The NB6M Miniboots
By Wayne McFee NB6M
Copyright 2000
Requests for an “outboard” version of the RF power amplifier used in
the 5 Watt Mod for the SMK-1 led me to further research and experimentation
which resulted in the amplifier circuit described in my article “A Mosfet QRP
Gallon”, published in the Fall, 2000 issue of QRPp, the journal of the NorCal
QRP Club. The RF amplifier described utilized the cheap, readily available
IRF510 Mosfet to produce 5 Watts of output with an input of from 1 to 1.5
Watts and was switched into and out of the antenna line from the QRP rig by a
DPDT toggle switch.
Since that article was published, I have added several improvements to that
basic RF Amplifier circuit. I call the new amplifier the “NB6M Miniboots”.
For those who are comfortable with building “Ugly Style” from a simple circuit,
this project is a snap and the parts are all readily available. For those who
would like the parts already gathered and the layout prepared for them, a kit
is in the offing.
40/20 Meter version shown, with attenuator input for 1 to 2 Watts drive
40 Meter version shown, with transformer input for 3/4 to 1 Watt drive
This RF Power Amplifier operates from a 12 Volt supply, can be set up to
accept an input of from about ¾ Watt to 2 Watts, and can deliver from 10 to
14 Watts of output into a 50 Ohm load. Tests thus far have been very
satisfactory on the 80 through 20 Meter ham bands, with output typically 14
Watts or more on 80 and 40, a bit lower on 30, and around 10 Watts on 20.
Initial tests on the higher frequency bands indicate that some circuit changes
may be necessary in order to provide for operation of the “Miniboots” on 17
through 10 Meters. Further tests are in progress.
Just as Rick Campbell, KK7B, called his small, high performance CW transceiver
the GQRP Rig, or Generic QRP Rig because it incorporated ideas from previous
generations of homebrew artists, this amplifier circuit could be called the
GPA, or Generic Power Amplifier because it makes use of ideas from contributors
like Michael Masterson, KA2HZA, Mike Kossor, WA2EBY, and from such all-time
greats as Wes Hayward, W7ZOI, Roy Lewallen, W7EL, and Doug
DeMaw, W1FB.
The additions to the basic QRP Gallon circuit include an RF-sensing relay
driver circuit to switch the amp into and out of the antenna line automatically,
output network filter values to allow for up to fourteen Watts of output with a
two watt input, and a drive level adjustment potentiometer which allows for
setting the amplifier at exactly the 5 Watt “QRP Gallon” level as desired.
Mike Gipe, K1MG, performed spectrum analysis tests on a 40 Meter version of
the “Miniboots”, which showed the output of the amplifier to be quite clean at
all levels of drive and output.
On-the-air tests of the “Miniboots”, involving many QSOs on the 40 and 20 Meter
ham bands, using an SW-40 and an SW-20+ as the driving QRP rigs, have resulted
in signal reports such as “nice sounding rig”, “very clean sounding rig”, and
“your rig sounds great”. Monitored on another receiver, the output CW note is
pure, and the keying very clean.
For a driving power level of from one to two watts, the amplifier utilizes a
resistive 3db attenuator at the input, in order both to provide a 50 Ohm load
for the driving rig and to prevent over-driving the amp. If the driving QRP
rig has an output of three-quarters to one watt, a broadband transformer input
is used which provides close to a 50 Ohm load to the driving QRP rig without
attenuation.
A simple DPDT toggle switch allows for two-band operation. A multiple-position
rotary switch could be used to select output networks for multiple-band
operation, if desired. The addition of an RF-sensing relay driver circuit
provides for hands-free T/R switching. The timing of the relay driver circuit
is such that there is full QSK for the slower CW speeds and semi-QSK for the
medium to fast CW speeds. The relay used pulls in quickly enough that, even
at the faster CW speeds, a single “dit” is not clipped short.
Although the relay specified in the circuit diagrams is Mouser part #
431-OVR-SH-212L, a 12 Volt, DPDT relay with a 900 Ohm coil, a Radio Shack 12
Volt DPDT relay with a 200 Ohm coil, Radio Shack part # 275-249A, has been used
very successfully in this circuit.
