Transmatch (Link Coupled Tuner) using Pine Core Coil Construction
2/02/R.Kwas-KA1-RBP
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You've heard of ferrite core and air core coils...well here is yet another use for trees (after antenna supports)...the coil form for your transmatch coil... I call it a Pine Core Coil!  N1KW advises me that the dielectric constant of wood is around 6-15 depending on moisture content...that may very well be...and I expect the permeability to be close to that of air...what I am sure of, is that I have yet to experience an arcing, or ANY kind of breakdown, due to the fact that I use wood, while operating at barefoot power levels.  I don't plan to use a pine core coil for full power use...commercial coils have been procured and will be used on full power tuners already in the works, but for power levels up to a 200W this is without a doubt my solution of choice!

Construction details and Notes:  Two pieces of 2X3 (as knot-free as possible...I'm not particularly interested in doing dielectric tests on them) were cut to 12" length (for a coil suitable for 160m, a longer form and more secondary turns may be necessary - the coil shown here does work fine 80-10m) and glued together - screws may be used, but would clearly have to be accompanied by some care in positioning to avoid problems once the coil is wound onto the form!  One inch max. length screws are used to mechanically terminate the coil ends.  Coils are wound at a pitch of approximately .2" (see pic), making constantly spaced marks might help.  While winding the coils, the  wire (14ga. tin plated in this case ) must be held with a fairly high tension.  This tension will deform the wood at the corners of the form which will serve to hold the wire in place well when the lumber shrinks further as it looses additional moisture in the shack.  Failure to do this will result in the coils being able to move around after the wood has shrunk, possibly touching as they do - clearly undesirable!...although a bead of RTV, applied after winding, would also hold the coil location and spacing well - forever!  The successful technique I use for winding the coils, is to leave the wire roll on the ground, pass the wire under a shoe, then onto the coil form held horizontally in the hands.  Using this technique, the wire tension can then be kept high (which also helps straighten the wire) but easily adjusted while winding, and even though I used the technique described above on the ugly tuner coil, I did notice a certain amount of slack still developed after a while, so to further compensate for the expected (future) shrinkage on the most recent coil I constructed, I placed some 3M adhesive backed foam weather-stripping on two corners of the form (see pic).  This foam is loaded up as the coil is wound and will further minimize slack occurring as the high quality lumber procured from "Home Cheapo" shrinks.  

Number of turns used.  Primary turns:  12   Secondary turns:  20 each coil

Pine Core Coil construction for tuner No. 3 in progress.

Once the three coils are finished, the center of the secondary coils must be connected together with a link as required per the classic tuner circuit.  Also, taps are soldered onto the primary to allow connecting to various numbers of turns (use heatsink to keep from melting foam at corners while soldering).  There is plenty of room for clips on secondaries, so taps are not necessary.

Coil detail showing secondary coil center jumper (Teflon tubing insulated), foam rubber on corner of form, and clip connections.  Characterization tests still need to be performed.

Once tuner construction is complete,  characterization tests need to be performed, to find the optimal matching points for each band (finding a perfect match should be possible!).  That's a bit tedious, and it varies as a function of antenna, feedline impedance and feedline length with respect to lambda, but this only has to be done once...(and only needs to be redone if any of the critical parameters are changed).  As the optimal tapping points are found, a color mark with a corresponding chart will allow rapid band changing and retuning.         

Coil in service in Transmatch No. 3, showing characterization marks, primary clips on taps, (orange wired) secondary and (dark) feedline clips directly on wires.

Link Coupled Tuner No.3 in service.  Note band chart with colors.  Also visible coming up from chassis behind coil are an insulated post and crossbar which serve to hold the (secondary) capacitor wires and feedline with nice spacing and symmetry before these connect to the coil.

Finally, it can be seen in the last two pics, that (for the current particular installation parameters), coils of a mere five turns would have been enough for the secondaries.  Then again, that's probably lucky...for some other installation this tuner gets used in, the entire 20 turns of secondary may be required...but undoubtedly a perfect match can and will be found!

1:1 Baby!

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