Antenna Tuning Unit (ATU) using the T-Network approach
By Chris Arthur VK3CAE
About a decade ago I built myself an ATU
similar to the Ultimate Match as described in the ARRL Antenna Handbook, here’s
how I went about this project..
At the time I’d been using a Z-Match unit as found in RSGB publications, however the Z-match does have some limitations and I needed a more versatile unit, the Ultimate Match looked like the answer. I had already collected a nice roller inductor and some Johnson make high voltage variable capacitors, the ceramic insulator type. The problem was that both of these capacitors were of mono stator construction and a split stator was required for the input capacitor of the original ultimate match and the output capacitors of the later SPC match, both shown in fig 1.
fig 1 From the Station Accessories section
of the ARRL Handbook
The later Series Parallel Network or SPC was
developed by W1FB to correct for poor harmonic attenuation of the
original circuit when used under extreme mismatch conditions. After talking to
one of the MDRC old timers (Bill VK3ATW Silent Key) I decided that this would
not be a problem at power levels of 100 Watts or less and opted for the simple
T-Network approach. A T-Network would have an almost identical matching range
to that of the Ultimate match making a split stator variable capacitor
un-necessary.
ATU Component Considerations
Prior to souring our ATU bits we should consider a few things like, what power will I be using? Will my ATU be used under severe mismatch conditions and do I need a balun for open wire feeders? These are all important factors as the unit may be exposed to quite high RF voltages or currents and you don’t want your ATU suffering from flash over or heating problems.
Variable Capacitors
Most manufacturers of air gap type capacitors
(variable or other) use a de-rated breakdown figure for the safe WVDC level.
The actual break down Voltage of dry air is around 3x10E6 Volts per Metre, or 3000
V / mm. So a variable Capacitor with 1 mm plate spacing could theoretically
take up to 3000 volts DC or Peak to Peak AC before arcing would occur, in
reality this figure is somewhat less. Things like the mechanical construction
of a capacitor and the relative humidity of air can have a profound effect on
brake over Voltage ratings, therefore a figure of 1000 V / mm is more realistic.
This break-over voltage is one of the factors that determines
the ATU's safe operating power and in-turn limits its impedance matching ratio.
On recalling Ohms Law, we know that for a fixed power level a higher resistance value will cause a higher Voltage. In a 50 Ohm system we would have around 200 Volts Peak to Peak for the nominal 100 Watts output from your average HF transceiver. However, for an impedance of 600 Ohms you would have around 700 V P-P at the same power level. So if you want your tuner to match over a wide range of complex impedance's then you should use around 1 kV rated variable capacitors for the 100 - 120 Watt level and 3 to 5 kV for the VK legal limit.
Variable Inductors:
Switched & Rotary
Inductors used in ATU applications will under
some conditions carry very heavy currents to ground, if the windings use a wire
of insufficient diameter there will be high Q losses and considerable heat
generated. A good rule of thumb would be "the heavier the inductor wire the
better" but then physical size and mechanical constraints may become a
problem. For most amateur type ATU applications a wire diameter of 2 mm
(No 12 AWG) would be fine, this would be suitable for powers up to the legal
limit. For low power to a maximum of 100 Watts you could get away with
using wire diameter of 1 mm but in the end one should always go for the
heaviest wire possible. Suitable values of inductance for use with the
circuits shown in this article would be in the range of 25 to 35uH. Inductors
can be of a fixed tapped type or a roller type, personally I find roller
inductors much easier to tune.
A roller inductor with 25 or more turns of 1.2 mm dia wire (No 17 AWG) on a 50 mm former would be nice for the 100 Watt level and would easily cover 160m through 10m operation. This is what I used in the T-Network described below but if you can't get your hands on a roller inductor then you can use a tapped and switched inductor as follows:
- 24 turns of 2 mm dia wire spaced at 6 TPI on a 50 mm dia former (air).
- Place taps (from non-earthed end) at 2 turns, 5 turns, 8 turns, 12 turns, 16
turns and 20 turns,
giving a total of six taps for switching to ground.
- A High Voltage ceramic insulated single pole multi-way switch is used to short the unused turns to ground.
In my test version this configuration gave
quite good results but the inter-connects between coil taps and switch block should
be kept as short as possible and made with heavy wire there-by reducing
additional inductance. I also found that a good match was difficult to obtain
under some load conditions when using 150 pF for C1. Use a 200 pF for C1 with a
switched type inductor, this should give some over-lap in each switch position.
The Simple T-Network type Antenna Tuning Unit
The basic T-Network ATU performs reasonably well
under a wide range of impedance mismatches and adjustment can be quite straight
forward if a roller inductor is employed for L. As mentioned previously the
only real problem with this T-Network circuit is that the ability to suppress
harmonics degrades somewhat under severe mismatch conditions. At powers of up
to 100 -120 Watts this is not really a problem but if you're planning on using
big power in suburbia with a wire antenna, then one should consider the SPC
match with its better harmonic suppression characteristics as an option.
All of the components used in constructing
my ATU were reclaimed from surplus type equipment found at amateur radio swap
meets and the like. Purchasing the Johnson make Variable Caps new here in VK
would set you back around hundred dollars each and then there's the roller
inductor, another $150, you can see why a good commercial ATU cost big dollars.
When laying out components you should allow a clearance of at least 15 mm in
order to minimise coupling effects. Interconnecting wires on caps and inductors
should be heavy enough to prevent inductive losses. I used the braid from RG-58
coax, when in place, solder was flowed through it to make a heavy rigid wire.
If you wish to add antenna or dummy load switching as
I did, then you must run 50 Ohm coax between all sockets and switches. The
coaxial braid must be earthed at any sockets and joined at all break points
around switch gear, this will minimise the introduction of Standing Waves when
switched through (ATU by-passed). A set of three wafers each having five way
switching (3 x single pole, 5 way) on a common shaft will allow selection of
two separate antennas (A or B) in a bypass or through ATU situation, plus a
spare contact for dummy load or off position.
If you follow the basic guidelines given here, you
should be able to build a useful ATU for your station at minimal cost.
This picture shows my ATU with the top panel removed, a pair of 50 Ohm dummy
load resistors were also removed to show the connector placement (right side
behind switch block).
This article is dedicated to Bill VK3ATW
Silent Key
E-mail VK3CAE: cja@rmit.edu.au