Thus far, several versions of the Miniboots have been built and tested, by the
author and by Richard Fisher, KI6SN, with similar, positive results. All
versions have been built “Ugly” style, as there are a relatively small number
of parts in the amplifier circuit, and, building the amplifier over a solid
ground plane, with short leads, improves stability and helps prevent spurious
output. Thus far, there has been no indication of instability in any of the
prototypes built.
None of the versions of the “Miniboots” built thus far have been laid out
exactly the same, due to the “ugly” method of construction. However, that
means that the individual builder can plan the layout so as to utilize any
of a variety of enclosures. One possible layout is suggested in the attached
drawing.
A parts list and two circuits diagrams are provided, one with the resistive
attenuator input to accept one to two Watts of drive, and another with the
broadband transformer input to accept a drive level of three-quarter to one
Watt.
If the broadband transformer input is used, insert a QRP SWR meter in the line
between the driving rig and the amplifier and check the reflected power back
to the rig. My antenna analyzer shows the amplifier with the transformer input
to have an SWR of 1.3 to 1 or less from 1.8 Mhz up through 10 Mhz. At 14 Mhz,
the SWR is 1.7 to 1. The number of turns on the primary of the broadband
transformer can be adjusted as necessary in order to provide a better match.
The IRF510 will require heat sinking, and the Drain of the Mosfet must be
insulated from ground. It is a good idea to use an Ohm-Meter to check for
shorts from the Drain to ground before supplying DC power to the amplifier.
The Miniboots could easily be built into an Altoids tin, if desired, making
for a very small package.
The drive level adjustment pot can be either a trimpot, with a screwdriver
access hole cut in the enclosure used, or can be a front panel mounted control
with a knob. The choice is yours. Whichever style pot is used, the input
circuit should be laid out so that short leads are used to connect the pot to
the gate circuitry of the Mosfet. Also, if considerable operation is planned
at Milliwatt levels, the pot should have a high enough wattage rating to be
able to absorb its share of the RF input to the amplifier.
Two component value charts are provided for the output filter network.
Table 1 has component values for ten to fourteen Watts of output.
TABLE 1
Table 2 has component values for a nominal 5 Watt output, should the builder
desire to build the amplifier for use only up to the QRP Gallon level.
TABLE 2
At this point, component values are provided for the 17 Meter through
10 Meter ham bands for information only, as initial tests indicate that
some circuit modification may be necessary to allow for satisfactory
operation on the higher frequency bands. As stated, further testing is in
progress.
An automatically switched, outboard RF Power Amplifier which allows for
operation at power levels from Milliwatts to several Watts is a very useful
addition to the QRP ham shack. Whether you decide to build the “NB6M Miniboots”
for operation only at true QRP output levels, or for optional operation at low
QRO levels as well, I am sure you will be more than satisfied with the results.
Enjoy!
MiniBoots Parts List
.1 uf 4
.01 uf 1
.001 uf 2
2.2 uf Electrolytic 1
100 uf Electrolytic 1
100 Ohm trimpot or panel mounted pot, as desired 1
33 Ohm, ¼ Watt 1
2.7 Kohm, ¼ Watt 1
10 Ohm, ¼ Watt 1
1 KOhm, ¼ Watt 1
4.7 KOhm, ¼ Watt 1
12 Volt, 1 Watt Zener Diode 1
1N914 (or 1N4148) 2
1N4004 Silicon Diode 1
2N3904 General Purpose Transistor (2N4401, etc) 1
IRF510 Mosfet 1
RF Choke, FT37-43 with 5 Turns # 22 1
12 Volt, DPDT Relay
Mouser # 431-OVR-SH-212L or Radio Shack RS275-249A 1
RF Jacks, BNC Type 2
Power Connector 1
For resistive attenuator input
10 Ohm, 1 Watt 2
150 Ohm, 1 Watt 1
For broadband transformer input
FT37-43, Primary 6 Turns # 24, Secondary 4 Turns # 24 1
If two-band operation is desired, add
DPDT Toggle Switch 1
If multiple-band operation is desired, add
Rotary switch, dual contact, multiple-position
as desired
For output filter component values,
see Tables 1 and 2
Enclosure as desired.
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