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Thoughts around antenna
tuners (published in
CQ-QSO) Introduction Regularly fellow
hams would like to know what antenna tuner to use in general or what type of
tuner to use with what antenna. Background to such questions is the idea some
hams have about antenna tuners. These are supposed to solve any matching
problem and to render the antenna plus feed line into a highly efficient
system regardless of impedances, losses etc. Many radio-hams do not even like
to think about any parameter around the antenna system and therefore regard
the tuner as a magical 'black box' to keep all antenna questions outside the
shack. Usually I first try
to find out what type of antenna one is going to apply for what type of
communication over what distances. These questions are regarded to be somewhat
tiresome since usually one has not decided on the antenna yet and furthermore
it is obvious that this antenna is to be used for all types traffic, 'as
everybody knows'. Also the fact that the antenna is not directly connected to
the antenna tuner in the shack, but through a feed line with certain
characteristics will easily be disregarded. Problem also is
that a very well designed antenna system and associated feeder line will look
not much different at the tuner site from a badly mismatched system with high
feeder losses; both systems may show moderate SWR figures that the tuner will
easily match to the transceiver. In the last situation one may even think to
be lucky to live in a quite EMC environment and blame a bad signal report to
'conditions'. The general set-up
of a transceiver to antenna system will usually look somewhat like figure 1,
which the rest of this short paper will refer to.
Let's start-off by
looking at the tuner function. At the tuner site of the antenna system an
impedance R + jX may be measured that is different
from the terminating requirements by the transceiver, usually some real
value, f.i. 50 Ohm. The antenna tuner therefore has
to compensate for the reactive part and transform the impedance 'R' into this
real value as required by the transceiver.
A quick look inside
the transceiver will exhibit a comparable transformation. Some years ago,
most amateur transceivers were equipped with tubes in the transmitter output
stage and this usually is still the situation in high-power RF amplifiers.
Such radio tubes like to see operating conditions with several hundreds tot thousands of volts
on the plate at currents of tens to hundreds of milliamps. For an efficient
system, this 'translates' into plate loading condition of a (usual) real
value of several hundred Ohm. At the beginning of
the antenna feed line, the impedance may have a different impedance , usually
between a few tens to several hundred Ohm (and higher) and a reactive part.
Therefore a tube output stage usually will look something like figure 2. This
so called pi-filter stage is capable to compensate for the reactive past at
the antenna terminal while translating the higher or lower impedance at the
antenna terminals to the specific requirements of the output tube. Notice
this pi-filter also to have a low-pass action which helps to attenuate higher
harmonics of the transceiver. Notice further that
most of the 'tuning' action of the antenna system to the output tube (and
vice versa) is performed as part of the 'normal' tuning procedure of the
transmitter; an external antenna tuner therefore is often not required.
At less than
optimal loading of the transmitter tube, total output power usually is less than
maximum and so the output power indicator is a good measure for optimal
settings of the pi-stage. In general a tube transmitter is somewhat
permissive towards less than optimal settings, provided total dissipated
power inside the tube does not become excessive. An additional SWR
meter in the output stage may tell something about the (mis-)match
of the impedance that is connected to the antenna terminals, which is an
indicator for losses in the feed-line and antenna. It will not provide an
indication for optimal matching conditions to the transmitter tube. At the receiver
side one will notice less than optimal matching to the antenna input as a
lower indication at the signal strength meter. Mind that as long as signal
strength is more than about 1 - S-point above the noise floor of the
receiver, readability will not suffer. Although total signal strength will be
diminished, the signal-to-noise ratio will not have changed, so a
less-than-optimal matching condition will usually go unnoticed. Contrary to a
tube-transmitter, a transistor output stage is operating with a collector or
drain voltage of tens to hundreds of volts, in combination with a collector /
drain current of many to tens of amperes. For optimal loading conditions,
this transistor stage therefore should be loaded with a few Ohms, now to be
matched to tens or hundreds of Ohms of the antenna (-tuner). Large
transformer ratios usually imply losses, therefore these large ratio's
usually are broken up into a two step action. Step
one transforms the very low transistor loading condition into a standard,
real value, usually 50 Ohm, with the user to perform step two with an
external tuner to compensate for any reactive part and also to translate the
unknown antenna resistance into this 50 Ohm. As it is less difficult to match
the low transistor loading condition over a wide frequency band, the
transistor output stage usually does not contain any tuning devices and will
look something like figure 3.
At a different from
optimal loading condition at the antenna terminals, the output transistor will
be loaded out of the original design space, often leading to an operating
condition outside the 'safe operating area' by the transistor manufacturer.
