[AMRadio] FT102 and HDVL for 160

[Donald Chester]

>On the HDVL for 160, yeah Geoff, I wouldn't have
>thought they existed, either. Is it printed on the
>jackbar?

Yes, they made them.  I had one at one time, but eventually it fell apart 
because the plastic self-destructed.  The HDVL series was made before WW2, 
starting in about 1938, so 160 was still a full-fledged amateur band then.  
They were also sold briefly after WW2, most likely until the old stock was 
depleted, and it became apparent we weren't getting full use of the band 
back any time soon.

The way to identify pre WW2 HDVL and similar coils is by looking at the 
ceramic bar.  The older ones had rectangular ends.  The later ones were 
thicker and heavier, and the ends of the bar were rounded.

The earliest version of the HD series coils did not plug into a jack bar, 
but had individual standoff insulators with a jack installed individually on 
each.  The whole assembly came mounted on a metal plate, which could be used 
in the final installation, or could serve as a template for re-mounting the 
individual standoffs directly on the chassis or breadboard.

The very earliest version of this coil series did not include the variable 
link coils.  No one had apparently figured out how to use a swinging link in 
the middle of a coil.  Variable end links were widely used, but balanced 
pushpull output coils always had fixed links.  It wasn't until just before 
WW2 that variable link coils for balanced tank circuits appeared 
commercially.

Link coupling was well understood from the early 30's, and I cannot imagine 
that no hams had figured out how to use variable links with a balanced coil, 
but apparently the commercial manufacturers didn't figure this out until 
just before the war.

The basic principles of the balanced swinging link appears in a rig 
described in the 1934 ARRL handbook, although strictly speaking it was not a 
variable link, but a secondary coil with variable coupling.  But the text 
fully explains the principle of keeping the variable coupling coil at a 
"cold" spot on the tank coil to maximise isolation between the two coils.

Don k4kyv