LF: 137.500 kHz ROS beacon

[Gary - G4WGT <g4wgt@tiscali.co.uk>]

Hi LF,

Having successfully bench & on-air tested a x2 harmonic amplifier method of driving my G0MRF Class D LF transmitter, I ran a beacon last night (Saturday 19th) on 137.500 kHz.

An extract from my previous e-mail description is shown at the end.

Using the equipment & method described below, the ERP from my 12 metre vertical antenna was around 150mW. The mode was ROS MF-1, 100 Hz bandwidth.

I received the following report from Michel, F5WK in JN18HP, a distance of 410 miles (661 kilometres) in daylight.

>> Not even a faint trace on a QRSS3 spectrum but:

>> RX1: 19:23 @ 5.9 Hz: 9 <STOP> -37 dB
>> RX1: 19:28 @ 5.4 Hz: O4WGT <STOP> -34 dB
>> RX1: 19:33 @ 5.4 Hz: G4WGT <STOP> -31 dB

The x2 harmonic low level amplifier is a simple way of driving a Class D type of transmitter which employs a divide by 2 drive chain. My limitation with multi tone drive is that my DDS VFO will only produce 2 frequencies ie. the main frequency & a CW/Rx offset which has previously limited me to 2 tones as with RTTY & DFCW modes.

Please bear in mind that it will not be suitable for modes like PSK were tones are transmitted simultaneously.

At the moment the 2 small PCB's are precariously lying on the desk, when I have cased them I will publish the circuit & pictures.

73

Gary - G4WGT.

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After a few days experimenting & bench testing a method of driving my
G0MRF Class D Tx on the 137.000 kHz band from my linear transverter I finally
constructed a harmonic amplifier producing a 274.000 kHz signal from 137.000
kHz input. An "on-air" trial was conducted resulting in a QSO with
Graham G0NBD on 137.000 kHz using ROS data mode software.
 
My thanks to Graham for his ideas & tests.
 
The mode used was ROS MF-7. The equipment used for the QSO was as
follows. Kenwood TS-440 to my LF-MF linear transverter producing the
137.000 kHz signal, followed by a tuned harmonic circuit & buffer
amplifier whose output was 274.000 kHz therefore at x2 drive for the Class
D Tx. The x2 signal is then divided by the Tx driver chain in the
normal way to provide 137.000 kHz to the PA.
 
This method should also work well using WSPR & other similar modes.

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