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450 watt 6m
Amplifier
A Description
The amplifier uses transistors that have been designed for VHF
SSB operation. The 'HF' aspect is important because, unlike VHF
devices, they are considerably more stable and do not have that
disastrous tendency to go into VHF' parasitic oscillations A
possible downside of this is that they may not have less gain at
50MIlz compared to a VHF transistor but, as can be seen in Figure
1, at least 12 to 14dB of gain can be obtained at 50MIlz (the
BLW96 performs well at 70MHz as well incidentally). The 'SSB'
aspect is important in that the transistor has been designed for
linear use and is sold with known inter-modulation performance,
unlike most VHF devices that have been designed for FM use. 'The
performance of these VHF devices in linear use is questionable,
especially so if driven to their power limit.
It is oft quoted that transistor amplifiers have worse IMD
performance that valve amplifiers; this is nonsense. The Philips
BLW96 is rather an old device by modern standards, but it can be
seen in Figure 2 that at 200 - 250 rd watts output it has an 3
order IMD of better than -40dB. This compares very well against
any well designed valve amplifier. (It is interesting to note
that in common with most transistors, the 3rd order IMD increases
as power is reduced, so don't assume that the best performance is
achieved by considerably under driving!).
To achieve the required output in this design, two BLW96s have
been paralleled to provide 450 watts with just a little over 20
watts of drive. The input power is split using a Wilkinson power
divider. The output of the two amplifiers are combined using the same,
only reversed, circuit. Under normal circumstances the 100 ohm
resistor connecting the two output (or input) ports does not
dissipate very much power, but during tune-up it is possible that
it could. As it is important that these resistors are truly
non-inductive, the design uses rather special devices. These are
'bolt-down' 100 Ohm terminating resistors with two flying leads
to connect to the circuit board - they look rather like
transistors in construction.
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Figure 3 - The
Circuit Diagram
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As shown in Figure 3, the two amplifiers use quite
conventional low-pass L-matching circuits to match to the 50 Ohm
inputs and outputs of the Wilkinson power splitter/combiner. 'the
design of these circuits is beyond the scope of this article has
been covered elsewhere. What I would like to concentrate on is
physical design issues that need to be taken into consideration
when building an amplifier of this power. Figure 4 shows the PCB
layout at half full size. To reproduce, just pass it through a
photocopier on x2 Figure 4 -PCB at 1/2 Full Size magnification.
The length of the board should be 27 cm. The PCB uses standard
double-sided copperclad board. The cross-hatched areas are cut
out holes in the PCB to allow the BLW96s, bias transistors,
thermostat, and the power splitter resistors to be bolted to the
underlying heatsink. The black areas are not etched but cut out
using a sharp knife. and a steel ruler. These gaps provide
isolation between the pads and the ground plane Bearing in mind
the high voltages in the output circuitry especially, make sure
they are not less than 2mm wide. A flashover... well I do not
need this further! The PCB is mounted on the heatsink as shown in
Figure 5. The BLW96 is bolted to the heatsink using 6BA screws
and heatsink compound. In this design especially make sure that
the transistors are firmly bolted to the heatsink, and that there
are no chamfers on the bolt holes. Going from zero to I kilowatt
in less than a microsecond presents a rather high thermal and
physical stress to the devices!
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The bottom of the
printed circuit board
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The top of the
printed circuit board
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Special Design Considerations
The underside of the PCB is left unetched but must be
electrically connected to the upper side ground plane over the
full extent of the board. This is achieved by drilling ]mm holes
about 10mm apart around all the pads carrying RF power. Short
pieces of wire are through these passed through the holes and
soldered to both sides of the board (Figure 7). It is especially
important to put many of these links around and under the
emitters of the BLW96s. The second unusual design technique is
shown in Figure 7. The upper and lower ground planes are also
shorted together all around the edge of the board. The best way
of achieving this is to strip the braid from short lengths of
UR-58 coaxial cable and fold this over the edge of the board as
shown in Figure 8. Because of the high RF currents circulating in
the amplifiers, especially in the output combiner, it is
imperative that the correct (or equivalent) fixed and tuning
capacitors are used. The ones quoted are low-loss types and do
not heat up at all at the power levels encountered in this
design. The trimmers are 57pf, Phillips foil trimmers type:
200-80908003 and the 47pf chip capacitors are 300V ATC
transmitting types. These look like a small 3mm on-a-side cube.
The PCB is mounted within a large Marston heatsink (available
from Famell Components) which is ideally shaped for use as an
amplifier (figure 9). The heatsink has two side skirts which
provide two sides of the enclosure. The heatsink has a thermal
resistance of 0.4o/watt, but as the amplifier dissipates 500
watts at maximum power a fan is needed to keep the heatsink cool.
The fan is thermostatically controlled by a small 40o closing
thermostatic switch (Marston part #147-092) placed in the centre
of the PCB and bolted to the heatsink.
