Peter Miles
Northam, Western Australia QRZ Page: VK6YSF
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Media
T-MATCH ATU
The T-Network Antenna Matching Unit was
constructed for use with my all-band HF dipole with the added feature of
switching various antenna and radio combinations . Install
July 2012.
Due to
the operational limitations of my Z-Match unit that had been in service for
number of years an alternate antenna system matching was required. The
Z-Match had shown to be problematic on a number of bands with particular
antenna configurations that manifested as arching across the variable
capacitors. While the Z-Match design is sound the availability of variable
capacitors that suit the Z-Match circuit configuration that had sufficient
plate spacing to deal with the voltages that can be generated across them
are very difficult to source. As it turns out I had accumulated most of
the key components over the years to construct a T-Match antenna matching
unit capable withstanding very high power operation, well within the
Australian power limitation of 400 Watts.
The T
Match antenna matching unit is a very accommodating circuit that is well
capable of matching almost any antenna and feed line combination impedance
that the radio amateur is like to confront.
With
the need to match a wide range of complex antenna loads between 1.8 and 30Mhz
at moderate power levels it was decided that a simple T Match network was
the most suited to the task and that all key components had been collected
over the years.
While
the T Match has a wide matching range the major draw back with this design
is that it can act as a high pass filter and therefore offer little
suppression of transmitter harmonics. While this would be a good
additional feature it is regarded generally as not necessary for modern
radios due the very high emission standards imposed on transmitting
equipment these days.
Theory
The
T-Network Antenna Matching Unit as do other types of Antenna Matching
units match a
transceiver with a fixed impedance of typically 50 ohms to the feed line
and antenna combination load impedance.The feed line and antenna impedance for the most part is unknown
and complex often with significant reactance.
To
achieve an ideal match the ATU brings the feed line and antenna
combination into resonance there by removing the systems reactance and
then correctly loads the transceiver to allow maximum power transfer.
Construction
This
has been a long term project that has had the key components such as 500pf
variable capacitors and a nice roller inductor procured at various
ham-fests over the years. The variable capacitors with 3mm plate spacings
are well capable of withstanding voltages of several kilo volts. Both
capacitors are adjustable by vernier planetary 6:1 reduction drives to
achieve very fine value adjustment. The roller inductor is adjusted by a
rotation counter crank handle.
Figure
1Schematic of the
T-Network Antenna Matching Unit including switching feature.
C1
50 - 500pf Variable
Capacitor
S1
Ceramic
rotary switch. 1P3T. Radio selection
C2
50 - 500pf Variable
Capacitor
S2
Ceramic
rotary switch. 1P3T. Antenna selection
L1
0 - 19uH Roller Inductor
Two
additional switches have been included in the circuit. The first is rotary
switch to select one of three radios and another rotary switch to select
one of three antennas.
No
balun has been included in the T-Match tuner as it is viewed that an
outboard balun gives the flexibility of trialling various baluns. Operationally
some balun ratios may be more suitable in matching more extreme impedances
presented on some band antenna combinations.
The
T-Match tuner has been assembled on a aluminium base as part of an aluminium
enclosure designed specifically to house matched component layout.
The
components were all scale drawn in a CAD (Computer Aided Drawing) package
to determine the ideal physical positioning to achieve the least possible
component interaction, layout practicality and control functionality.
While not essential for the construction of the T-Match tuner the CAD
approach allows for various component configurations to be precisely
positioned and accessed for practicality with accurate construction
measurements determined from the final layout.
Figure
2CAD (Computer Aided Drawing)
to determine the ideal physical layout and control functionality.
Photo 1T-Network Antenna Matching Unit roller
inductor and tuning capacitor fitted.
Photo 2Front and top view of the
T-Network Antenna Matching Unit.
Photo 3Side view of the
T-Network Antenna Matching Unit.
Photo 4Rear view of the
T-Network Antenna Matching Unit.
Photo 5Top view of the
T-Network Antenna Matching Unit.
With the key components fitted the values of the Variable
Capacitors and Roller Inductor were measures with the AIM 4170C
antenna analyser to plot
of relationship between the tuneable components values and the front dial
indications.
Figure
3Plot
of relationship between variable capacitor values and front dial
indication.
Figure
4Plot
of relationship between roller inductor values and front dial counter
indication.
Operation.
Tuning
technique
1/
Adjust 'C
Load' to near
maximum capacitance.
2/
Set 'C
Source' to the half
value scale.
3/
Adjust the roller inductor
for a SWR dip. This dip may be barely noticeable.
4/
Slightly increase or
decrease 'C Source' and readjust the roller inductor
for a SWR dip.
5/
If the SWR is
lower than it was in step 3, slightly vary 'C Source' in the same
direction as in sep 4.
If the SWR is higher than before,
adjust 'C Source' in the opposite direction to that taken in step
4. Alternately, inch 'C Source' in the sep 4 direction and redip
the SWR with the inductor until you obtain an SWR near 1:1
6/
When you've almost
reached the match point, the SWR may start to go up as you adjust 'C
Source', but make the change anyway and redip the SWR with the
inductance.
7/
Continue to adjust
'C Source' in the same direction until adjusting the inductor produces
a higher SWR than before. Inch 'C Source' back to the previous
setting.
8/
If you cannot obtain a 1:1
SWR, reduce 'C Load' and repeat the process, beginning with sep 2.
If you cannot acceptably minimize the SWR at some settings of 'C
Load' then the antenna impedance is out of the range of the tuner.
The above matching technique
described by Andrew S. Griffith
W4ULD
in his article Getting
the most out of your T-Match Network Antenna Tuner has always produced
excellent results with my T-Match Network and antenna system.
Video
of the T-Network Antenna Matching Unit, features, construction
and operation.
This
is a simulator for the popular, series capacitor, parallel inductor
T-network tuners. The three knobs across the bottom adjust the three
components.