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LCT Coil Set Design

Link Coupled Tuner

Home Brew Link Coupled Tuner for 160-40 Mtrs.

Link Coupled Balanced Tuner with Plug-In Coil Sets for 160,80,40,  and 20 meters

Also known as a Link Coupled Matching Network

 

This home brewed balanced link coupled tuner is designed for 160 - 20 meters, and is rated for maximum legal power on the amateur radio bands.  This type of tuner is specifically used for open wire transmission lines with balanced antenna systems, generally in the range of 450 - 600 ohms.  It utilizes a parallel tuned secondary. The secondary is normally tapped for connection to the transmission line.  It is similar in design to the E.F. Johnson Matchbox in that it is link coupled.  Although I have made provision for tapping the secondary, I have not found it necessary to use taps while using individual plug-in coil sets designed for each band.  Plug in coil sets were once quite common and have been used for over half a century, and now are rarely seen, even in homebrew tuners.  I thought I would share this one for those individuals that might be interested in a simple and fun homebrew project that will enable the operator to use one balanced antenna on more than one band for efficient power transfer.  The circuit and associated wiring is simple enough for the new comers to this hobby, as well as the seasoned operator seeking a near perfect match to the transmitter across an entire band or bands with one balanced antenna. 

Schematic of legal limit Link Coupled Balanced Tuner
In depth information for this type of tuner and it's theory of operation as well as it's configurations, may be found at W4RNL's web site http://www.cebik.com/. Also, Mr. Cebik has been very generous in sharing optimized link coupled tuner component values for all amateur bands in table form.  The physical design of the coil sets for this tuner were derived from Hamcalc software by VE3ERP which can also be found and downloaded from the internet. Hamcalc is another generous donation to the amateur radio hobby.  Free downloads for this software may be found at http://cq-amateur-radio.com/cq_ham_calc/cq_ham_calc.html I am grateful for their contributions as crunching the numbers for this type of project would have required much more effort on my part.

Since I am a die hard link coupled tuner user, and needed a tuner for 160 meters for  station operation at another QTH, the choice was simple. Fabricating the coil set was relatively easy.The coil sets are made of soft 0.25 inch copper tubing which may be  wound on PVC stock and cardboard tubes used for concrete forms. Forms for specific diameters can also be fabricated from plywood, using hole saws to cut the circular end pieces to the desired inside coil diameter size, and thin strips of pine used to join the circular end pieces together.  A router and patterning bit would be another solution. This latter method enables you to design any size coil based on your most demanding specifications. These coils sets were all wound at two turns per inch with 0.50 inch spacing. Depending on the band or coil size the 0.25 inch copper tubing will cost approximately 25 to 50 dollars per coil set for each of the bands if purchased from Home Depot.  I'm sure there are better deals out there and smaller tubing is also available at a much reduced cost.

Plug-In Coill Set Assembly with male mount    Female mount for Plug-In Coil Set

The coil set shown above is designed for 7.0 mcs. L1=1.2mh has an inside diameter of 5 inches and L2=12mh with an inside diameter of 5.25 inches. See schematic for air variable capacitor values as well as Cebik's chart. I was able to obtain C1 & C1a from a ham fest and radio supply company for about 50 dollars. C2 was already in my collection. The coil sets for this tuner are all mounted on perflex (Plexiglas) and connected to the plug-in base also made of perflex. The male and female connectors or banana plugs are from Pomona Electronics. Their web address is:http://www.pomonaelectronics.com/index.php  I purchased my banana plugs from RSE electronics.  They have a retail store in S.E. Michigan.  

Since the 160 meter coil set for this tuner is about 9.5 inches long with an inside diameter of 8.5 inches I made the 0.25 inch thick Plexiglas coil mounts 2.5 W x 9.5 L inches. This enabled me to mount all three coil sets (160, 80, 40 mtrs.) on the same Plexiglas base. For mechanical support, and ease of construction the coils are radial mounted with L1 inside of L2.  Wire for component connection is all 8 AWG stranded.  For connection to the ends of L1 & L2,  the 8 awg. wire ends were flattened and tinned, then inserted into the copper tubing.  The ends of the tubing were placed in a vise and crimped (crushed) flat, then heated with a torch and soldered.  Shrink tubing was applied to all joints, as well as the 8 awg. un-insulated ring terminals used for component connections.

