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.
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:
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.
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.
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.
The 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.
Home 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.
Page last updated on 01/17/2011
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