Friday, 19 January 2018

Adding a 50 MHz (6m) antenna to the remote site

The Red Pitaya, with its 125 MHz ADC clock, can function on the 50 MHz (6m) band. As the remote site is at relatively high elevation of approximately 200 metres ASL the decision was made to install a 6m antenna.

After some research, and reading the blog of GM4FVM, I purchased the Diamond A502HBR. This 2 element Yagi of the HB9CV type promised moderate gain, light weight and a reasonably wide F/B ratio which made it a good candidate for a receive antenna intended for activity detection and monitoring.

A502HBR on the workshop floor

Construction was rapid with so few parts and the use of colour coding on the elements. An initial continuity check of the coax after fitting a PL259 plug showed a connection between centre and braid, which was accepted after acknowledging that the Gamma match will produce this phenomenon with a multimeter - thanks to G0MJI for confirmation.

To mount the antenna, I purchased another L.G. Harris 731 5m pole which, for around £16, offered adequate strength and height for a temporary installation.

The A502HBR on the 5m mast

Not having an antenna analyzer to hand which covered 6m, the initial plan was to start receiving in a variety of modes using the Red Pitayas and see what could be heard. The antenna was fixed on an azimuth of 134 degrees, with the main lobe towards pointing towards Italy.

After a number of hours of silence, I was extremely fortunate to experience a winter Sporadic-E opening that afternoon to test the antenna. First impressions are that it is working satisfactorily.

Signal processing flow

Red Pitaya for WSPR & Red Pitaya for CW and FT8

Stations received on 18 Jan 2018, CW (including beacons) and FT8

Monday, 15 January 2018

Optimising audio bandwidth on the KiwiSDR

After comparing the KiwiSDR's audio with an SDR from a different platform, I decided to look into the filter settings of the KiwiSDR.

The default settings on USB (and LSB) are a receive bandwidth of 2400 Hz, with a low of 300 Hz and high of 2700 Hz. While this is a good setting for weak signal intelligibility, on strong local signals a wider filter will produce more pleasing audio - particularly through headphones.

To change the filter, tune in then zoom on the waterfall to your signal of interest. The ends of the yellow filter bar above the signal on the waterfall are draggable by the mouse when clicked and held, enabling the high and low setting of the filter response to be changed to suit the signal of interest, resulting in more rounded audio.

80m LSB, audio response changed to 0 - 2900 Hz

Thursday, 4 January 2018

NO3M decoded in JT9 mode on 630m using the KiwiSDR

Propagation on the low bands has been good in recent weeks. While it is now not unusual for WSPR to be heard from across the Atlantic most nights in the winter season, the digital conversational modes such as JT9 or FT8 are more challenging as they require a greater signal strength for a successful decode.

That some decodes were achieved indicates that the continual optimisation of antenna and processing paths, coupled with efforts at noise reduction, are producing results.

Setup: KiwiSDR - Firefox - Virtual Audio Cable -  3 x WSJT-X (WSPR, FT8, JT9)

Monday, 1 January 2018

Reducing Ethernet interference

Ethernet switches, routers and cables are a known potential source of interference to reception. The effect was visible using the KiwiSDR on the 20 metre band with spikes every 60 kHz.

To mitigate this interference source, the switch was changed from a TP-Link unit to a Netgear GS305. It was hoped that having a switch in a metal case would be better than one in plastic. In addition, all LAN cables were changed to shielded CAT6 and the switch placed 20 metres away from the receivers.

A subsequent check displayed significantly reduced interference.