[AMRadio] Classic Link Coupled Tuner

rbethman rbethman at comcast.net
Sat Aug 15 11:19:06 EDT 2009


The bumfuzzlement is being caused by something that your calculations 
don't have the ability to deal with.

All your other tuners do NOT use link coupling!  The commercial tuners 
are usually an "outboard" PI-Network.  Although the UT-2000 is a T-Network.

The link coupling does this in a manner different than a PI or T, in 
that the link coupling has that little quantity of the field that the 
coupling depends upon.

I do NOT have numbers to perform a mental gymnastic exercise, although I 
have had the experience of using the three of them.  The link coupled is 
much like the BC-610 PA Coil with its rotating link.  It actually can be 
set to different coupling positions and STILL achieve resonance by 
retuning the variable capacitor.

Bob - N0DGN

WA5AM wrote:
> 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:
> http://www.possumnet.com/Graphics/Diagram.jpg
> I don't do this.  My variable is is paralleled directly to the feedline
> taps.
> 73
> Brian / wa5am

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