Weak Signal Digital Modes on HF-Bands

Many open source programs are available for Windows, OS-X and Linux, if you want to become active on the amateur radio HF bands using digital modes. Modern coding and modulation schemes allow for long distance contacts with low power.

Below you can  download my presentation about PSK 31 (Phase shift keying) and JT-65/JT9 modes as presented in our monthly SARTS meeting August 2016 in Singapore.

Presentation (pdf): WeakSignalModes


DU1- Dasmariñas – Aug 2016

This time we stay longer than a week in our second home, and I set up a 12 m fibreglass pole with a G5RV multi-band dipole antenna for 40 m to 10 m. The pole is located at on corner of our area, the dipole endings are fixed at two other corners, so the antenna has a form of a 90 degree inverted vee.

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fibre glass pole

The antenna analyser shows clearly 3 resonant frequency points: around 40 m, 20 m and 10 m. All resonances seem to be below the amateur radio frequency bands, probably because of the inverted vee form.


Using my “old” AT-130 antenna tuner, I can operate on all bands from 40 m to 10 m including the WARC bands.

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Assembling the antenna:

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Operation in digital modes using my home-brew interface:

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Time to disassemble, as we are going back to Singapore. Heavy duty 40 ft fiber glass pole and G5RV dipole:

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Server down – repairing disk drive

After more than two years 24/7 the volume on my server degraded. One of the three hard disks crashed. Luckily there is some redundancy using Synology Hybrid Raid, and I am now trying to repair the volume. 5 TB of data at risk …

Once successful, will check the defect disk. Still should be under the 3-years warranty from WD.


IC-7300 and Fldigi

For digital modes I am using fldigi on a MacBook and the IC-7300 built-in USB interface. This is my running configuration:


On the IC-7300 press Menu -> Set -> Connectors

ACC/USB AF Output Level to 75%

USB MOD Level to 25%

Under CI-V CI-V Baud Rate is set to Auto.

CI-V USB Port: Unling from [REMOTE] and
CI-V USB Echo Back: ON


You need to get the IC-7300.xml file written by W1HJK to use the CAT.
After Installation start fldigi and go to
Configure -> Rig ->RigCat
Select the downloaded xml file. Under Device select “SLAB_USBtoUART” (driver name).
Set Baud rate to 19200.
Tick CAT command for PTT
Audio Port: Audio USB Audio CODEC
The “sweet spot” for the IC-7300 in data mode is 1500 Hz.
Configure -> Misc -> Sweet Spot PSK et al.: 1500


Reduce the output power to 20% … 35%. When you click on “Tune”, ALC should be at zero. In my station setup I can “hear” the computer on 15 m. Levels vary depending on the USB connection cable. I still need to experiment with some ferrite to reduce interference.
The same configuration can be used for JT-65 (WSJT-X). I changed the IC-7300 CI-V address to  88h and select IC-7100 as transceiver.

ICOM IC-7300 – SDR radio in a box

Finally the new IC-7300 arrived, and I am still in the process to get familiar with it. I could not resist, as my IC-718, though reliable, is quite “outdated” in its technology, developed more than a decade ago.

Unfortunately, the first IC-7300 unit I picked up from the retailer in Singapore, had a DOA (defect on arrival): the CW keyer always sending dashes. Luckily, there was a second unit available, and it was exchanged without any problems.

Now the new unit is in operation for a week and doing its job without any problems. Size and weight are the same as my IC-718, but the features are not comparable, and it is a real advance in technology. The menus are intuitive and signal reception is excellent, as I can adjust all parameters – gain, filter bandwidth and response, audio processing etc. – according to the actual band conditions.


Two days ago, I setup the configuration for digital modes, connecting my Apple MacBook via USB, using fldigi and the IC-7300 built-in USB sound interface and serial control. So there is no need for any additional interface box, just one USB connection between computer and radio. It works fine, and I was able to make my first contacts in PSK-31.

Minami-Tori-shima JD/M and Ogasawara JD/O

Minami-Tori-shima, also known as Marcus Island belong to the easternmost territory of Japan on the pacific plate past the Japan trench. It is an isolated Japanese coral atoll in the northwestern Pacific Ocean, located some 1,848 km southeast of Tokyo and 1,267 km east of the closest Japanese island, South Iwo Jima of the Ogasawara Islands, and nearly on a straight line between mainland Tokyo and the United States’ Wake Island, 1,415 km further to the east-southeast. (Source: Wikipedia)

Before my first radio contacts with JD1BOW and JG8NQJ/JD1, I even didn’t know that Japan has a couple of small island so remote from the main island.

MinamitoriDistance from Singapore approx 6000 km.

Linear loaded short dipole

For my amateur radio activities on the HF bands I only have a 3.6 m wide balcony available for antennas. After some study I decided to experiment with a shortened dipole for the 15 m band. The full size 21 MHz half wave dipole would require a span of approx 7 m, therefore I searched for a solution to shorten the full size length by 50 percent.

Finally I decided for a linear loaded dipole; the layout and resulting dimensions are shown below.


First measurements at the resonance frequency showed a low input impedance at the dipole’s feed point with values around 10 Ohm. The theoretical radiation resistance of a 2 x 1.6 m short dipole would be 10 Ohm, so it is an indication of low losses in the system. In order to match the antenna to the 50 Ohm of the transceiver, I use a quarter wave transmission line transformer consisting of two parallel connected 50 Ohm coaxial cables.


After fine tuning the length of the antenna wires, I achieved an SWR below 1.5 for the CW segment of the 15 m band.


The antenna’s bandwidth is around 200 kHz; I could already work DX stations around the world.

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Dipole feed point and 1:1 balun