VHF Aircraft Scatter

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After the brief excitement of a small winter Sporadic-E opening last week, all has been predictably quiet on the newly installed 6m antenna. When trying to dial in the correct frequency offset for receiving WSPR I noticed Aircraft Scatter on my transmitted signal from home (IO83LS) to the remote site (IO83QV).

Numerous decodes were noticed from reflections. After further investigation, it is clear that I am fortunate (?) to live in an area of extensive aerial activity.

Trans-Atlantic traffic originating from Western European hubs such as Schipol and Munich passes over in an upper airway at high altitudes. More exploitable traffic regularly originates from Scottish airports travelling South at lower altitudes. A number of tests were performed, making use of the excellent Airscout Software by DL2ALF and Flightradar24.com.

1) FR24 was monitored until an aircraft appeared with an anticipated course that would pass through the area of interest at a useful height. EZY 1806 from Reykjavik to Manchester at approximately 19,300 feet on a track of 155 degrees looked a good candidate.

2) As hoped, the aircraft descended as it became closer to its destination. As the aircraft passed over the receive site (IO83QV) its altitude had dropped to approximately 12,150 feet on a track of 146 degrees.

3) The aircraft is now visible in Airscout, and it’s time to start transmitting. The transmit antenna is a 4 element Yagi with a beam heading of 130 degrees.

After transmitting three sequential WSPR transmissions, the results were available for review in WSPR-X (n.b. I’m only using WSPR-X for the purposes of this test – the most up to date and recommended software incorporating WSPR is WSJT-X).

No other stations decoded the primary or reflected signals – which was unsurprising due to the current flat conditions and lack of activity. Still, there remains optimism that from Spring the use of this technique could bring success, perhaps coupled with FT8.

For further background reading, I recommend reading the blogs entries of G3ZJO, G0ISW and G3XBM which contain much more detail of successful tests and to which I give credit for starting my interest.

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

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The Red Pitaya 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

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Optimising audio bandwidth on the KiwiSDR

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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

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NO3M decoded in JT9 mode on 630m using the KiwiSDR

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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)

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Reducing Ethernet interference

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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.

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Using the KiwiSDR to provide receive audio to WSJT-X

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Listening to modes such as CW, SSB and AM using the KiwiSDR is straightforward as the default output of the HTML5 sound from the browser goes to the default soundcard, which typically powers speakers or headphones.

A challenge in using digital modes is that the output from the browser needs to be sent to a third party application, such as WSJT-X, using a virtual audio cable. A limitation of the common browsers is that they do not provide any options to re-route the sound to other devices. This post pertains to Windows – Linux provides various other options.

After some research, a solution was found in the form of a Firefox Add-on:

The following steps were followed:
1) Downgrade Firefox to release 56 (the Add-on is not yet compatible with Firefox Quantum).
2) Once installed, disable automatic Firefox updates. As you now are not running the most recent release, there are security considerations. In my configuration these concerns are minimized as the Firefox instance is running on a separate machine and used only for accessing the KiwiSDR (no general browsing).
3) Install the Add-on.

4) Configure the Add-on. Select your virtual audio cable in ‘Options’.
5) In WSJT-X audio settings, select the correct virtual audio cable for audio input.


As a test, a QSO was attempted in JT65 mode on 20m using WSJT-X.
The KiwiSDR was started in Firefox and set to the correct frequency.
After verifying that the receive audio from the KiwiSDR was being decoded in WSJT-X correctly, I chose to reply to the CQ of S53PM to test the configuration.
1) There was a significant disparity in sent and received signal reports. This was anticipated due to the large difference in effectiveness between the respective transmit and receive antennas.
2) The internet adds delay, which is manageable within the generous DT (Difference in Time) tolerances of JT65/JT9, but it made FT8 a challenge. The receive audio delay was typically between 1 and 2 seconds. In FT8 you have to be really fast to double click the callsign of interest to start the QSO (as otherwise your first transmission will start too late and not be decoded) – however if you can do this the auto sequence works normally from that point onwards.
KiwiSDR (over internet) on the left, local (IC7100) on the right
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AM Bandstop evaluation

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I had my first opportunity today to assess the AM Bandstop, which should reduce the strength of signals on the Medium Wave band and hopefully clean up the receiver.

378-1880 kHz – 3 dB attenuation
526-1710 kHz – 9 dB attenuation

KiwiSDR1 on the left, KiwiSDR2 with AM bandstop on the right.

Unfortunately, the addition of the bandstop was not enough to stop ADC overload on the KiwiSDR when the attenuator was removed. As this was the primary purpose of its procurement its continued retention in circuit will have to be justified by improvements in other areas, which will require further observations.

Other strong signals outside of the Medium Wave band could be the cause of the ADC overload or it could be because of the aggregate strength of all the signals being fed in from the loop across the LF/MF/HF range.

Reducing the circumference of the loop would give the KiwiSDR an easier time, however this would affect the Red Pitayas which can tolerate larger signal inputs. A compromise is therefore needed.

Further observations were made in the early morning of Sunday 17th December. A snapshot taken at 0610z of the 80m band shows a real improvement with the bandstop fitted. The improvement is so marked that continued retention of the AM bandstop is justified.

KiwSDR1 on the left, KiwiSDR2 with AM bandstop on the right.
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Lightning protection installed / AM Bandstop / Second KiwiSDR

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Protection from lightning discharge is an important consideration at a remote installation. This morning I installed an earthing rod and a Nagoya lightning arrestor near to the antenna feed point. No degradation in performance was noted.

A second KiwiSDR was installed onsite, for additional user capacity and side-by-side experimentation.

Currently, the second KiwiSDR has a newly purchased AM bandstop in circuit to determine its effectiveness. Currently, both KiwiSDRs have a 10 dB attenuator on their inputs to avoid constant ADC overload. I would like to find a way to remove these as this solution is not optimised. Users should be aware that this second KiwiSDR will be used for experiments and will have a lower quality of service.

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Adjusting the KiwiSDR display for optimum spectrum presentation

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New users of the KiwiSDR often comment that the spectrum display seems noisy. It is necessary to change the display settings to get optimum results once you have set your frequency and display bandwidth.

Suggested steps:

1) Select ‘Spectrum’ if you have not already done so.

2) On the upper display, note the noise floor.

3) Adjust ‘WF min’ until the noise floor just disappears below the display.

4) Adjust ‘WF max’ to raise the peaks of the visible signals.

A KiwiSDR session optimised for good presentation
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WSPR Reception

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One of my primary interests recently has been WSPR reception. I have found that WSPR is an excellent mode for receiver and antenna optimisation.

The input from the antenna is distributed to the receivers by an Elad ASA-15 antenna splitter. Two Red Pitaya boards act as WSPR receivers running Apline linux software by Pavel Demin. Each Red Pitaya can decode 8 bands simultaneously.

Presently, no impedance transformation solution is in circuit for the Red Pitayas and they are directly connected to the Elad. The Red Pitaya’s antenna ports are high impedance inputs. The current receive performance across the frequency range seems satisfactory in the present arrangement.

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