By Carl E. Luft
07/26/04 - RDF, or "radio direction finding" is tracking and locating an RF source or transmitter. During the past 25 years of flying RC sailplanes I have seen many a plane lost off the field never to be found.
A few years ago, the ESL decided to invest in the Walston retrival system where ESL would purchase the receiver/antenna equipment and the flyers would buy the transmitter that would be taped inside their plane with the hope that in the event the plane flies away, it could be tracked and found.
I bought a transmitter, but soon realized I needed to verify the transmitter's operation. There is no way of knowing if the thing was working, let alone being able to track it.
I decided to do research on the problem, and have found a wealth of information in two areas on the world wide web. The amateur radio folks have a lot of information on RDF. They call it "fox hunting" or "T-hunting" and they have contests to hunt down hidden transmitters. Another hobby that uses RDF is rocketry. They use RDF to find their rockets. One web site in particular has a lot of information and can be found at:
http://home.att.net/~jleggio/projects/rdf/rdf.htm
Receivers
The best receiver that money can buy for tracking the Walston transmitter is the Walston receiver. It is designed just for tracking the Walston transmitter. It has a built-in attenuator, an RF gain control, fine tuning control, and band and channel selection switches. The audio is loud and clear and the signal strength bar graph meter is easy to see.
The down side of the Walston receiver is the cost. Walston has four models to choose from. A two hundred channel receiver that will receive any Walston transmitter down to a three channel receiver. The two hundred channel receiver will cost you over eight hundred dollars and the three channel receiver will cost you around four hundred. It does the one thing it was designed to do and does it well.
There is an alternate to the walston receiver that will track the walston transmitter and it's called a communications receiver. Although communications receivers do have the ability to "scan" frequencies like a fire and police scanner does, their main function is to receive any type of signal over a wide frequency spectrum. Police scanners for the most part just cover the frequencies used by two way radio services(police, fire, amateur radio, ect.)
What to look for when choosing a communications receiver to be used for RDF:
1- Will it tune the frequencies you are interested in tracking?
2- Does it have a signal strength meter (s-meter)?
3- Can you hear the sound of the signal you are tracking and not just see a readout of the signal strength?
4- Does it have an attenuator?
These following receivers are worth consideration. Prices listed are approximate as of Oct. 2003. You can read reviews on these receivers at www.strongsignals.net
AOR AR8200B MkII 500kHz - 824 MHz, 849-869, 894-2040 AM NAM WAM FM SFM WFM LSB USB CW$579.95
YAESU VR-500 100kHz - 1300MHz AM FM WFM LSB USB CW $310.00
ICOM IC-R2 495kHz - 1310MHz AM FM WFM $180.00
ICOM IC-R10 100kHz - 1300MHz AM FM WFM LSB USB CW $310.00
ALINCO DJ-X2000 100kHz - 2.15GHz AM FM WFM LSB USB CW $500.00
I listed the Icom IC-R2 because it's a nice small receiver that will fit in any toolbox and is the least expensive of the five listed. It does not have USB, LSB and CW receive modes and this will limit your ability to track with it.
The receiver I use is the Yaesu VR-500. It has all the capabilities needed to track the Walston transmitter:
1 It will tune in any frequency from 100 KHz to 1.2 GHz (except cell phones)
2 It can be set to any receive mode-AM, FM, WFM, USB, LSB, and importantly CW. CW is used to hear Morse code where the transmitter is keyed on and off for the dits and dahs. The Walston transmitter turns on for just a split second- once a second so when you receive its frequency and set the mode to CW, you'll hear the beeping or chirping sound. If your receiver only has AM, FM, or WFM, you will only be able to see the change in signal strength as you track (provided your receiver has an s-meter), and not hear the sound. This is important because you'll hear the transmitter before you see any signal on the s-meter.
3 It can be fine-tuned in 1KHz steps, unlike other scanner/receivers of its size. This is useful for attenuating the signal when you get close, or to find the exact frequency for maximum range.
4 It has an attenuator, numbered keypad to enter the frequency in directly, common BNC type antenna connector, good audio and long battery life.
Antennas
Rubber Duck- I don't think the rubber duck antenna that comes with the VR-500 is useful. I have found that for tracking, your antenna should be tuned(adjusted in length) to the frequency your tracking. Rubber duck antennas are not designed for any one frequency but to cover an entire range of frequencies. I have a stubby antenna that's about 2 inches long that I use for checking my Walston transmitter's operation. I would not want to have to track with it.
