Q-Mac TA-90 User manual

No part of this manual may be copied, transcribed, translated or reproduced in any manner or form
whatsoever, for commercial purposes, without obtaining prior written permission from Q-MAC
Electronics Pty Ltd. However, limited copying is permitted for private use providing authorship is
acknowledged.
© Copyright Q-MAC Electronics Pty Ltd, 2003.
142 Hasler Road
Osborne Park, WA 6017
PO Box 1334
Osborne Park Business Centre, WA 6916
AUSTRALIA
Australia: Phone [phone], Fax [phone]
International: Phone [phone], Fax [phone]
TA-90
Automatic Antenna Tuner for Vehicle Whip
First Edition
Print date: December 1997
Author: Rod Macduff
Literature Reference Number: TECH04.PUB
Part Number: QM1021

TABLE OF CONTENTS
Section 1 Introduction ...................................................................................................... 2
Section 2 Product specification.................................................................................... 3
Section 3 Mechanical assembly.................................................................................... 4
Section 4 Functional overview....................................................................................... 5
Section 5 Block diagram.................................................................................................. 7
Section 6 Circuit description and Schematic Diagrams ....................................... 9
Section 7 Fault finding.................................................................................................... 14
Section 8 Hints and tips................................................................................................. 15
Section 9 Parts list........................................................................................................... 16
Section 10 PCB overlays.................................................................................................. 19
Section 11 Installation Guide.......................................................................................... 21
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1. INTRODUCTION
The TA-90 Automatic Tuning Antenna System is designed specifically for use with the Q-
MAC HF-90 Transceiver allowing it to deliver maximum power to a range of short whip or
wire antennas. It is optimised for electrically short antennas and is normally used in vehicle
installations with the Q-MAC QM7131 continuously loaded 1.8 metre whip.
The HF-90 is significantly different from other Autotune Antenna Systems in the following
respects.
1The TA-90 allows a variety of mounting configurations as the antenna and tuner are
separate. On a vehicle the tuner can be mounted behind the bull-bar affording
considerable protection.
2The system is more economic to repair in the event of a vehicle rollover or antenna
decapitation as the whip is relatively inexpensive and readily available.
3Road visibility is not impaired by the tuner when fitted to a vehicle.
4The TA-90 is versatile. It can be used on trucks, buses, small vessels and aircraft or
helicopters (24volt version). Wire antennas may be used.
5Efficiency is comparatively good when operating into short whips on low frequencies.
Other autotuners are either unable to tune for these conditions or achieve poor
efficiency. Improved efficiency is due to the special low-capacitance switching
design.
6Antenna current and power transfer to the antenna are maximised. This ensures the
best radiation efficiency when compared to VSWR based tuning systems.
7The design uses Surface Mount Technology on a single PCB with toroidal inductors.
This ensures maximum reliability in a compact unit. It also minimises cost.
8The TA-90 autotuner box mounted on the stoneguard plate weighs only 2.2 kg.
-2 -

2. PRODUCT SPECIFICATION
Application:-Vehicle, Fixed, Marine or Aircraft used with HF-90 Transceiver.
Frequency range:-2 to 20 MHz continuous
Power rating:-70 watt PEP
Input impedance:-50 Ohm
VSWR:-Typically 1.6:1, Maximum 2.5:1 (antenna current always maximised).
Current drain:-500mA typical
Voltage:-10-15volt, from auxiliary connector on rear of HF-90 ( a 20-30volt model
is also available).
Tune time:-Less than 3 seconds.
Tuning method:-Binary weighted relay switched inductors perform tune, whilst transformer
achieves match.
Dimensions:-Tuner housing:-221 (L) x 146 (W) x 62 (D)
Tuner with mounting plate:-242 (L) x 206 (W) x 59 (D)
Whip Antenna:-1.8m (L)
Whip antenna with spring and insulator:-2m (L)
Weight:-TA-90 tuner box on plate 2.2kg.
Antenna mounting:-1/2” BSW screw
Environment rating:-IP67 (fully waterproof, limited immersion).
Antenna:-Continuously loaded 1.8 metre heavy duty CB type whip. Alternatively
may be used with wire antenna (up to 10 metres) or Marine HF whip.
Specifications are subject to change without notice
-3 -

The TA-90 tuner box comprises a single 2.4mm thick PCB which contains all the active
electronics mounted within a waterproof, aluminium diecast box with 6mm polymer lid.
The PCB is mounted in the box on four 20mm pillars using M3 screws.
DC power and input RF are taken to the PCB via a 10 way IDC connector.
The high voltage antenna feed is via a soldered connection to the PCB.
External RF input to the TA-90 is applied on a standard UHF S0-239 socket.
DC power from the HF-90 transceiver is taken to two 5/16 inch circular lugs. (this is also the
tune activation signal).
A 4mm high voltage binding post allows feeding of whip or wire antennas.
All connectors are protected by a heavy duty aluminium plate (stone guard) which has
mounting holes for U-clamp fixing to bull-bars.
The TA-90 can be mounted in any orientation but is best configured with the connectors to
the side and with the antenna feed topmost (see installation guide).
3. MECHANICAL ASSEMBLY
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The function of the TA-90 autotuner is to allow maximum power transfer between a Q-MAC
HF-90 transceiver and the 1.8m helical whip or a wire antenna.
The 1.8m continuously loaded whip when mounted on a vehicle has an equivalent circuit as
shown in Figure 1.
Figure 1 Equivalent circuit of 1.8 metre CB whip.
Below 8MHz the effect of the 1µH inductor and 440R resistor is negligible and the whip is
equivalent to 47pF in series with 6R. This is a highly reactive load.
In the frequency range 8-15MHz the L-R parallel circuit starts to raise the resistance and
cancel the reactance. At about 20MHz the LC elements are rear resonance and the reactive
value is very low and resistance is approximately 40 ohms.
In order to transfer maximum power to the above network the antenna tuner must:
Cancel the capacitive reactance.
-jXc = 1,000,000 (f in MHz)
2 x PI x 47 x f
Match the 50 ohm feed impedance of the HF-90 to the lower resistance left when the
reactance is cancelled. This resistance lies in the range 6-40 ohms.
Figure 2. Equivalent Circuit for TA-90.
The TA-90 contains a series of binary weighted toroidal inductors which are switched by
relays under control of the onboard microprocessor. These series inductors are adjusted to
exactly cancel out the capacitive reactance presented by the electrically short antenna. There
is also a 2:1 turns ratio step down transformer which is used on the lower frequency range to
obtain the requisite impedance match.
4. FUNCTIONAL OVERVIEW
-5 -

