A symmetrical antenna tuner by Mans Jansen, PA0MBJ
With an inverted-V wire antenna, a symmetrica feed line and a truly symmetrical antenna tuner, all HF bands between 160 meters and 10 meters can be easily covered.
After almost 30 years of "radio silence", I decided to re-enter the HAM radio world and especially the HF bands. The next question was which antenna is suitable to explore as many as amateur bands between 160 and 10 meters. And this is not an easy task, especially because I wanted to keep it simple. No high antenna towers (XYL must not get upset from the beginning), no very expensive antenna tuners (better build it yourself) and the chosen antenna must have a reasonable good efficiency. After plowing through many antenna books it became obvious there is no such thing as an ideal antenna. If you want to cover all amateur bands between 160 and 10 m AND antenna space is limited, an inverted-V antenna with some funny bends in it (remember the limited space) may be a solution. Forget coaxial cables as feed line (antenna will not be resonant on several bands) but use 450 Ohms ladder line and a truly symmetrical antenna tuner. The length of the inverted-V dipole halves depend on the available space in your backyard. So, in my case, I could hang out 2 x 18 meters of wire (with bends to keep it within my garden area). Good for the 80 to 10 meter bands, short in length for 160 meters but still usable on that frequency but with reduced efficiency. If you have less space, just use shorter lengths of antenna wire. You will loose 160m, perhaps 80 m as well but you will still have much fun on the remaining bands. Because you may end up with an arbitrary antenna length and also an arbitrary length of feed line, the impedance at the end of the feed line will vary wildly with frequency and must be matched to a 50 Ohms asymmetrical impedance for proper connection (and avoiding smoke) to your transceiver. And this is the point where the symmetrical antenna tuner comes in. The principle is certainly not new. Our grandfathers in the 30's of the previous century already used this type of tuner with great succes but the principle has been forgotten.....
The type of antenna tuner presented in this article uses a resonant circuit as coupling element for the feed line and is tested wit RF powers up to 100 W. In case of a low feed point impedance, a series resonant circuit is used, in case of a high feed point impedance a parallel resonant circuit is used. Coupling to the transceiver is done with a coupling coil in series with a variable capacitor. Because of the use of ladder line, high SWR on the feed line gives only little attenuation. The tuner resonates the whole antenna system, feed line included. In this way, it is possible to cover all HF amateur bands with a single antenna. With the help of an antenna analyzer, tuning for a specific amateur band can be done rapidly and easily. A set of plug-in coils has been constructed for different (groups of) amateur bands. The schematic gives an impression of the tuner set-up. The transceiver is connected to the 50 Ohm coax connector. L1 is the coupling coil which is oriented in the middel of L2. C1 is a 4 x 500 pF receiver-grade variable capacitor. One section is in series with L1 and the three other sections can be switched on if a larger capacitance is required. You can also use a capacitor with 3 x 500 pF or even a 2 x 500 pF type and switcheable fixed additional capacitors. L2 is the secondary coil and this one is split in two identical sections, close together side by side. C2 is a QRO-type of split stator capacitor with wide plate spacing. This is necessary because high voltages can be present across the secondary circuit, especially when the antenna length is less than a half wave. Don't use a single capacitor here. With a split stator type the RF current does not need to flow through a wiper contact which can give problems with the wiper's contact resistance. The top-left schematic shows the configuration for low-Ohmic matching. The two halves of L2 are in series and also in series with C2. The feeder is connected to the ends of L2a and L2b which are close together in the middle of the coil assembly. Impedance matching can be done from a few Ohms to about 600 Ohm.The bottom-left schematic shows the high impedance configuration. in this case, L2 and C2 form a parallel resonant circuit with the feeder connected to the ends of L2. Impedances of about 400 to 3000 Ohms can be easily matched in this way. In one special case, it was necessary to connect the feeder to taps a few windings from the ends of L2 to obtain a 1:1 SWR. To make quick band switching possible, the coil assemblies are mounted on a multipole connector for a quick change. Note that there is no electrical connection between the primary and secondarty circuit. The whole setup gives a "quiet" impression when listening to the bands. A directional power meter and an antenna current meter are included in the design (to be discussed later). The picture on the right shows the bottom side of the tuner. In the top left corner the directional power meter assembly is visible. Below this unit the antenna current transformers are located. The large tuning capacitor is operated via a 6:1 ball drive reduction gear. The shaft between the ball drive and the capacitor is made of insulated material to avoid an electrical connection between the capacitor shaft and the chassis.