Simple Dipole- If your receiver has a telescoping antenna you'll be able to adjust it for 220MHZ as well as the first harmonic at 440MHz. Radio Shack has a good telescoping antenna that works very well(RS#20-006). I built my own by using a right-angle BNC connector(RS#278-126) and a replacement telephone antenna(RS#270-1413A). Just hollow-out the inside of the connectors plastic case so the antenna element fits inside and connect the element using the tiny screw in the center of the BNC connector. This homemade antenna can be folded down behind the receiver which makes it nice to drop in your pocket while climbing through the brush. There is a method to using this type of antenna that may make you think why you would ever want to use a yagi. More on this later.
Yagi- A yagi antenna looks something like a small TV antenna. It receives more signal when it's pointed at the transmitter then from the sides or back. It will also pick up a signal at a greater distance. The more elements it has, the better. At 220MHz, three elements are about all you would want to drag through the brush.
Walston makes a good quality yagi that has folding elements. I have also seen designs for 3 element yagi antennas on the web. One design uses the tape from a tape measure for the elements. This allows for the elements to flex and bend while getting in and out of the car, or hacking your way through the jungle. I have built a tape measure antenna and it works well. Here are the dimensions I have calculated for 220MHz using a program called Quickyagi:
Reflector Length = 26 1/4" spacing 11" from the driven element
Director length = 23 5/8" spacing is 7 3/4" from driven element
Driven element length is 25 1/4"(12 5/8" each side)
Beta match (Hairpin) length of wire is 3 3/4" long
Width of tape measure elements is about 11/16"
See the web page construction article at
home.att.net/~jleggio/projects/rdf/rdf.htm
or search- "tape measure yagi"
Another site to look at is Arrow Antennas at:
www.arrowantennas.com/" target="_blank">www.arrowantennas.com/" target="_blank">http://www.arrowantennas.com/
Arrow Antenna has hand held yagi antennas tuned to the frequency of your choice from 120-500 MHz.
Time Difference of Arrival RDF- This method of radio direction finding is popular with the amateur radio "fox hunting" folks. It works by switching your receiver between two antennas at a rapid rate. When both antennas are the same distance from the transmitter, the RF "phase" received is identical. If, however, the two antennas are at a different distance from the transmitter then the RF will have a different phase at each antenna. By switching between the two antennas, utilizing an electronic circuit at 500 times a second, this phase change will be detected by an FM receiver as a 500Hz tone. By turning the TDOA antenna array for a null in the tone, the two antennas will be perpendicular to the transmitter.
If your receiver only has AM and FM receive modes then this type of setup may be what you’re looking for. The web sites I visited that describe these units call for an FM receiver. They also give instructions for building the units. I have not tried this method of tracking so I cannot comment on how well it works but it seems to be the "hot setup" in amateur radio fox hunting.
Attenuation
An important part of tracking is the need to reduce or attenuate the signal. As you get closer to the transmitter, the signal strength may become so strong that you won't be able see any difference in signal strength in any direction you point the antenna.
There are several ways to attenuate the signal while tracking:
1-tune your receiver slightly off frequency
2-tune in the first or second harmonic of the transmitter
3-turn on the attenuator in the receiver
4-add an external attenuator to the antenna leadin
5-a combination of the first four.
Do not de-tune your antenna to attenuate the signal.
An external attenuator is a must if your tracking with a yagi antenna. I found several plans for attenuators on the web and the one that I built is called a "passive attenuator". It uses an air gap in the feed line to reduce the signal. I simply add it to the antennas leadin when needed. Arrow Antenna has a step attenuator available in kit form or ready to use.
As an attenuator, another method of reducing the signal strength is to attempt to receive a harmonic (multiple) of the transmitter frequency. The first harmonic of the Walston transmitter frequency is 440MHz. This signal will be significantly weaker than the actual(220MHz) frequency. If your tracking with a telescoping antenna, this trick can be quite effective if you know you are close to your plane but can't see it in tall grass or brush.
