OH2FTG Labs‎ > ‎

137kHz experiments

I have lately had an interest on these low bands like 137Khz, 472kHz and 8.9kHz. As one of my goals is to work a 2-way QSO on all the ham bands that exist, I'll be building some gear for 137kHz. This band is also interesting because cheap SMPS fets will give significant output cheaply. Sadly that output is needed due to the low efficiency of antennas that the average hams have on this band. I doubt that I'll be breaking the 1W EIRP barrier.

The first few steps on the road to 137kHz include having a receiver capable of covering the band, and a signal source. As I don't have a signal generator that covers 137kHz I'll have to build one.

I have various junked CB radios I occasionally raid for parts and cases for projects. When I heard from Tatu OH2EAT about his experiments about using a CB radio output divided by 200 for 137kHz, I had found my signal generator.
The rig I'm planning on using is a junked KusoJet CB transceiver that has some mods done to it, but the synth seems to be working. It should give me 23 output frequencies to use for experiments.
I'm planning on removing the final and predriver from it and tapping the output in AM mode from there. As the rig uses a mask ROM (useless for modifications) LC71xx series PLL, the TX LO runs at transmit freq/2. I could tap it at the LO tank, or before the doubler for a more simple divder assembly. But I want both 137KHz and 274kHz outputs so it gets done this way. I'm also not interested in reverse engineering the CB more than I have to.
The divider topology is going to be two chained 4017's used to divide the input by 100. The output is then buffered and both fed outside and into a 74HC74 used to divide it by 2. The second D-flipflop in the 74HC74 is left unused.

The transmitter part is going to be a simple single ended deal with either a SSP520 or some NXP fet for the first experiments. Later on the plan is to build a +100W final with a half-bridge using some NXP fets. The driver for the half-bridge is yet to be decided upon. Either some NXP SMD driver stuff will be used or a IR2113. The NXP stuff is easier to design with, but due to being SMD a real PCB needs to be designed and manufactured. The IR2113 will need some additional electronics than just a 74HC74 to create the high side and low side control signals.


The antenna I'm planning on using is my 40m dipole fed as Marconi-T with the appropriate matching.
I'll likely also have to put out tons of radials for it.
Comments