Antenna coil building
For this type of antenna tuner, air core coils are a good choice. but the problem is how to obtain them (at fair cost). I decided to build them myself and I found a way to produce reasonable good looking types of different diameter and self inductance. At the end of this article, I will give you the dimensions, number of windings and inductance of the coils I use for the different amateur bands. Here is the recipe for coil construction: strip off the insulation of a suitable length of solid 2.5 mm2 AC mains wire or other wire of the same diameter or thicker). Twist this wire with the help of a drill and stretch it at the same time. This makes the wire straight and stiff. Take a short length of plastic plumbing pipe with a 5 to 10 mm smaller diameter than the diameter the finished coil must have. Drill two small holes at one end of the pipe to attach the end of the wire and wind under tension the coil on the pipe. Make sure you will get more windings than intended for the finished coil. When finished, just let the coil go (it will expand a bit in diameter), and cut it loose from the pipe. Now comes the trick: take another short length of plastic pipe wit a larger diameter (larger than the coil just wound). Just "help" the coil on the larger pipe, turn after turn. Do this carefully to avoid bends in the wire. You will notice that the coil somewhat clamps on the pipe. Stretch the coil to get enough spacing between the windings and make sure that these spacings are correct at all places. Slide three or four strips of stiff plastic sheet between the coil and the pipe. Use a hot melt glue gun to fix the windings on to the plastic strips. Let the whole assembly cool down and slide the coil off the pipe. Apply more hot melt glue on the inner side of the coil to make the whole assembly more rigid. Repeat the whole procedure to build the coupling coil that will be located inside the secondary coil. Its diameter can be about 50 to 70% of the secondatry coil's diameter. Now it is time to prepare the secondary coil for final assembly. We have to cut the winding in the middle to obtain the two halves of the coil for connection to the feeder line in case of low Ohmic matching. Make this cut in the center winding and between two of the glued strips. Now define the exact number of turns the coil will get and make sure the whole thing will be symmetrical. Also bend the coil ends and make sure that the ends and the center cut are in line with each other. Now it is time for the final assembly of the coils. I used a rectangular 20-pin connector. But any connector with enough pins and the right dimensions will do. The smaller coils can directly be soldered to the contacts. As you can see in the last picture of this chapter, this coil assembly is for low-Ohmic matching. The coil's two center taps are soldered to two pins in the center row of the connector. The end connections of the secondary coil are soldered to contacts on row 3 and 8 and the primary coupling coil is soldered to the contacts on row 1 and 10. If the coil assembly is meant for high-Ohmic matching, the cut in the center of the secondary coil is closed and the feeder contacts are connected to the ends of the secondary coil. In my case, I can operate on all 9 HF amateur bands with 6 different coil assemblies. When changing bands, all I have to do is to change the coil assembly and to tune in for SWR 1:1. With the help of an antenna analyzer, this can be done in the blink of an eye. But before you have reached this comfortable situation, you have some work to do. When you have your antenna and the feeder in place, measure (if possible) the impedance at the end of the feeder for each band. So you will get a global impression if the impedance is hig- or low-Ohmic. Build a coil assembly as described before with the right specifications and start trying to get a low SWR by adjusting the controls of the tuner. IF you can't find an SWR close to 1:1, the secondary coil must be smaller or larger, or change from serial to parallel tuning. If a coil won't do the job for one band, it is quite well possible that is works fine for another band. Just try! If you have your coils ready and everything works fine, make a list with all amateur bands, with the used coil and the settings for C1 and C2. In this way band changing goes very fast. I wondered what the dielectric loss of the hot melt glue would be. Tests with 60W output in digimode on several bands gave no noticeable increase of coil temperature. So I suppose that the hot melt glue is fine for this purpose. The results with the combination of the inverted-V antenna and this tuner are good. I started with 5 W QRP power digimode and worked USA, South Africa and Australia with it.
Directional Wattmeter and antenna current meter
Those two meter circuits are very convenient in daily operating practice. The directional Wattmeter circuit is well known as the tandem coupler principle. It provides good power readings between 1.8 and 30 MHz. It's always nice to see your transmitter power flowing in the right direction and it is comforting to see no reflected power. The coupler is build from two current transformers, each with a short length of RG58/U or aequivalent coax cable fitted through the core of each toroid. The 1M resistors R5...R8 only have a mechanical purpose. They support the main line in the coupler. The 100 Ohm resistors R1...R4 do the same job but also form the 50 Ohm load for the secondary line in the coupler. The rest is very straightforward. A range switch provide convenient reading from 1 W to 100 W full scale. The next picture shows the mechanical buildup which is not very critical. Make sure the braid of the short coaxial lines through the cores are only grounded at one side! A piece of PCB is used as a substrate for the coupler. You can put a vertical shield of PCB material between the two lines in the coupler. Keep connections short. I have built the power meter in the tuner as a unit with separate input/outputs. In this way the meter is also useable with other tuners/antennas. For normal use, a short length of RG58/U cable connects the meter at the back of the unit with the input of the tuner circuit.
The antenna current meter only has one meter which can be switched from the left to the right line of feeder. The current transformers are wound on FT82-43 toroids. The bigger aperture of these cores is used to put more insulation between the feeder line and the secondary windings on the core. This is done because in some cases, especially with parallel tuning, the line voltages can be very high. And we don't like sparks in our tuner... The meter readings give a good impression of the balance in your dipole or inverted-V antenna system. You can also observe that with a high-Ohmic match the antenna current is rather low while still almost all of your transmitter power is going into the right direction. But since the proof of the pie is in the eating, the actual results on the HF bands will confirm if the whole contraption is working satisfactorily. In the beginning, I have worked many stations in Europe, Asia, the USA and even one time in New Zealand with only 5 Watts of RF power. Also tests with a small WSPR transmitter with only 1W RF output on 14 MHz were heard within 24 hours on all continents as the picture below will show you. This result was obtained on April 12, 2015, in only 24 hours.
tuner coil table
And here is the coil table for this tuner. Remember that every antenna may require different coil parameters, dependent on the antenna lenght and the height of it. So this table is a starting point. Just wind a couple of coils and experiment with it. You can use lesser windings by connecting the capacitor stator connections to a tap a number of turns from the end. After obtaining the proper coil inductance for a specific antenna and band, just remove the loose turns on both ends of the coil. There will also be differences between standard dipoles and inverted-V type of antennas. Inverted-V antennas have their ends closer to ground which gives an increased capacitance. It is often possible to use another coil, meant for an adjacent amateur band as well. Just look how well the matching goes for every coil and keep using the best one for a specific band. There is something odd in the table. The 30 meter band coil has an higher inductance than the 40 meter band coil. I will have to look into it why this is the case. But in the mean time, the tuner is doing well on both bands.