Organizer: Peter Sheerin, K6WEB


K6WEB, Peter Sheerin
Direction-Finding Equipment

Directional Antennas
Compass with a rotatable bezel and declination adjustment.
Consider a compass that can be mounted to your directional antenna, and in a bearing that is aligned with the true directionality (not the visual alignment) of the antenna.
The ideal protractor for plotting a bearing on a paper map is a “course protractor” (a.k.a. “one-arm protractor”).

Using APRS for ARDF

Some APRS software packages can be used for direction finding. DosAPRS, being the first APRS software program, set the standard methodology, and offers three distinct modes of ARDF. One goal of this SIG will be to determine which APRS software is capable of supporting whichever of these DF modes we choose to use, and well enough for it to be easy for us to operate.

These descriptions are adapted from Bob Bruniga’s documentation:

Omni-Direction Finding (Omni-DF)
APRS incorporates a whole new aspect to direction finding by permitting the plotting of signal strength contours. This permits stations with only omni antennas to participate and provide valuable information! This is possible since APRS has a line-of-sight Power-Height-Gain (PHG) reporting format and display format that it can use to draw range circles around each station showing his relative communication range. For stations not reporting the PHG format they are assumed to have the default parameters of 10 Watts, 20 feet HAAT, and a 3 dB antenna.
Equal Fade Circle Direction Finding (Fade Circle-DF)
The Fade-Circle technique is a classic technique used by the Civil Air Patrol for rapidly finding a signal using only your omni 2-way radio. All you need is to find 3 places of equal signal strength. In an airplane with no signal strength meter, this is best found by where the signal fades out. Thus the Fade-Circle name. But with Ham radio, if you have a signal strength meter, just drive to 3 equal signal points. This can save a lot of gas.
Beam Heading Direction Finding (Bearing-DF)
This mode, which can only be done by people with beams or specialized equipment, is the classic DF-ing technique. Let’s call this Bearing-DF, because bearing is the right word; heading applies to the direction of a moving vehicle, while bearing simply indicates a direction. —K6WEB But Forget It if you need to find something now in a come-as-you-are scenario, unless you have everyone planned and ready in advance. Most people do not have DF equipment, and most mobiles do not have beams. So it may be more accurate, but it leaves 98% of everyone on the air out of the game. APRS is a come-as-you-are system designed to collect and display data quickly and from every available source. Learn to use it!

ARDF-Related Standards

Agrelo Format Static Bearing Report
Although no longer in production, the Agrelo DFjr Doppler direction finder set the standard for reporting DF bearings to ARDF software. The format consists of a bearing and a quality factor, in this format:
The %bbb specifies the absolute, averaged bearing in whole degrees to the detected signal (but a value of 000 has the special meaning of “no direction information is available”, so 365 is true north), the / is a separator, and q is a single digit that specifies the quality of the signal. A 0 specifise a report of unsuitable quality, a 9 represents a manually-input report, and values of 1–8 indicate increasing confidance in the direction report. This bearing data may sent continuously (at up to 20+ bearings/sec.) for logging or compass rose displaying or in timed intervals for APRS packet use.

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