[AMRadio] Classic Link Coupled Tuner

WA5AM ars.w5ami at gmail.com
Sat Aug 15 06:05:56 EDT 2009

I'm bumfuzzled...   This is not exactly AM, but I use an AM tx with it...

Several years ago, I built a large link coupled tuner from parts I managed
to get off ebay.  A large coil with rotating link in center which came out
of a vintage BC transmitter, and a large Johnson split stator variable,
about 35 to 475 pF.  I breadboarded these two components together and have
been using it since on 75 and 40 meters with great results using 450 ohm
ladder line to a doublet cut for 75 meters.

After I built this, I found tapping points on the main coil that worked well
on 75, ultimately giving me a standing wave of near perfect on a given
freq.  I did that simply by trial and error...

About two years ago I purchased an inductance/capacitance meter.  Never
thought to measure the coil taps on the tuner with it, etc., until
yesterday.  I disconnected the cap, coil and feedline from one another and
checked the inductance of the main coil where I had my 75 meter taps.  To my
surprize it was only ~7.2 uH!!  For the heck of it, I measured the variable
in the range it normally is adjusted to and it showed about 220pF.

According to formula, my  values do have a resonant value near the upper end
of 75 meters, and in real life do well around 3.885Mhz with my conditions.

Looking at other tuners, homebrew and commercial, it appears most will tap a
balanced coil like this at about 22uH and set a capacitance at around 75 pF
to get resonance on 75 meters.  Since I have plenty of coil on this, I did
exactly that and can not get anywhere near where I need to on my tuner.
What gives??  As a reference; the lowest SWR I could achive was about 5:1,
and there was a noticeable attenuation on rx too.

Since I can get it to work just fine with my original 7.2uH tap, it really
is no big deal to me, but I am curious why others I've seen use (and can
use) a lot more inductance.  I know using more C gives better 'Q', but why
is my real life parameters so far from the norm, and why will it not tune
using the normal parameters of L and C?  There must be something not right
on feedline length, or the length of the doublet itself, maybe?  By the way,
the measured inductance of the link coil is about 3uH.

One other thing I've noticed is that many seem to indicate the variable is
paralleled to the entire length of the coil, not directly to the points
where it is tapped for a given band.  See this diagram:

I don't do this.  My variable is is paralleled directly to the feedline

Brian / wa5am

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