting News from around the world. Results of recent international competitions and announcements of upcoming ones.
- Equipment Ideas for Radio-Orienteering. Inexpensive transmitting and receiving gear for international-style ARDF.
- Sprints and foxoring, two new events at international ARDF competitions.
- Electronic Scoring for ARDF, what it is, how it works, and why more and more local events and championships and are using it.
- Try ARDF on 80 Meters. If your club does radio-orienteering only on two meters, you're missing half the fun. There's another international foxhunting band, too.
- Foxhunting for Scouts, how to introduce youngsters to RDF and foxhunting at the Jamboree-On-The-Air and other Scouting events.
- ARDF in the Radio Merit Badge. New in 2017, Scouts are encouraged to build ARDF antennas and go on hidden transmitter hunts.
- Extenders Aid Handicapped Foxhunters, a novel way to include persons with disabilities in on-foot foxhunts.
- Tips for Hosting ARDF Championship Events, to bring the best foxhunters to your home town.
Results, stories and photos of ARDF sessions, large and small
- Current events in southern California -- Results of 2021 sessions (there were none in 2020 due to COVID) as well as announcements of upcoming ones.
- Year 2019 events -- ARDF sessions in Mt. Pinos, San Diego, Fullerton, San Dimas, Goleta, Mission Viejo, Yuma (AZ) and Irwindale.
- 2019 USA National Championships -- It's hard to beat Carolina hospitality as Backwoods Orienteering Club again hosted the USA ARDF Championships in late July and early August. This site has a wrap-up report and links to lots of of photos.
- Year 2018 events -- ARDF sessions in Mt. Pinos, San Diego, Fullerton, San Dimas, Goleta, and Irwindale.
- 2018 IARU ARDF World Championships -- Over half of the Team USA members won medals in these competitions in Sokcho City, Korea. This site has a report and two pages of photos.
- 2018 USA National Championships -- The mountains of northern California made a perfect site for four ARDF competitions, plus a practice day. This site has a wrap-up report and a big batch of photos.
- Year 2017 events -- ARDF sessions in Mt. Pinos, San Diego, Fullerton, San Dimas, Goleta, and San Luis Obispo.
- USA Participates in Asian ARDF Events -- Reports from the 2017 and 2015 IARU Region 3 ARDF Championships and the 2015 China International ARDF Championships.
- 2017 USA and IARU Region 2 Championships -- Four events plus three days of training near Harrison, Ohio. This site has complete results and lots of photos.
- Year 2016 events -- ARDF sessions in San Dimas, Mt. Pinos, San Diego, Fullerton, Los Angeles, Goleta, San Luis Obispo, and Rowland Heights.
- 2016 IARU ARDF World Championships -- Team USA brought back seven medals from individual and national competitions on the Black Sea coast of Bulgaria. This site has a report and two pages of photos.
- 2016 USA National Championships -- A youth ranch near Killeen, Texas attracts ARDF fans from all over the country. This site has a wrap-up report and lots of photos.
- Year 2015 events -- ARDF sessions in Goleta, Long Beach, San Pedro, Mt. Laguna, San Luis Obispo, San Dimas, Agua Dulce and Rowland Heights.
- 2015 USA and IARU Region 2 Championships -- ARDF in a Colorado Scout ranch with over four dozen Chinese youth. This site has complete results and lots of photos.
- Year 2014 events -- ARDF sessions in Los Angeles, Goleta, San Dimas, Mt. Pinos, Placentia, Rowland Heights, Irwindale and San Luis Obispo
- 2014 IARU ARDF World Championships -- A small Team USA went to Burabay, Kazakhstan and brought home a medal.
- 2014 USA National Championships -- Back to Beantown for more New England ARDF fun. This site has complete results and lots of photos.
- Year 2013 events -- ARDF sessions in Irwindale, San Luis Obispo, Goleta, Mt. Pinos, San Dimas, Rowland Heights and Placentia
- 2013 USA and IARU Region 2 Championships -- Nothing finer than ARDF in Carolina during October. This site has a wrap-up report and more than 90 photos.
- Year 2012 events -- ARDF sessions in Irwindale, San Luis Obispo, Goleta, Mt. Pinos, San Dimas, Solana Beach, Mission Viejo, Los Angeles and Placentia
- 2012 IARU ARDF World Championships -- Team USA won thirteen medals in individual and national competitions at the resort city of Kopaonik in central Serbia. This site has the story and two pages of photos.
