Denon TU680 NAB stereo AM tuner, antenna I made for it for AM

Denon TU680 NAB stereo AM tuner, antenna I made for it for AM

Post by Robert Cas » Thu, 03 Oct 1996 04:00:00

 ordered the last Denon NAB special AM stereo tuner from a local chain,
and got it last night.  It was a floor model, but it looks like few
hands played with it.  Of course, the AM loop antenna was missing, and
the salesguy gave me one off of another Denon set.  Those little
loop antennas hardly pick up anything anyhow.  I made myself a much
bigger loop, using shielded wire that was designed for RS232 signals
or similar (2 twisted pair inside common outer shield) length about
7 to 8 feet.  Also used 3 feet of RG59 coax as the antenna lead
in to the tuner.  So, make a single turn loop with the RS232 cable,
at one end tie all the wires and shield wire to the coax shield.
At the other end of the loop, connect all inside wires to the coax
inner conductor, but leave the shield wire not connected there.

Then the coax shield connects to the tuner's "loop antenna" GROUND
terminal (if it's not marked, touch the wire while listening, if
you get a signal, the other post is the ground).  The center conductor
goes to the live post.  Lots of signal strength now.  

The shielding on the loop is to minimize electrostatic fields coupling
onto the loop, the magnetic field part of the electromagnetic radiation
(radio waves) gets picked up.  Idea is that noise is more electrostatic
than magnetic.  Sure, you loose some signal, but just make the loop
bigger to capture more magnetic signal to make up for it.

Just be careful you dont get so much signal from a local station
that the front end gets overloaded and cause intermod trash all over the
AM band.  The Denon front end appears to have lots of dynamic range
here.  

One design quibble with the selector buttons: the AUTO/Manual selector
for tuning frequencies also disables the Stereo mode if set to manual
tune.  I thought the tuner was defective because I haden't known
about this (of course there was no manual so I couldn't RTFM!).
I found it by "hacking", that is, holding a button down as a shift
key, and trying other buttons to find this "undocumented" stereo
enable feature.  Eventually I got the STEREO indicator to light, and
all I really needed to do was go into AUTO tune mode.  For $400,

Other than that, it's a good tuner.
Though the 10KHz hetrodynes can get annoying (narrowband mode a bit too
narrow for my tastes).