[AMRadio] Classic Link Coupled Tuner

Bernie Doran qedconsultants at embarqmail.com
Sat Aug 15 09:44:24 EDT 2009

HI Brian: Q is usually not a factor at all unless someone wants a narrow 
bandwidth tuner for some reason.  The issue is very likely your feedline 
length, the only time the feed point "looks" like the antenna Z is if it is 
multiples of electrical 1/2 waves long. If multiples of 1/4 wave lengths it 
will basically invert the antenna Z, IE a high Z will appear as low at the 
feed point, a low z will appear high. Now, anything in between these lengths 
also introduces reactance, some conditions producing a large amount that 
requires rather extream measures to adjust out.     I would wager that yours 
is in that catagory.  If you place a second tuning cap in series with the 
link you will find that adjustments are a little easier, however what you 
have will work fine if the coupling link can be turned easily.    also, 
usually one would expect to see the cap across the entire coil if it is not 
too large, then it could have taps for the cap as well as the feedline. It 
is also common to see two caps, both isolated from each other and ground, 
connected to taps on the coil, with the feed points connected to the caps. 
IE each of the caps appear  as in series with the feeders.  This is used to 
feed a low Z load, probably what you have.   You system will do fine, but it 
probably is a little critical to initally adjust.  with a BC coil the losses 
are probably small. I assume the is no detectable heating in the coil at 
all.   I have used these tuners since the 1950s and they work great, used to 
use the B&W plug in coils, tapped for each band.  Now I have gotten lazy and 
use a Johnson KW matchbox, The best antenna matching device ever made, in my 
opinion. Always found that a high Z was easier to match.  73 Bernie W8RPW
----- Original Message ----- 
From: "WA5AM" <ars.w5ami at gmail.com>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
Sent: Saturday, August 15, 2009 6:05 AM
Subject: [AMRadio] Classic Link Coupled Tuner

> 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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