The insulated coil spacers are made of 0.5 x 0.75 inch red oak cut from 1" x 2" stock on a band saw. A small plywood template was made to accurately drill the 0.25 inch holes in the spacers, then the spacers were split length wise in half on a band saw.  Do not try to use plywood for spacers as it will self destruct while clamping the pieces to the coil.  Any hardwood will work fine.  Plexiglas spacers would be an excellent choice, but the set time for Plexiglas adhesives is too short to enable you to assemble and tighten the clamp.  After winding the coils,  the two piece spacers were assembled to the coil with carpenters glue and clamped with a homemade clamp from 1" x 2" red oak shown below. They were allowed to dry for about 12 hours before clamp removal. I found this to be the easiest way for me to properly space the coil turns. There are several other methods of coil fabricating that I have used in the past but this method is my favorite.  Besides, if you are not satisfied with your assembled coil, you can submerge the assembly in water, which will enable you to disassemble the tubing from the spacers.  A full size template for drilling the mounting holes for the male and female banana jacks is shown below should you wish to utilize the above coil set.   Also shown below is the clamp used for holding the spacers while the carpenter's glue dries.   The threaded bolts are placed through the clamp every other coil turn.  The clamp size will depend upon the coil design and turns per inch.  If you wish, please feel free to copy and download.

Full Size Template for Male and Female coil set mounts    Clamp for holding Coil Spacers to Coil.

With the exception of C1 & C1A, banana plugs and Coil sets, I had everything else I needed to build the tuner. I did purchase the knobs new/old stock for 6 dollars each on e-bay, which match the Johnson tuner knobs with a total tuner cost of just under two hundred dollars with one complete plug-in coil set. Obviously one could just build a coil set for 160 meters and tap L1 & L2 with a band switch for bands up to 15 meters with the listed component values if so desired.  

The tuner worked perfectly upon completion. I had no difficulty in tuning an extended lazy H with 88 foot elements spaced at 44 feet fed with 600 ohm transmission line.   It also tunes the antennas described on my other web page. As stated earlier, I did not have to tap L2 even though I made provision for tapping if needed. The transmission line is electrically connected directly across L2.  For experimentation purposes, I did tap the coil and found that I could resonate my non-resonant dipoles on 20 and 15 meters with the 40 meter coil set.   At a later date,  using the Hamcalc program and the same parameters used for the 160 - 40 meter inductors, I wound a plug in coil for 20 meters. 


The two section C2 rotors are not grounded, but you might want to experiment by doing so if you have any difficulties in tuning your balanced antennas. I used an aluminum strip with the banana jacks purchased from Pomona Electronics to fabricate a shunt instead of using a switch for C1 & C1a, which enabled me to use C1 or C1a or both for tuning the transmitters 50 ohm input. The many pros and cons of the link coupled tuner can also be found at Cebik's web site. I have used other network tuners occasionally, and found that they certainly have many positive applications.  It is not my intension in this brief construction article to criticize other proven tuning methods.  I have been using  link coupled tuners for balanced antennas with no difficulty since the 1960's,  and the LCT will probably remain my choice of manual tuners.   

Additional information regarding coil set L1 & L2 design for this tuner can be found on the LCT Coil Set Design web page.  The information is in table form and covers the 160 - 40 meter bands.  

 

Here are some additional images of the tuner that may be of interest:

40 mtr. coil set installed on Link Coupled Tuner base.  Side View of Capacitor C1a  used independently or paralleled with C1  Coil Set to Female base connector 

 

Unbalance To Balanced:

 I would like to introduce Mike, WZ5Q.    He is talented and very serious about his hobby.   Mike has cloned my coil set designs for 160 - 40 meters, and added a few goodies (improvements that will enhance your tuning experience).  He has also taken the LCT tuning network to a new level by balancing it to his asymmetrical 160 meter antenna with an oscilloscope.  We can only strive to make our balanced antenna systems as symmetrical as possible.  We have all experienced life out of balance.  As Mike has discovered, the same situation occurs with  even the best laid plans for a balanced antenna, and link coupled tuning network.

Mike has spent a great deal of time, and effort in making sure his tuner and antenna system is working at peak efficiency. You will find this information as well as many pictorials on Mikes Tuner web page.  Don't miss out on viewing this well assembled,  and easy to follow semi-tutorial of  balancing a high powered link coupled tuning network to an antenna system with an oscilloscope.

 

I was impressed with Mike's procedure and decided to do some balance checks of my own.  