The TA-90 also incorporates a high to low impedance matching amplifier which is selected
to provide good receiver signal matching when the unit is untuned or the transceiver is in
scan mode. Tuning is performed purely by measuring the antenna current and by hill
climbing until a maximum value is obtained. This technique is superior to minimising
VSWR as it optimises the radiated power. Antenna current is measured using an improved
proprietary galvanic isolation technique whereby a current transformer excites a light
emitting diode coupled optically to a photo transistor via an optical fibre. The
phototransistor is connected to an A to D convertor via a dynamic range extension network.
The A to D convertor comprises an up-counter, D to A converter and comparator. A straight
up count is used (proceeding from high antenna current down to nil antenna current due to
inversion) because a successive approximation algorithm would give wildly erroneous codes
if the RF level changed slightly during the count. The counter is reset by a clear pulse from
the microprocessor and then the micro issues clock pulses to the counter until the comparator
goes low.
Circuit Operation
In the TA-90 the tuning sequence is initiated by a supply interruption of 400ms.
On turn-on all inductors are switched out and the matching transformer is switched out.
The TA-90 then goes through its tuning sequence.
The scan amplifier has RF protection. If RF power is applied when the scan amplifier is active thyristor
TH1 triggers and switches out the scan amplifier.
On the HF-90 transceiver when the TUNE button is pressed or on the first occasion when PTT is pushed
on a new channel, the +12volt auxiliary supply to the TA-90 is interrupted for 400ms. Following this, a
CW signal of three seconds duration is produced at approximately 20watt power level.
During this three second period, the TA-90 microprocessor monitors the antenna current and determines
the optimum inductor and transformer combination.
The analog to digital converter is a single ramp type. The measurement cycle is as follows:
First RST pin 11 on U9 is taken high momentarily, then counter U9 is incremented by pulsing pin 10 of
U9. This continues until the output of comparator U8:B pin 1 changes state on reaching the actual
current value level.
The resultant ramp can be seen on R27. The ramp proceeds from a low to high voltage with low
corresponding to high antenna current and low corresponding to low antenna voltage.
One measurement of antenna current is made for each relay setting chosen by the convergent tuning
algorithm.
-6 -

The RF chassis of the TA-90 auto-tuner comprises a selectable 2:1 matching transformer or 47pF series
capacitor, a selectable impedance matching receive preamplifier, a series of fifteen binary weighted
relay switched toroidal inductors and a current sensing transformer at the antenna output.
The control section of the TA-90 comprises of a sensor section, microcontroller section and output drive
section.
The sensor section consists of an optically linked antenna current sensor, dynamic range extendor and
analog to digital converter with -10volt charge pump.
A microprocessor with high order address latch and EEPROM are the programmed control element in
the circuit.
Output to the relays is effected via three darlington driver blocks.
A +5volt regulator supplies power to the PCB.
5. BLOCK DIAGRAM
-7 -

6. CIRCUIT DESCRIPTION + SCHEMATIC
DIAGRAMS
RF Section
The RF section of the TA-90 comprises fifteen separate relay switched toroidal inductors. These
inductors are approximately binary weighted. This means that up to 2 ↑15 inductance combinations are
possible with a minimum inductance of 144nH and a maximum inductance of 100µH.
Use of toroidal inductors ensures minimum coupling between adjacent coils and eliminates “suck outs”
or tuning holes.
All inductors have relays arranged to isolate them in bypass mode. This minimises the stray capacitance
and maximises the efficiency. The top four inductors in particular use a pair of relays resulting in
further capacitance reduction and improved high voltage handling.
A 2:1 turns ratio matching transformer T1 switched by relay RL1 steps down to 12 ohm allowing
effective matching in the 2-15MHz range when using short whip antennas. When the transformer is
switched out by RL1 a low loss 47pF capacitor C67 is switched in series with the RF input to allow the
tuning of antennas which are inductive.
A scan amplifier comprising JFETs Q2 and Q3 with associated passive components and protection
diodes D9-D14 provides impedance matching and gain in receive mode when the antenna has not been
tuned and when the HF-90 is scanning.
Antenna current is monitored using current transformer T2 and optical emitting diode D8. Galvanic
isolation is achieved by converting the current level to a light level and passing this down an optical
fibre. Optotransistor Q1 then converts the light signal back to a current level.
As the light level spans several orders of magnitude and it is necessary to detect very small currents and
yet be able to see change in very large currents then a dynamic range extender is necessary.
This network of D6, D7 and D15 with associated resistors R28, R29, R30 and R71 presents a very high
value of load (100K) for small currents and a very low value of load (1K) for high currents. In this way
the dynamic range is extended.
(see schematic diagram opposite)
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