The Marathon - A low power
transmitter for 136 kHz
As is common on LF, I usually run high power on 136 kHz (400 watts). But,
in May 2001, I started wondering whether the 136 kHz band would be suitable for low power
operation (i.e. operating with a TX power output of just 5 watts). It certainly seemed
unlikely because of the poor antenna efficiency achievable at a wavelength of 2200 m, when
using an antenna just a few tens of metres in length.
But I decided to build a QRP TX for LF, and give it a try! The Marathon LF TX to
be described uses: a FET VFO; FET buffer; 2N2222 amplifier + BC212 keying transistor;
2SC2166 driver; and a pair of 2SC2166 transistors in parallel.
On the first morning of operation, I made three QSOs using this little TX. None of the
QSOs was pre-arranged. The first contact was with Tom G3OLB in Devon - 97 km away. I was
so excited that it was hard for me to maintain control of the morse key! But I managed to
rattle out Tom's 599 report, and was surprised and delighted to receive my 569 report from
Tom. After my QSO with Tom, I worked G3YXM (105 km) and G8IK (101 km) - all using 5 watts
RF to my simple 12 m vertical (no top loading).
One week later, I worked John G4CNN (Caversham, Berkshire) for my best QRP DX, at 122 km.
I believe that a simple low power transmitter for 136 kHz would make an excellent club
project, with the very real prospect of not just making LF QSOs across town - but over
significant distances too!
The 50 pF tuning capacitor provides a tuning range of 10 kHz - enough to cover the
135.7-137.8 kHz amateur band, and yet also act as a useful source when 'searching' for
antenna resonance, or measuring antenna bandwidth. The VFO drives a very effective FET
buffer which presents the VFO signal to the driver stages. (For improved voltage
stabilisation, see the section 'Updates & feedback
from other constructors' below.)
4.7 mH RF chokes work very well in this circuit. In the UK, suitable
RF chokes may be obtained from Maplin (light green body, 50 ohms DC resistance, of rather
poor construction which is prone to failure) and Sycom (brown body, 25 ohms DC resistance,
of robust construction).
The pre-driver stage uses an untuned 2N2222 common emitter amplifier,
keyed by a BC212 PNP transistor. In my experience, many QRP TX designs fail to provide
adequate shaping of the keying waveform. To reduce the likelihood of transmitting key
clicks, the keying circuit in the Marathon provides a rise time of 3 ms,
and a fall time of 6 ms. The result is a very pleasant T9 note.
The keyed signal is coupled via a 0.1 uF capacitor to the 2SC2166 driver
transistor. The gain of this stage is set by the 1200 ohm feedback resistor.
For some brands of 2SC2166, you may need to increase the gain of this stage by increasing
the feedback resistor to 4700 ohms. The drive level to the PA transistors is set by
adjusting the emitter resistor, Rx. If variable output power is required, a variable
resistor of 250 ohms may be used in place of Rx. The driver transformer, T1, is
wound on a 25 mm OD 3C85 ring core,
which has been found to provide excellent characteristics in LF transformer applications.
An alternative core material is 3C90.
The PA uses two 2SC2166 transistors connected in parallel. Rx in the
driver stage should be set so that, on key down, each PA transistor draws 600 mA. This can
be checked by measuring the voltage across one of the 1 ohm emitter resistors: 600 mV
corresponds to a current of 600 mA. To reduce the PA emitter current, increase the value
of Rx in the Driver stage. The gain of the PA is set by the 470 ohm resistor.
For some brands of 2SC2166, you may need to increase the gain of this stage by increasing
the 470 ohm resistor to 1200 ohms. The PA transistors will require a heatsink. Note
that the tab of the 2SC2166 is internally connected to the collector, so be sure to use an
The PA transformer, T2, uses a 25 mm OD 3C85
Because the VFO runs at the TX output frequency, some 'pulling' of the VFO frequency might
be expected on key down. The degree of VFO shift will depend upon output power, and
the magnetic coupling between the VFO coil and T2/L1/L2. This effect can be
minimised, through effective screening between the VFO and the PA; and single-point
earthing of all stages. The use of ring cores for L1 and L2 would also help (see
'Constructing the low pass filter inductors' below).
