Radio Direction Finding  by KH7O

Click Here for Main Internet Site on ARDF

 

Radio Direction Finding comprises of RDF equipment and the art or skill of using various pieces of radio gear. It requires discipline and skill in the  interpretation of Radio Signals. The basic components are the Receiver, Antenna, Attenuator, compass and Map.  It is one of the more fun things to do in Amateur Radio that presents challenges of the operator's skill and ability to "feel" the radio waves and get a sense of how radio signals radiate, reflect, and attenuate.

There are a lot of web sites already on the internet that can be googled. This page is not intended to duplicate a wealth of information already on the internet but rather a summary of what , when, and why certain equipment is used, what can be built in the Ham Shack. This is from being in Ham Radio for 32 years and first hand knowledge of DFing. A good video 

Most Difficult is in a dense building area where signal reflections make it difficult to get proper bearings.

Least Difficult is on the open ocean.

WHAT YOU NEED:

1) I first take multiple Signal strength readings looking for maximum peak signal value not direction

2) with that knowledge in hand then proceed to directional beam readings only from ridge lines or spots less likely to have a stronger reflected signal, never low spots where you can get reflections, EXAMPLE: use places like round top, KCC to get the 2 intersect readings, then go to airport end of lagoon drive and take a reading back in toward the land mass, after carefully plotting the path on a map with beam headings you should narrow it down to within a mile of the actual transmitter if its in Honolulu area (good for bearings between Red Hill and Diamond Head) if you are in town and have to split the difference fast then go to Magic Island or similar, somewhere where you are away from the building and can get a reading 50/50 between signal from the East or the West from that point,

3)get to the general area from step 2 switch to near field detectors / antenna which should place you essentially on top of the signal.. being in a small search area, its a matter of just narrowing it down to the exact spot by again using Signal Strength combined with Directional antenna.

4)rely more on relative signal strength rather then actual bearing of yagi antenna when you are in the city area with a lot of reflections.. Although the beam headings taken within the city will give you more WRONG directions then correct directions, one thing for sure is that signal strength in a general area will always be stronger in an area where the transmitter is located rather then a reflected signal from a building...

If you are driving from one area of condensed buildings taking signal strength readings and move to another area also strong with signal readings you can safely say the transmitter is where the signal is the strongest. This doesn't mean taking a signal strength reading from a static position using a beam!! This means taking the signal strength from a wide area like several blocks or miles say with a mobile omni antenna.. so as you drive just mark down the signal strength averaging what the strength is in an area, then the next area etc.. FOLLOW steps 1 and 2 above.. don't convince yourself that a signal is somewhere that it isn't!!.. take the logical approach

 

  NATEC NY144X recent purchase from Akihabara

Antenna similar to HB9CV, have not tried it yet.. Still waiting for the next Hawaiian Fox Hunt?  

  

Roanoake Doppler type mobile direction finder, needs strong signal for bearings and may still give bearing on reflections. Built these two different models with different circuitry but did not have favorable results when in areas of reflections from buildings or mountains. I haven't tried to build the kit at Byonics but it sure looks good, and I have built many kits from Byonics for APRS that worked great.

 

Used when getting real close to the transmitter. Your receiver will start to pick up the beacon even without an antenna when you get close to the transmitter. Once that happens you will not be able to get a bearing since you are dependent on the antenna pointing in the right direct. This Offset Attenuator is a must have to attenuate strong signal when near to the transmitter. Easy to build, just use a clock oscillator module from an old computer board. Any frequency will do, just tune your receiver above or below the transmitter frequency by whatever the clock oscillator frequency is.  Put this inline between the receiver and directional antenna and continue to narrow down the location which should be in walking distance at this point.  The Offset oscillator in the picture has a 16 mhz clock module so you would just tune in Receiver 16mhz off the transmitter frequency and adjust the gain control. You can make the signal weak enough to find the high power transmitter feet away.  Or buy one from the web from such places as ARROW ANTENNAS

This is a TDOA Time Differential of Arrival home brew DFer. Basically 2 quater wave whip antennas are placed on the roof of the vehicle that are fed into the antenna switching box (silver chasis) which has PIN diodes that are electronically toggled and fed via a single coax to the main control box (black with meter) All this box is doing is switching the antennas fast and measuring the phase relationship between the signal hitting both antennas which is displayed on the meter on the black box or if you are driving I got a "BIG" microamp meter that can be placed on the dash of the vehicle and plugged into the black box to the MTR connection which displays the same thing just on a bigger display. 

Above are various attenuators that can be found in surplus stores, Dayton, Ebay, old surplus equipment or can even be built from kits available. You gotta be a pack rat like myslef to collect this stuff when needed.

Above is a Chinese made 80 meter RDF receiver, how can I describe it.. well it should sell for $20 at best.. cheap plastic case.

This is a Japanese near field detector for 2 meter frequencies only . Very good selectivity, sensitivity AND that big black knob will take the signal down to nothing if there is a strong transmitter. It is as good as the offset oscillator / receiver combo but the advantage of this one is that it has visual LED indicators, Beeper on the top of the unit, BIG Signal meter on front.. When getting close just switch this puppy on and your there. very light weight, comes in Kit form, found in Akihabara for around $60 US.  Chassis is very strong aluminum precision made. The chasis comes with the holes punched and everything except the battery itself was included in the kit.

  FRXWebsite here

This is a fantastic fox hunt DFer, Manufactured by Mizuho it is very accurate, frequency agile. Notice the 5 LEDs on the front. Just so happens in Official fox hunts there are 5 sequential transmitters that are spaced by time. The LEDs will toggle by internal timer once the fox hunt starts, therefore you will have a visual indicator of which fox transmitter is on the air.. How sweet it is.. Now for the bad news, I got this rare Mizuho off of interent. It is no longer being manufactured.  ugh....

 

AD8307 by Analog Devices Low Cost DC–500 MHz, 92 dB Logarithmic Amplifier, fantastic for a cheap near field detector. I used a Radio Shack remote control project box that has a sliding door battery compartment to replace the battery.. I think I could sell this gadget on the market. August 2000 QST.

Click here for manual VK3YNG

VK3YNG Foxhunt Sniffer is a specially designed synthesised VHF direction finding receiver covering 120MHz and 144MHz bands.The receiver is designed for quickly finding the direction of beacons or hidden transmitters. Anything from distant weak signals to very close "sniffing" of transmitters running many watts of output power can be pin pointed accurately without suffering "overload" problems that plague other designs.

  ROX-1 RECEIVER FOR ARDF by G3ZOI

"This project started in the autumn of 2002 with a search for a 'simple', low cost but practical design for an AM receiver to be used in 2m IARU ARDF ( Radio Orienteering )."

  DF1FO's ARDF-Receivers 

Gallery 1 Gallery 2 Gallery 3

Chinese 80m kit 

Chinese main site in English

Russian ARDF

 

Japanese web site of Chinese ARDF 

 

 

Homing In US Web site