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Mounting the
Transistors
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The thermostat controls a small 60mm +48 volt fan which is
mounted in Marston heatsink 300mm long, 60DN03000A100, Famell
Electronics P148-129 an aluminium enclosure on one end of the
heatsink (Figure 10). To force the air through the fins the top
of the fins are covered by a piece of aluminium. Two pieces of 10
gauge aluminium sheet are screwed to the ends of the heatsink and
an aluminium lid is screwed to the bottom thus fully enclosing
the amplifier.
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Figure 7 - Wire
shorting links
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Figure 8 - Edge
shorting links
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The input to the amplifier is via a chassis mounting BNC
socket at one end of the housing, while the output is taken via a
N-type socket mounted at the other end. Power, 4 50V @ 20A, + 12V
for the fan, and switched +50V for the two bias generators are
fed in by multi-pole socket. Because of the high RF field inside
of the case, all power supply leads that are greater than 10cm
long are screened. In the case of the main +50v supply wires,
these are screened using 5mm diameter solid brass tube by feeding
the multi-strand cable down the centre as shown in Figure 10. A
benefit of using brass tube is that the supply wires are held
rigidly and neatly in place.
Figure 11 shows the ancilliary circuits for the amplifier. The
two bias circuits are driven from a switched +50V supply. The
four transistors are bolted to the heatsink as shown in Figure 6.
The other components of the bias circuit are mounted on the PCB
above, and beside, the transistors. The two 5-watt 100 Ohm
resistors are placed against the side wall of the heatsink and
held in place with an aluminium bracket. All of the amplifier
ancilliary circuitry is be built into a separate cabinet which is
connected to the amplifier by a short umbilical cable.
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Figure 9 -
Heatsink Cross-section
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It is recommended that each amplifier is initially tuned
individually by disconnecting each in turn from the Wilkinson
splitter and combiner and connecting to a 50 Ohm dummy load. Once
each amplifier is working correctly, they should be reconnected
to the input/output circuitry and final adjustments made. Be very
careful at this stage and or) not detune too much. Just move the
trimming capacitor a little bit at a time to achieve the required
matching. It is all too easy to seriously detune an output
circuit which could cause an explosion. Once aligned, and used
with a suitable ALC circuit to prevent overload, the amplifier
should provide excellent service with IMD performance at as good,
or better, than a valve based design. It is interesting to note
that I have never seen a need to use phased switching of the
power supplies and changeover relays to prevent hot switching.
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Figure
10 - Fan housing assembly
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Figure
11 - Screening the 50V supply cables
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Figure 11 -
Amplifier Ancilliary Circuitry
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20amp +50V power supply
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| A regular circuit
that can be used to create a +50V @ 20 amp power supply. |
A Long-wire Aerial for Six Metres
Having had the time to evaluate it, I find this aerial very
useful for both semi-local and sporadic-E on six metres.
The antenna is about 65 feet of ordinary aerial wire, insulated at the
far end. The near end connects to a matching unit situated in the loft of the house. The
aerial wire enters the loft under a tile near the ridge of the roof of the house, through
an insulating sleeve.
 The theoretical diagram.
The aerial is voltage-fed (high impedance) and the antenna matching
unit (AMU) transforms this to match 50 ohm cable. The two trimmers are ‘polycon’
variables - these are OK up to 50 watts. For operation at higher powers, obviously more
substantial trimmers, such as mica compression types, will be needed. Old radio receivers
of the 1950 vintage or before are often a source of these types of trimmer.
‘Cirkit’ can supply them, but they are not cheap! The trimmers are likely to be
the only ‘expensive’ parts required for this antenna, however.
The method of adjustment is to set C2 to half capacity and then adjust
C1 for best noise on receive. Then feed in a carrier (or an audio tone on SSB) and adjust
C1 for a low SWR reading. 1:1 is achievable at this QTH.
No problems with ‘RF in the shack’ have been encountered at
50 watts SSB. The counterpoise were effectively ‘earths’ the bottom end of the
parallel-tuned circuit; it hangs from the AMU in the loft. It is 4ft 6in in length.
The AMU is constructed on a tobacco-tin lid. The coil is nine turns of
18 SWG copper wire taken from old coax cable. It is 3/8 inch in diameter, spaced over
approximately ¾ inch.
Increasing C2 too much will prevent the tuned circuit from resonating
at 50MHz. This, in turn, depends on the capacitance of the whole aerial wire to earth -
but no problems were found and it was easy to get unity SWR. Shorting out C2 might be
acceptable if a low-capacity system can be constructed, but this did not work at my QTH.
The wire gives a gain of 4 dB in four directions at 35 degrees to the
run of the wire. There are also several smaller lobes which are useful. This is a good
‘listening’ antenna with gain that can be arranged in desired
directions.
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