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symmetrical antenna tuner

symmetrical antenna tuner

Last Updated: 28 September 2016

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 (http://www.mansrfdesign.nl/ham-radio/antennas)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.....

Antenna tuner

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.

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