This 'overload' condition usually is more easily reached at these transistor
stages compared to tube amplifiers. Therefore transistor output amplifiers
often are equipped with safeguarding feed back
mechanisms that shut-down input drive in case of high(er)
SWR loading. Nevertheless, care should always be taken when tuning HF
amplifiers and a low drive power should be applied only until optimal
matching is reached; this is going for transistor and tube amplifiers alike. To a transistor
receiver the same remarks apply as we have seen at tube receivers. As long as
total signal at the antenna terminals is around one S-point above the noise,
bad matching conditions will not degrade readability. It should further be
noted, that many modern ham receivers exhibit a much different input
impedance from 50 Ohm, which also depends on the particular tuning condition.
This should be kept in mind when calibrating the S-meter; at a different
tuning condition this calibration may deviate considerably. At this stage we
now may conclude that an antenna tuner will not change anything to a bad
antenna system, a less than optimal or otherwise lossy
transmission line or high SWR conditions anywhere at the antenna system with
associated additional losses. A tuner will optimize matching of whatever
impedance is to be 'translated' to the antenna terminals of the transceiver,
thereby maximizing total signal at the receiver input and protecting the
transmitter from overload conditions; it will not improve any losses in a
badly designed antenna system and any power (receiving or transmitting) lost
herein will not be recovered anywhere in the rest of the 'chain'. Losses in the antenna system From the above it
is now clear that before we look at the tuner, we first have to take the
complete antenna system into account. An other
article on system losses over frequency may be found on this web site at
"Where does the power go",
that has been published before (in Dutch) in the ham magazine Electron. In this article one
may appreciate the importance of cable losses. Looking for instance at
The figure of SWR 1
: 17 at 7 MHz. is already obtained at a connecting
impedance of 400 Ohm in series with a capacitor of 52 pF (j.400 Ohm). A
dipole antenna will meet and surpass these values easily when tuned outside
resonance. At the tuner site, this SWR is brought back to around 1 : 9 due to
the same cable losses so you may not have been too much worried about as yet.
You will further probably not have noticed anything unusual when tuning this
system since the tuner will match this impedance with a series inductance of
around 1,9 μH and a capacitor to 'ground' of
around 712 pF. In a well
designed tuner, these power losses will now be around 1,5 % so this is
beyond compare to the 50 % we already were losing in the transmission line.
This example is showing directly the importance of considering the total of
the antenna system at first before even starting to think about any type of
antenna tuner. Values will be
different with a symmetrical transmission line with penalties in other
systems parts; I will come back to this later. A second aspect of
a non-characteristically terminated
transmission line is that it will transform the impedance at one end
of the cable to a different impedance at the other end. The above value of
400 Ohm in series with 52 pF at the antenna end of our
Depending on the
specific transformation (low to high or high to low) the capacitor should be
on the other site of the inductor. This may be accomplished by means of a
(good quality!) switch or doubling the capacitor to both sides of the
inductor. In this last situation one easily will recognise
the pi-filter, already described with the tube amplifier stage. This example
clearly shows that the choice of the feed line (transmission line) may be
just as important as the selection of the specific antenna. Bottom line is to
end up with a low mismatch between antenna and feed line. A at the same time
one should not to be too extreme about this; for moderate line length and
frequencies, SWR < 1 : 3 certainly is tolerable without too much cable
loss. Keep in mind that even the best decision on transmission lines is lost
when connecting to the wrong antenna, or, what works out the same, operating
the right antenna at the wrong frequency. After the
transmission line, let's look at the antenna. A dipole is a
simple and useful antenna that at the right antenna height may be operated
for local and DX traffic. In resonance this antenna exhibits a terminating
impedance between 40 and 80 Ohm, depending on ground type and at an antenna
height of around 1/5 wavelength. This is easily accomplished at 7 MHz., with
More antenna's than
just this dipole exhibit a 'practical' termination impedance while keeping
other antenna parameters at a nice level as may be appreciated from my 'Duoband antenna' and 'Five band antenna', both
also published (in Dutch) in the ham magazine 'Electron'. Both antennas
exhibit low termination impedance at the design frequencies at a 'practical
antenna height of This leads to the
observation that a sensible choice of antenna and matching feed-line does not
require an antenna tuner to match the antenna system to the transceiver! Always better with open feed lines? Let's go back to
our first selection of coaxial transmission line for feeding the antenna.