Those with lots of experience of building homemade rigs will have no trouble building the Marathon
into a smart equipment case.
Those with less experience would, perhaps, benefit by starting with a
simpler approach. A prototype version can be constructed quickly on copper clad board
using 'ugly construction' techniques. For such an approach, start by fixing some off-cuts
of copper clad board to a piece of wood or 'chipboard'. (Prior to fitting the copper-clad
board, be sure to remove any oxidation using metal polish, followed by a rinse with soap
and water.) Odd scraps of aluminium sheet, or lids from tobacco, confectionery or biscuit
tins can be used as the front and rear panels by screwing them to the front and back edges
of the chipboard. Start by getting the tuning capacitor; antenna sockets; TX/RX switch;
NET switch; 12 v connector; and PA transistors/heatsink mounted in convenient positions,
and in relation to their position in the circuit diagram.
The rest is easy: solder those components needing a connection to earth directly to the
copper-clad board; and solder the remaining components directly to one another. Don't
worry too much about lead lengths; but be sure to provide enough spacing between
components to prevent short circuits, and to allow voltage measurements to be made during
testing. To support larger components, high value resistors can be soldered to earth at
one end, and the other end to the component - a sort of poor man's 'insulated' terminal
post. The bird's nest in the picture below worked just fine!
For more information about ugly construction techniques, click here.
Constructing the low pass filter inductors
L1 and L2 are 55 uH inductors, wound on separate formers and mounted at 90
degrees to each other. For each coil former, I used a 50 mm length of 22 mm outside
diameter PVC pipe, purchased from a plumber's merchant. (Note that such pipe is often
identified by its internal diameter, rather than its outside diameter.)
The coil was wound with wire salvaged from a 5 m length of internal telephone cable - the
type used for permanent wiring along skirting boards etc.. This type of wire is
plastic-coated, having an overall diameter of about 0.9 mm. First, wind a single layer of
33 turns on the 22 mm pipe, and hold the turns in place using one layer of insulating
tape. Then wind a second layer of 27 turns, centrally over the top of the first layer, and
hold the turns in place with insulating tape. To complete the coil, solder the end of the
first winding (B) to the start of the second winding (C).
The inductance may be checked using a dip oscillator: when a 270 pF capacitor is connected
across the 55 uH inductor, resonance should occur at about 1.3 MHz.
Alternatively, L1 and L2 may be wound on ring cores, using Amidon type
T130-2 cores (coloured red and grey). 60 turns of telephone cable wire (about 2 m in
length) will do the job nicely.
Winding the transformers
The driver transformer, T1, and the output transformer, T2, are wound on
25 mm OD 3C85 ring cores. The wire
gauge is not critical: hook-up wire capable of carrying one ampere would be fine.
For T2, it helps if the wires are colour-coded. I used telephone cable wire for both
T1 and T2.
For T1, wind fourteen closely-spaced turns in a single-layer on the toroid, and hold them
in place using small cable ties, or insulating tape. Then wind seven turns, centrally over
the first winding, and retain in place.
For T2, twist three wires together at about one twist every 15 mm. Wind twelve turns on
the toroid, and label each of the three wires at the start of the winding with the
identification numbers 1; 3; 5. Then label the other end of each wire with 2; 4; 6
respectively. Refer to the PA circuit diagram to ensure correct installation. The number
of turns is not critical in this design: during testing, I found no difference in
performance between a transformer of 10 turns, and another wound with 14 turns.
Putting the Marathon
in a box
The original 'bird's nest' version has proved to be effective and stable,
but I wanted to build a more rugged transmitter - something that I could take out
portable, if required.
Consequently, a second Marathon transmitter was built using ugly
construction into a small diecast box measuring 150 mm (length) x 50
mm (height) x 80 mm (depth). To maintain adequate screening
between the VFO and the PA sections, it was necessary to install the PA transformer (T2)
and low pass filter ring cores (L1 & L2) in a separate tin plate box, mounted on the
lid of the diecast box. Note that, for the tin plate box. I chose to use an
'Altoids' peppermint box - a favourite project box used by many QRPers!