Ground test
Connect the antenna to the receiver. Put the batteries in the transmitter. Walk away from the transmitter and dial in the frequency on your receiver. If you don't hear the transmitter, check your batteries. Make sure your frequency is tuned in and attenuation is off. The important thing here is that you know your transmitter is working and that the batteries are good. Write down the frequency of your transmitter. Calculate the first harmonic(frequency x 2) and tune that in and write that frequency down also. My transmitter frequency is shown as 220.014 but it tunes in at 220.013 and the harmonic is 440.026
Tracking using a simple dipole antenna
A dipole antenna is omnidirectional, that is, it receives well from all directions when it is held vertical. To make the antenna directional, hold the radio against your chest with the antenna horizontal to the ground. Swing the antenna by turning around in a circle while noting where the lowest signal is received. The signal will be best with the antenna pointed 90 degrees away, (to your sides) and lowest behind you. You may notice the signal almost completely disappears with the antenna pointed directly away from the transmitter. Using this method along with tuning the first harmonic can get you very close to your plane. This is known as the "Body Fade" or "Body Block" method. Tracking with a dipole or telescoping antenna is different then with a yagi because your looking for a drop in the signal rather then a peak in the signal. This method of tracking is susceptible to reflected signals from hidden metal objects that can give you false readings. You should get several bearings on the signal from open areas and use triangulation as you hunt for your plane. You will also spend more time stopping and turning while looking at the receivers s-meter.
Tracking using a Yagi Antenna
Hold the antenna with the antenna elements horizontal to the ground. Turn slowly through a complete circle to determine in what direction the signal is from. You should be able to hear and see a difference in the strength of the signal received. Just walk in the direction of the strongest signal. Attenuate the signal as you get closer so you can see the peak in the s-meter. Try to keep the signal peak less then full strength on the s-meter.
Yagi vs Dipole
Using either a yagi or a dipole will do the job of tracking. Deciding what type antenna to carry along in the car depends on your tracking preference. A simple telescoping antenna is the simplest and least expensive way and it gives you the freedom to move about in dense brush or wooded areas. If you just dumped your plane just off the field, you could just grab your receiver and head out to find it and get back in time for the next round.
The yagi would have to be assembled and connected and if you don't have any idea how far off your plane is then I would take the yagi. The yagi is the quickest way to your plane. As your walking you can hear the signal peak in the speaker while checking the s-meter now and then and your eyes are always looking for the plane and not the signal on the receiver. If Walston thought a telescoping antenna was the best, he would have inserted one in his receiver. He chose the yagi because of it's ease of use and range.
One last chance
There is another source of RF that is transmitted from your plane that you may not be aware of. The receiver in your plane is also a transmitter and the RF it puts out can be tracked. By using my VR-500, I found that my receiver on CH46(72.71) was transmitting at 73.165(single conversion IF+). The first time I tried this I was somewhat disappointed with the amount of signal received and I only got about 200 feet away using my telescoping antenna. I then tried the first harmonic of that frequency at 146.33, I could get about 400 feet away. Then I tried the second harmonic at 292.66MHz and I got 600-700 feet away. Using a yagi antenna from arrowantenna.com tuned to 293MHz, I could track the signal from over 1000 ft away You'll need to set the receive mode to "CW" to track this signal because the signal is so weak you can only hear the sound. I also set the frequency step to 1 KHz so I could fine tune for maximum signal and tone from the speaker. A 600 foot range may not be much, but it could find your plane.
I tried this idea when I first acquired my VR-500 receiver and I settled on a piece of wire cut to 41 inches. I simply plugged the wire into the center of the VR-500's antenna connector. I figured that someday I would build a telescoping antenna that would extend to 41 inches. The telescoping antenna I now have extends from 24 inches down to 5 inches and that is within the range of the first and second harmonic frequencies. This, along with a lower static level at the higher frequencies, is what I believe is giving me the greater range.
A flier at one of the contests last year asked if I would check his plane to see if it could be tracked. I couldn't detect any RF from it. It may be because I was looking at the wrong frequencies or his receiver was of a different design or make. For this to be a reliable method for tracking each receiver you use would have to be checked and the exact frequencies documented. After all, the receiver's job is to receive and drive the servos, not to transmit a tracking signal.
Final Thoughts
Armed with a communications receiver and a simple telescoping antenna, you should be able to find your plane if it flies off. In a hobby that uses radio, it only makes sense to have a receiver available so you can listen to and track if necessary, the frequencies that we use and the interference that may do harm. Since learning what I know about RF emissions that receivers can generate, I plan to test my RC receivers for RF emissions and document all those frequencies. I hope others will do the same. |
RDF-finding a lost plane
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Posted - 12/26/2003 : 11:15:42 PM
