by tonytran2015 (Melbourne, Australia)
#AM, #broadcast, #emergency, #radio, #radio direction finding, #find North, #Medium Wave, #radio navigation.
Every natural disaster emergency kit is recommended by authorities to include a battery operated Amplitude Modulation Medium Wave (AM MW, also called AM Broadcast) radio receiver to receive critical information and instructions.
The radio receiver can actually do much more than receiving information and instructions. It can find the direction to nearby AM broadcasting stations and even find your location on any map showing the stations.
The technique is applicable when there are nearby (closer than 100km) AM broadcast stations. It is useful to regions near the terrestrial magnetic poles where magnetic compasses do not work well, to desert crossing and to coastal boat travel when visibility is very poor and other modes of navigation are not available (such as when in bad or extreme weather, during volcanic emergency and without any working magnetic compass).
1. Basis of Radio-direction-finding.
An AM broadcasting station in Medium Wave sends electromagnetic waves out from its antenna at the speed of light. The invisible EM waves propagate outwards from the broadcast station’s vertical antenna. The antenna continuously releases consecutive, expanding circular magnetic lines of alternating (clockwise then anti-clockwise) orientations. The circles spread out in all directions. When a receiver receives the signal, the alternating magnetic field is at right angle to both the spreading (propagation) direction and the alternating electrical field which is almost in the direction of the broadcasting antenna.
Most AM radio receiving Medium Waves (0.5MHz to 1.6MHz) use ferrite rod antennas to capture these magnetic field lines. The ferrite rod of an AM MW receiver is usually placed inside and along the top edge of the (usually flat) receiver.
Figure 1: A battery operated multi-band AM radio that can be used in emergency. This battery operated multi-band AM radio with a fair sized signal strength indicator in a small window on the left of the dial was made in the 1960’s. Its pulled out rod antenna is only for Short Wave reception, an internal long ferrite rod antenna is used for Medium Wave reception.
Figure 2: A compact radio receiver WITH an AM internal ferrite rod antenna that can be best for emergency use. This compact AM/FM receiver has an internal ferrite rod antenna for AM MW reception and can properly receive AM signals even without using any earphone.
When an AM MW (also called broadcast band) radio receiver has been tuned to a station, you can notice the signal strength varies when its ferrite rod antenna is rotated on the horizontal plane: The received signal is weakest when either end of the rod is pointed directly at the transmitting station, and it is strongest when the rod is at right angle to the line of sight to the station. The technique has been widely used for ships until the advent of low cost GPS. Reference  gives a really interesting long list with pictures of AM radio receivers specially built for the application of this technique on ships.
CAUTION: Many compact AM MW radio receivers do NOT have ferrite rod antennas, they use their earphone wires to receive the electrical component of the broadcast waves. This method of finding directions does NOT work with such receivers.
2. Simple direction finding.
Rotate the top edge (with the ferrite antenna) of the receiver by a full horizontal turn and notice the two opposite directions where signal receptions are weakest. When this happens, one of the ends of the ferrite rod is pointing exactly at the station and the station lies on the line through the top edge of the receiver. Additional information from somewhere else is needed to tell in which of the opposite directions the station may lie.
3. Finding the receiver on a line through the station shown on a map.
If the direction of the top edge of the receiver relative to true North is known, the locus of the receiver on the map is a straight line through the broadcasting station.
Your location can be found using the intersection of two such locus lines through two broadcasting stations shown on a map. (The intersection R of the lines RS1 and RS2 of the following section).
4. Finding the receiver on a circle through two stations shown on a map.
Figure 1: Finding your location using intersecting angle bearing circles.
Any map showing the locations of the broadcasting stations S1, S2, S3,… can be used to locate the position R of the radio receiver. Any receiver with the angle from R-S1 to R-S2 being 40 degrees (an example value) in the clockwise direction must be on the circle drawn through S1 and S2 for that example value of 40 degrees. The example circle is called the circle bearing the angle of 40 degree in the clockwise direction for viewing the line S1-S2.
If two stations S1 and S2 are shown on a map and A is a directed (signed) angle from the line to S1 to the line to S2 then the locus of the radio receiver is the circle through S1 and S2 bearing that directed angle.
For example, if the angle from R-S1 to R-S2 is 40 degrees (an example value) in the clockwise direction, the receiver must be on the circle drawn through S1 and S2 on the map for that example value of 40 degrees (The circle is the smallest one drawn in the figure of this section.). The example circle is called the circle bearing the angle of 40 degree in the clockwise direction for viewing the line S1-S2.
5. Finding the receiver as intersection point of three circles through three stations shown on a map.
If three stations S1, S2 and S3 are shown on a map as in the figure of the last section and the directed (signed) angles between the directions to S1, S2, S3 are all known, the receiver is on the intersection of all three circles through each pair of S1, S2 and S3 bearing the respective directed angles between them. Usually, only two circles are needed to draw the intersection point, which is the location of the radio receiver R.
6. Limitation of the method.
a. The method works well if there are non-interfering AM broadcast stations within 100km of the navigator and the terrain is nearly flat.
b. When there are high atmospheric electrical activities it may be hard to find the direction for weakest reception by the ferrite rod of the broadcasting signals.
c. Many compact AM MW radio receivers do NOT have ferrite rod antennas, they use their earphone wires to receive the electrical component of the broadcast waves. This method of finding directions does NOT work with such receivers.
d. The method does NOT work with Shortwaves as shortwave signals are not received through ferrite rod antennas.
e. Note that Philips from Holland had a famous radio model, L4X00T, with a fold up/fold down rectangular antenna loop for shortwaves, which can also be used to find out the direction of the incoming shortwaves in the same way as described in preceding sections. However shortwaves bounce and they do not accurately show the directions to the stations.
f. For the method to be useful after an Electro-Magnetic-Pulse event, the receiver and the broadcasting stations should all survive the event. You may have to wrap your radio receiver in many layers of Aluminium foils and the broadcasting stations must know how to protect themselves against EMP events.
. Radio Direction Finders, angelfire.com, http://www.angelfire.com/space/proto57/rdf.html, accessed 06 Feb 2017.
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