- 2012 USA National Championships -- The mountains east of San Diego were a perfect setting for ARDF, sprints and foxoring. In this site are complete results and four pages of pictures.
- Year 2011 events -- ARDF sessions in Yuma (AZ), Irwindale, San Luis Obispo, Rowland Heights, Goleta, San Pedro, Mt. Pinos, San Dimas, Topanga and Placentia
- 2011 USA and IARU Region 2 Championships -- Back to Albuquerque for another well-organized event in forests that had not been used before. In this site are the story and five pages of pictures.
- Year 2010 events -- Southern California ARDF sessions in Irwindale, San Luis Obispo, Rowland Heights, Los Angeles, Mt. Laguna, Mt. Pinos and Placentia
- 2010 IARU ARDF World Championships -- Team USA brought back three more medals from this event near on the Adriatic seacoast. This site has a report and two pages of photos.
- 2010 USA National Championships -- Soggy running through the forest and skipping through a creek in western Ohio. In this site are complete results and five pages of pictures.
- Year 2009 events -- Southern California ARDF sessions in San Dimas, Topanga, Placentia, Rowland Heights, Fullerton, Irwindale and Goleta.
- 2009 USA National Championships -- Our first national championships to be on the east coast brought out the largest number of competitors so far. In this site are complete results and five pages of pictures.
- Year 2008 events -- Southern California ARDF sessions in Fullerton, Rowland Heights, Placentia, Long Beach, San Dimas, Mt. Pinos, Huntington Beach and San Diego.
- 2008 IARU ARDF World Championships -- Team USA brought back another medal from this event near Seoul, Korea. This site has a wrap-up and two pages of photos.
- 2008 USA National Championships -- Texas hams put on well-planned and well-executed comptitions in a pine forest near Austin. In this site are complete results and dozens of pictures.
- Year 2007 events -- Southern California ARDF sessions in Fullerton, Los Angeles, Placentia, Long Beach, Brea, Goleta, Topanga, San Pedro and San Dimas.
- 2007 USA National Championships -- A June wildfire didn't keep radio-orienteers from ending their summer by tracking transmitters in the Sierras of California. This site has complete results and dozens of action photos.
- Year 2006 events -- Southern California ARDF sessions in Rancho Santa Margarita, Topanga, Goleta, Placentia, Long Beach, San Dimas and Huntington Beach.
- 2006 IARU ARDF World Championships -- USA took home its first World Championships medal from this event on the Black Sea coast of Bulgaria.
- 2006 USA National Championships -- A beautiful North Carolina forest was filled with radio-orienteers on an April weekend. This site has a wrap-up report and dozens of photos.
- Year 2005 events -- Southern California ARDF sessions in Rowland Heights, Rancho Santa Margarita, Los Angeles, San Dimas, Placentia, Huntington Beach, Goleta, Glendora and Agua Dulce.
- 2005 USA National Championships -- Albuquerque hams surpassed their 2001 hospitality as they put on this event, which was combined with the Third IARU Region 2 Championships. This site has the results and dozens of photos.
- Year 2004 events -- Southern California ARDF sessions in Los Angeles, Fountain Valley, Rancho Santa Margarita, Placentia, San Dimas, and Fullerton.
- 2004 IARU ARDF World Championships -- A 21-member delegation from USA went to Czech Republic to compete and be part of the international jury.
- 2004 USA National Championships -- A 2m hunt in a boulder-filled desert park and an 80m hunt at 8000' in the mountains were the main events of these championships in California. This site has a complete wrap-up including the medal winners, plus an album of 40 photos and a first-person account from a first-time medal winner.
- Year 2003 events -- Southern California ARDF sessions in Los Angeles, Fullerton, Placentia, Goleta, and Anaheim.
- Hamcon/Foxhunt-2003 -- Another great ROCA in San Pedro, California, with lots of transmitters and prizes.
- 2003 USA National Championships -- Over forty radio-orienteers converged on Oxford, Ohio in late July for training and foxhunting fun. This site has the results and an album of 60 photos.
- Year 2002 events -- Southern California ARDF sessions in Fullerton, Fountain Valley, Long Beach, Huntington Beach and Thousand Oaks.
- 2002 IARU ARDF World Championships -- Team USA performed better than ever before at this premier event in Slovakia. This site has USA's results and first-person accounts.
- 2002 USA National Championships -- Over two dozen stateside foxhunters braved difficult courses to win medals near beautiful Pine Mountain, GA. This site has the results and an album of 40 photos.
