The manufacturer will set one null as the "to" null. For example, if both the loop and sense antenna voltages are in phase and add to each other prior to the null position, the loop antenna is facing towards the transmitter. This will be set as the "to" null. The ADF needs both antennas to correctly point to the station.
This method of operation has been around since the 1930's and we still use it, virtually unchanged, today.
Operation and controls
ADF receivers have several operating modes that the pilot can select. In the ADF mode, both the loop and the sense antennas are used, the pointer is activated, and the ADF tries to point to the station. (This is the normal mode of operation.) Rotating the loop antenna control switch in either the left or right direction will cause the loop antenna and pointer to move left or right. Releasing the loop antenna control switch causes the loop antenna and pointer to rotate and point to the station again. The ADF indicator (RMI) consists of a compass card and pointer. In dual ADF systems, the RMI will have two pointers (for systems one and two). The pilot manually sets the compass card or, if the airplane has a flux valve, the compass card is slaved automatically to the aircraft's magnetic heading.
In the ANT mode, the loop antenna is disabled and all receiving is done through the sense antenna. The pointer should not move even if you turn the loop antenna control switch left or right. (This is the best mode for listening to your favorite ball game.) It is also good for identifying the station.
If the loop mode is selected, the sense antenna is disabled and all the receiving is done through the loop antenna. Rotating the loop antenna control switch in either direction will cause the loop antenna and pointer to rotate in the selected direction for as long as the switch is held. Releasing the control switch causes the loop antenna and pointer to stay where it is. You may also notice that the reception gets better when the pointer is 90 degrees away from the station and worse when the pointer is either pointed directly at or 180 degrees away from the station.
Some ADF control panels have a test position. Rotating the mode control switch to "test" will perform a self-test of the ADF system. In the self-test mode, the pointer will move to 45 degrees left of the lubber line to a heading of 315. The lubber line is a little arrow or line at the 12 o'clock position on the RMI.
The BFO switch
If you have a digital tuner, you can skip this paragraph.
Most ADF systems have a BFO switch mounted on the control panel. BFO stands for Beat Frequency Oscillator. With the BFO switch on, the ADF receiver generates an audio tone to help you tune in an NDB. On the control panel, select ADF mode and turn the BFO switch on; then rotate the tuning knob.
As you approach the station, you will begin to hear a high pitched tone. Continue rotating the knob and the tone will gradually decrease until you can no longer hear it. At this point, the ADF is tuned to the station and the signal strength meter should also be at its highest peak. You should be able to clearly hear the station's Morse code identifier. Rotate the knob in the same direction and you will begin to hear the tone again. Adjust the tuning knob so that you can not hear the tone.
The VBR filter switch
Transport category aircraft has a switch that is found on the audio select panel. These aircraft usually have multiple radios, nav aids, and audio integration systems and this switch has no effect on the ADF pointer.
Nevertheless, the VBR Filter switch is important if you want to identify a station. It is usually a rotary switch but some audio select panels have toggle switches.
Basically, it allows you to select either Voice or Range or Both audio filters in the ADF, VOR, and DME receivers. In the V position, the DME audio is grounded and voice audio can be heard on the ADF and VOR receivers. (This is another good choice for listening to that ball game.) In the B position, both Voice and Range filters are bypassed and all VOR, DME, and NDB audio can be heard. In the R position, DME audio is again grounded and the VOR and NDB Morse code identifier signals can be heard. If you are tuning in your local AM broadcast station, use the V position. If you are tuning in an NDB station, use the R position.
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