RHF wrote:
BURNIA3,
.
The RadioShack DX-398 (Sangean ATS-909) has both a
Whip Antenna and External Antenna Jack:
.
The RadioShack DX-398 / Sangean ATS-909 is noted for
being some what deaf off of it's built-in Whip Antenna.
.
Generally the RadioShack DX-398 has a reputation for being
better using an External Antenna connected via the 1/8" Jack
on the Side of the Radio. This is a Stereo Jack and you should
use a Stereo 1/8" Plug with the End-Tip connected to the External
Antenna Wire and the Rear-Barrel connected to the Ground Wire.
NOTE: The Center-Middle-Barrel of the 1/8" Stereo Plug is NOT Used.


These fine people disagree.

http://www.mods.dk/mod/radio_shack/dx-398antjack.gif



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The radio accepts a 1/8" stereo headphone jack for its external antenna.
It's wired a little differently than you think.

The tip goes to the antenna terminal and the sleeve goes to ground, just as
you would expect in this unbalanced antenna input. Shortwave signals are
routed from the external antenna jack when a connector is plugged in.

The ring--the terminal in the middle of the plug--has a special purpose.
When you ground it, medium wave signals are also routed from the external
jack instead of the radio's internal loopstick. This is why some people have
reported poor MW reception when they use regular 1/8" mono patch cables to
connect their antennas.

But for medium wave DXers, this is an opportunity to connect their favorite
antennas to the radio; the lack of a good MW antenna connection has made
some otherwise good radios less than desirable.

If you prefer to use the internal loopstick for MW, simply leave the ring of
the plug unconnected.

Ingo Truppel has another approach:

An important note to the MW/LW - external antenna. It is not useful to
connect the ring terminal of the antenna plug to ground. Instead of that it
should be connected also to the antenna terminal and parallel via a 1 kOhm
resistor or an inductance (some mH) to ground - that way you get more
sensitivity for MW/LW (look at the scheme for that). I use an self made
active antenna (150kHz-30MHz tuned) with an output transformer which grounds
the antenna for DC. So I only connect the ring of the antenna plug to the
antenna terminal.

[I haven't tried this. The antenna I use is also DC grounded, so Ingo's idea
should work for me...DM]
How do I disable the internal MW antenna?

Using a 1/8" stereo connector, ground the ring terminal to the sleeve. Or
else use a 1/8" mono connector. The FAQ author much prefers the former
method as this can be controlled by a SPST switch to disable and reenable
the internal antenna, thus saving the antenna jack inside the radio from wear.

http://www.mods.dk/view.php?ArticleId=2301


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mike