- Year 2001 events -- Southern California ARDF sessions in Fullerton (2), Rowland Heights, Huntington Beach and Placentia.
- 2001 USA National Championships -- Ten states and three foreign countries were represented at this historic event in Albuquerque, NM.
- Year 2000 events -- Southern California ARDF sessions in Fullerton, Fountain Valley, Pasadena, Huntington Beach and Placentia (2).
- 2000 IARU ARDF World Championships -- A dozen foxhunters from six states made the trip to Nanjing, China to compete against the best.
- Team USA 2000 Qualifying Runs -- A difficult 2-meter hunt on a hot day, plus California's first 80-meter ARDF event.
- Hamcon/Foxhunt-1999 -- Southern California's first ROCA, 20 foxes to find, prizes for everyone.
- 1999 IARU Region 2 ARDF Championships in Portland, Oregon -- Eight states and seven other countries were represented. This site has the results and an album of 47 photos.
- Schabarum Park 1999 -- Preparing Team USA for championships in Portland.
- Antennas In The Park 1999 -- Informal, but so much fun that it has become an annual tradition.
- 1998 IARU ARDF World Championships -- In Nyiregyhaza, Hungary. This was USA's first time to take on the radio-orienteers of the world.
- Schabarum Park 1998 -- Preparing Team USA for championships in Hungary.
- West Coast VHF Conference 1996 -- 1995 was so much fun that we had another one.
- Hamcon/Foxhunt-1995 -- Southern California's first big international-style foxhunt.
- Friendship Radiosport Games 1991 -- The first international-rules ARDF event in the USA.
Volunteer Opportunities -- Use your RDF skills to help researchers and protect wildlife
- A tribute and memorial information for Mike Obermeier K6SNE and David Gordon-Ross N6IDF, who died accidentally on May 27, 2006 during a "Mini All Day" mobile T-hunt.
- Almost 300 RDF-related links and Homing In Correspondents to help you find T-hunts and ARDF events in your own area. Equipment sources, too.
- Where to find RDF-related mailing lists (reflectors), to bring transmitter hunting news directly to your electronic mailbox.
- Search this site by word, name, callsign or phrase.
Spending a few minutes at this Homing In site will give you a jump-start into the world of transmitter hunting. After that, you can find out how to get involved in mobile T-hunts in your area by visiting local T-hunt/foxhunt web sites and contacting nearby Homing In Correspondents listed on the links page. You'll find manufacturers and suppliers of RDF gear there, too.
Who is KØOV? A registered professional electronic engineer and an active Amateur Radio enthusiast since age 11, Joe Moell KØOV has 55 years of experience designing radio-frequency circuits and systems for broadcast, communications, and radar, ranging from near-DC through microwave frequencies. He has designed new devices for radio direction finding and has written about RDF and other topics for almost every ham radio publication in the USA. In February 1998, he was appointed by the American Radio Relay League (ARRL) as the USA's first ARDF Coordinator, to promote international-style foxhunting and to organize Team USA for international ARDF competitions. He also conducts the annual CQ Worldwide Foxhunting Weekend.
Joe collaborated with Tom Curlee WB6UZZ to write TRANSMITTER HUNTING---Radio Direction Finding Simplified, a comprehensive text on RDF, and has written over 280 published articles on the subject, including his monthly Homing In columns that ran for 15 years in 73 Amateur Radio Today magazine, then for ten years in CQ-VHF magazine, for one year in CQ-Plus digital magazine and now appears four times per year in CQ Magazine. As a Technical Advisor to ARRL Headquarters, he authored a chapter on RDF for The ARRL Handbook and has made more than 100 presentations on transmitter hunting to clubs, conventions, classes and seminars. As time permits, he is available for private engineering consulting.
Surfing suggestion: For a quick start into the world of RDF and mobile hidden transmitter hunting, jump to Let's Go T-Hunting.
Please note that this Web site is built and maintained independently by
Joe Moell. It is not sponsored by or affiliated with CQ Publications, 73 Amateur Radio Today, Wayne Green Enterprises, TAB/McGraw-Hill, ARRL, or any other commercial or non-commercial entity. All content is protected by applicable intellectual property laws.
Entire site Copyright © 1996-2021 Joseph D. Moell. Text, photos and original graphics may not be served or reproduced elsewhere without permission.
PO Box 2508
Fullerton, CA 92837
This page updated 16 November 2021
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