[AMRadio] Classic Link Coupled Tuner

Gerald Stockinger cliogunsmith at centurytel.net
Sat Aug 15 10:30:20 EDT 2009

Brian, I believe the problem is your connection. The Capacitor is NOT 
connected across the feedline points. It should be connected across the 
coil ends OR as much of the coil as is needed to resonate on the band in 
use. A high L(inductance and a low C(capacitance) will give a broader 
tuning of the coupler. Use the second capacitor in series with the link 
as it makes the adjustment simpler. With a SWR bridge in the coax to the 
coupler start with the coil taps about 1/3 of the turns out from the 
center towards the ends of the coils. Put 20-30 Watts RF into the 
coupler and adjust both capacitors for the lowest SWR. Now move both 
coil taps out a couple of turns and readjust. If the SWR goes lower than 
the first time repeat the coil tap adjustment in the same direction 
until you reach 1:1. If the SWR went up  from the first tap setting it 
means the taps were moved in the wrong direction. For a parallel tuned 
coupler (like your drawing) think of the tuned circuit as an adjustable 
impedance transformer (which it is).  At the center of the coil it 
starts at 50 ohms. The outer ends of the coil where the capacitor is 
connected is very high like 12-15,000 ohms. Between these two points you 
can find any value between the two. What you are doing by connecting the 
capacitor across the feed points is always having your feedline 
connectecd to the highest impedance on the tuned circuit. Now before 
anyone jumps all over my explanation of the impedances here I know that 
the statement is not absolutely technically correct but about 90% and it 
will serve to make the whole thing understandable to someone whom is 
just starting out with this type of coupler. This type of coupler is one 
of the handiest things around. If you need a coupler to match anything 
this is it. Brian once you get the hang of it which will come quickly 
you won't even try anything else Good Luck  73

Gerald D. (Jerry) Stockinger


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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