Fortunately,  my current  (temporary) wire anternna system is for the most part physically symmetrical, which meant that balancing my Coil sets would be minimally intensive.   I took care in routing my feedline from the tuner to the antenna.  I have only five feet of open wire line inside the structure, and once outside, the line moves directly away from the structure in the clear.  The feed line is routed vertically from the Antenna feed point to about 6-8 feet above ground.  

The following images were made from my Tektronix 465 oscilloscope:  Antenna = 88 ft. Vee  with apex at 40 feet.  Feedline =  80-90 feet of home made  600 ohm ladder line

The first image is what each feed line signal looks like at the output of  the  unbalanced  Link Coupled network.  Each feed line exhibits an out of phase condition as well as differences in amplitude.  In addition to a host of other problems, this condition will allow  RF  at higher power levels,  to radiate from the feed line onto other conductive surfaces, usually resulting in various RFI conditions. 

unbal.jpg (192514 bytes)

The second image is what each feed line signal looks like with a balanced Link Coupled network .  The signal on each feed line is in phase and of equal amplitude, which will allow cancellation of  their surrounding fields .  For  reference, this 15 watt signal was generated from my transceiver at 7200 kc  The transformation from Unbal to Bal was made by moving the taps one turn from each end on L2  of the LCT.   RFI will be nearly eliminated and the transmitted signal will be radiated at the antenna.  

7200 kcs.JPG (748645 bytes)

 

I have aligned all of my coil sets to this antenna.   When switching to another antenna system,  it will be necessary to re-align or balance the coil sets to the new antenna.  I would also like to add that the LCT can exhibit an SWR of 1:1 at its input, that does not necessarily mean the system is balanced.  

In both preceding images the input to the tuner exhibited an SWR of 1:1.

 

 

 

Restricted Space Antennas used with the Link Coupler Tuner

 

I have used a center fed 88 - 140 ft. long wire inverted vee with this type of tuner for many years, and had no problems tuning all amateur bands 80 -10 meters.  Of course this antenna requires a bit of real estate.

Antennas that work well for me, and require a minimal amount of space are shown below.

Sigma 40XK fed with 450 ohm feedliine via Link Coupled TunerThe Sigma 40 XK vertical dipole is designed for all band use with coax, and requires manual tuning and interchanging of elements in order to operate all of the amateur bands.  For some, this might be educational and fun, and of course, necessary for all coax cable users.  I purchased this antenna because I have never owned a vertical antenna system and was interested in making some comparisons of my own.

With the LCT this antenna was fed with 450 ohm ladder line and tuned with the above antenna tuner on 80/40, and with the modified Johnson matchbox 80-10 meters including WARC bands and no trips to the back yard. 

A comparison in signal strength  between the  Lazy H and Sigma 40XK , depicts the Sigma consistently 2-3 S units lower than the Lazy H antenna.  Often times, I found that distant signals were slightly stronger with this antenna.  I recommend this antenna for portable work or for stations short on real estate.  It is very well constructed.

In my humble opinion, this antenna is overpriced.   After a year or two of occasional use, this antenna was sold to a fellow amateur.

 

 

 

 

Extended Lazy H for 10 meters fed with 600 ohm homebrew Ladder LineHome brew Extended Lazy H designed for 10 meters, uses 44 foot elements vertically spaced at 22 feet and fed at center with Home brew 600 ohm ladder line.  The ladder line is made from #14 stranded wire and 0.5 inch pvc spacers.  I am currently using this antenna on all bands from 80 -10 meters.  A very inexpensive, and easy antenna to assemble.  I have built one of these for 20 meters which uses 88 foot elements and 44 foot spacing for use at another QTH.  I have also modeled an Extended Lazy H for 40 meters using 176 foot elements spaced at 88 feet, but unfortunately don't have room for the vertically spaced elements.   This would be a fun antenna for someone with a pair of 150 foot vertical supports.

 

My preferred antenna to use with the LCT is the above 10 meter Extended Lazy H because of the lower take off angles on the higher bands ( 20 -10 ), and it's additional gain over a dipole on those bands.  This is an ideal antenna for restricted space, provided you have two suitably spaced  supports.  For average heights of about 30 - 40 feet this antenna is a cloud burner on 80 & 40 meters, with a progressively lower take off angle on the higher bands. 

 



If you have any questions, please feel free to e-mail me.

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Page last updated on 01/17/2011

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