This type of line makes an easy connection to the antenna, has no problems
when leading along metal roof trimmings, attaching to the outside wall and
leading into the shack. This type of cable unfortunately exhibits a low
characteristic impedance and relative high loss figures when compared to
open, symmetrical transmission line. When putting the 'mechanical'
consequences of this latter type aside, we are looking to a different
situation. An earlier example
around a dipole antenna at 7 MHz. exhibiting an
impedance of 400 Ohm in series with 52 pF (400 Ohm), resulted in SWR 1 : 17
when connecting to RG58 cable. This leads to an associated cable loss of 50%
of input power already at a cable length of When we connect the
same antenna at the same frequency to an open, symmetrical transmission line
with a characteristic impedance of 600 Ohm, we notice a SWR 1 : 2,5 with an
associated loss of 0,37 % of input power for the same At line input the total
impedance now looks like 430 Ohm in series with a reactance of 462 Ohm and
this is almost the same as at the antenna side of the feed-line. To transform
to 50 Ohm, the tuner now requires a series inductor of 4,7 μH and a capacitor of 131 pF to ground, which is a
considerably lower value than in the coaxial situation. Since we are now
dealing with a symmetrical situation, the tuner also should have a
symmetrical lay-out and so the series inductor has to be split up in two
sections of 2,35 μH each. To retain symmetry,
also the tuning capacitor should have symmetrical (parasitic) capacitance of
both sections to the environment.
Keep in mind that
the transceiver side of the tuner now is showing a real 50 Ohm value, but
this is still symmetrical with respect to ground. To connect to an
a-symmetrical transceiver a good, high impedance balun is required that
exhibits a series impedance, a number of times higher than the antenna
connection impedance in order to preserve the symmetrical nature of the
complete system. More on baluns in general and high-impedance
baluns in particular may be found elsewhere on this site. In the above
example calculations were made on a off-resonance
dipole antenna. Looking at antenna resonance at 3,6 MHz.,
the antenna impedance is real and 50 Ohm. When connecting to the same 600 Ohm
open transmission line, SWR now is 1 : 12, with a total line loss of 2 % of
input power. This still is much less than in case of the coaxial transmission
line but already five times as high when terminated characteristically. At the tuner side
of the symmetrical transmission line, the 50 Ohm antenna impedance has now
been transformed into 6540 Ohm in series with 12,4 pF.
To translate to 50 Ohm, the tuner needs a series inductance of 13 μH and a capacitor to ground of 39 pF. Because of this relatively high inductor value, more
power will be lost herein even at a reasonable inductor quality (7% at Q =
100). The tuning capacitor will be much more (voltage) stressed than before
as already 1150 Volts will be present at an input power of 100 Watt. In general, a high
impedance antenna may be best connected to a high impedance transmission
line. The antenna tuner will be less simple and the internal components will
be more (voltage) stressed. Also the balun needs specific attention. A low impedance,
usually resonant antenna may be best connected to a low impedance
transmission line, with all characteristic impedances below 100 Ohm as a good
choice. Tuner will be less complicated with lower voltage stress. To avoid
outside braiding currents, connecting a current choke is always a good idea. Summarizing the
above discussion, the following steps may be a recipe for designing your
antenna tuner - At first you
should determine what type of communication (distance, cross band) your
antenna should be optimized for and the possibilities your particular 'real
estate' situation has to offer. A practical tool for antenna design is the
free version of Mmana (Mmana-Gal now).
Using this program you may quickly find the radiation angle (azimuth and
elevation) to give you properties for local and DX communication. You will also
find antenna impedance, that you should calculate for all frequencies you
intent to use. - Next step is the
application program "Transmission lines for Windows" (TLW) that is
distributed for free with the ARRL Antenne
Book (starting from 20th edition). Using this program you may calculate
cable loss for you particular transmission line / length together with the
transformed antenna impedance at the other end of the line. These latter
values you need for calculating tuner characteristics. The free application
program TLD may provide the
same information. - Again using TLW
you may directly calculate all tuner components, together with component
losses and stress. This information unfortunately is not provided by the TLD
application. Repeat these steps for all frequencies you intent using to
obtain an overview of minimum and maximum values of all tuner components.
This actually is all that is needed for your tuner design. - As a last step,
you again read all balun info on this web-site to get
a good idea what type of balun(s) to design for your particular antenna
system, either at the antenna or at the input or output of the tuner. When designing your
tuner or evaluating a design keep in mind that all components in this tuner
will dissipate (some) power. This power you prefer to keep low, so your first
aim is to use as little components as possible. As a consequence you may
regard complicated tuner designs with some mistrust, especially when
advertised as 'wonder machines'. Not only "less is more", but it
will also keep unnecessary heat out of the shack. Bob J. van Donselaar, on9cvd@veron.nl |
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