Click on the thumbnail to enlarge the pictures:
Updates & feedback from other
Sources for transistors (added 07/10/2001)
Paul G0ODP (G-QRP Club member #9428) asks: Where do you buy the 2SC2166s, and what is the
I know of two sources for the transistors:
1) Grandata Limited, K.P. House, Wembley, London, HA9 0HB
Tel: 020 8900 2329 Fax: 020 8903 6126
2SC2166 - 80p
2N3819 - 29p
2N2222 - 23p
2) Sycom, PO Box 148, Leatherhead, Surrey, KT22 9YW
Tel: 01372 372587 Fax: 01372 361421
2SC2166 - 150p
2N3819 - 45p
2N2222A - 25p
BC212 - 10p
Also good for: 4.7 mH chokes; T130-2 ring cores; capacitors; TO220 insulating kits;
Results at G6RO (added 10/10/2001)
Ron G6RO (G-QRP Club member #1269) has been using a Marathon at 5 W RF and B28
receiver from his QTH in Shipley, Yorkshire to work the following stations on CW:
Mal G3KEV (100 km); John G3CCH (80 km); Tom G3OLB (349 km); Steve GW4ALG (253 km); and
Dave MM0ALM (354 km). In fact, on 7th October 2001, Ron worked three countries in
Also, Ron's QRP CW signals have been heard by:
Walter G3JKV (310 km); Mike G3XDV; Dave G3YMC (283 km); and Ko Versteeg, NL9222 [SWL] in
JO22KE (482 km). Amazing, simply amazing.
Sources for 3C85 cores (added 10/10/2001)
George G3ICO (G-QRP Club member #6406) wrote to ask: Where can I get 25 mm OD 3C85 ring
The complete description of the ring core includes the material type; outside diameter
(OD); inside diameter (ID); and height (all measured in mm). The material type is
3C85; and the OD/ID/height of the required component is 25/15/10 mm. For more
information, click here.
Possible sources are:
1) Arrow Electronics (UK) Limited, Edinburgh Way, Harlow, Essex, CM20 2DF.
Tel: 01279 626777. Ask for Arrow Stock Number 058377R, and confirm the
component description with the sales person.
2) Hawnt Electronics, Birmingham, UK. Tel: 0121 784 2485
Fax: 0121 783 1657
More results from G6RO (added 14/10/2001)
Today, Ron had two QSOs with Finbar EI0CF this morning! On the first occasion he received
RST 419 from Finbar; and then got RST 519 for the second QSO. That makes four countries
worked on QRP - and heard in five countries!
GW4ALG goes QRT on 136 (added 16/10/2001)
Steve GW4ALG was active on the 136 kHz band from March 1998 through to October 2001.
For Steve, the arrival of the first RSGB 136 kHz repeater on 10th
October 2001, located in southern England, took away the main attraction of the band.
Naturally, the repeater brought with it the usual bunch of pirates, and lid
operators - and even a certain amount of 'cheating' by those claiming to have 'worked' DX
on the band. Click here for more
Improved voltage stabilisation (added 03/01/2002)
I'm grateful to Jim G3PCA (G-QRP Club member #9277) for pointing out that the two 5.1 volt
zeners may fail to provide adequate stability the VFO voltage. As a result of Jim's
comments, I am now using a single 9.1 volt zener diode.
Jim writes: "I found with 11.4 v on the '12V N' line (12 v less the
0.6 v drop across the 1N4004), the two 5.1v zener diodes could not stabilise the VFO
voltage (11.4 - 10.2 = 1.2v which, when taking into account component tolerances, we can
easily drop out of the zener range). The change is very simple, as follows:
1) Remove the 120 ohm resistor and the two 5.1v zener diodes.
2) Add a 78L05 regulator; two diodes; and a capacitor, as shown in the following diagram.
The 12V N line can now vary from 8 to 20 volts input, with a very stable 6.3 volts on the
The VFO works 100% on 6.3 volts and is very stable. Left on the bench with a window
open (approx 10ft from the VFO), the drift was no greater than 15 Hz in 5 hours!!"
Feedback from G4CNN (added 08/04/2002)
At the web site of John G4CNN, you will find some detailed information about
the approach that John took when building his 'Marathon' transmitter. All
those interested in building a Marathon should visit John's site at: http://www.qsl.net/g4cnn