[AMRadio] Classic Link Coupled Tuner

John Coleman jc at pctechref.com
Sat Aug 15 13:09:14 EDT 2009

	In a link coupled assembly the coupling actually has an effect on the Q, the effective load.  But increasing the Q of a parallel tank (less coil more capacitance) will increase the coupling.  Using taps on the coil for the antenna connections reduces the coupling while increasing the Q.  The fewer turns there are between the antenna connections the greater the Q of the tank but less voltage is coupled to the antenna.  The coupling between the link and the main tank increases because the Q of the main tank went up as it was loaded less.  The voltage across the ends of the main tank will go up as the taps are moved towards the center.  You can get a different run of coupling by tuning the link in parallel instead of series.  But this will double the voltage across the link capacitor. You can get yet another run of different coupling factors by using a swinging link.
	It all boils down to this:  The tuner should be adjusted so that there is no L or C reactance at the antenna terminals. Then the coupling is adjusted to pull the current from the rig so that Erf/Irf = desired Z-load (generally 50 ohms).  
	If the Q of the circuit is not high enough you will not be able to couple enough energy out of the XMTR to draw enough RF current to get the desire E/I (LOAD resistance) low enough.
	If the Q is too high in the wrong place then the circulating currents in the tanks of the tuner will either have high loss or begin arcing.  If it aint getin hot and it aint arcin then it's OK.
	There always a number of ways to do it.

BTW:  I was in your neck of the woods a couple of weeks ago visiting with Kids in Searcy and drove out to the Peti-Jean park.  I guess we must have gone right by you place.
John, WA5BXO    

-----Original Message-----
From: amradio-bounces at mailman.qth.net [mailto:amradio-bounces at mailman.qth.net] On Behalf Of WA5AM
Sent: Saturday, August 15, 2009 5:06 AM
To: Discussion of AM Radio in the Amateur Service
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:

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

Brian / wa5am
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