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I am seeing this in a simulator, so I don't have a manual for it…

On the Bendix King KR87 NDB receiver, the leftmost button labelled “ADF” switches an indicator above it from “ANT”

enter image description here

to “ADF”

enter image description here

When pushed, the needle on the direction indicator indicates towards the NDB (if in range). When popped, it does not, at least for the NDB stations.

So I am wondering, what does this switch change and why it is needed?

(screenshots taken from FlightGear simulator by me)

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    $\begingroup$ I suspect it means “Antenna”, for when you just want to listen to an AM radio station. Maybe disabling ADF improves the range for that purpose? $\endgroup$
    – StephenS
    Dec 12 '20 at 23:11
  • $\begingroup$ @StephenS, it might be; the simulator does not simulate the audio quality (basically it starts when you get in range), so that effect can't be observed in it. $\endgroup$
    – Jan Hudec
    Dec 12 '20 at 23:18
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The ADF consists of two antennas: the loop and the sense antenna. We already have an excellent answer by mins on how the ADF works, so I'm not going to repeat it here. The important thing for this question is the pattern of the two antennas:

antenna patterns
(modified from source)

As you can see, the sense antenna has a spherical pattern and therefore sees the same signal strength from all directions. The loop pattern does not (which is what gives the direction information when used as an ADF). The ANT button disables the loop antenna, which means all the signal comes from the sense antenna. This improves the audio quality when listening to a Morse code identifier (or something else like AWOS):

Most ADF receivers have several modes that the pilot can select. If the "ANT" mode is selected, the loop antenna is disabled and all receiving is done through the sense antenna. This mode provides the clearest audio reception, so you’d normally use it to identify a station or listen to Rush Limbaugh. On King ADFs, the needle should park in the 90-degree position when the receiver is in "ANT" mode; other brands may work differently.

In the "ADF" mode, the pointer is activated and the ADF tries to point to the station.

(AVweb - ADF Basics)

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ANT mode improves the audio reception when listening to a station's morse code identifier or whatever else is being broadcast. Normally the directional needle gets parked at 3 o'clock when in ANT. NDBs can also have other broadcasts besides station ident, like ATIS or AWOS weather data, etc., and in some cases use of ANT may make the difference between making out what is being said, or not. It may also improve AM radio audio reception, but not sure. I've used the ADF to listen to music back in the day, but never had to go to the trouble to switch to ANT to hear it.

Here's a link to a KR 87 manual.

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