SW Data Net Experiment, Monday 14th December 2020 13:00 GMT

7 minute read

This is a follow-up to the 80m SW wide net we trialled last Tuesday, we aim to work with the grain of propagation to see if we can improve regional coverage to be suitable for the Wessex Ham initiative. Apologies that the time is not convenient for people of working age, sadly the vagaries of propagation wait for nobody.

This was successful. Results are here

We will start using the Olivia 16/500 mode, which can work well with poor signal to noise ratios, so it gives us the best opportunity to cover a wider physical area. Please note the settings - I have defined these in terms of USB settings of the suppressed carrier. This is because some rigs seem to do datamodes only in USB mode. See the Olivia groups.io page, where dial frequencies on 160m and 80m are similarly 1500Hz below the signal centre frequency, indicating USB on these bands.

I am aware that common practice for frequencies below 10MHz use LSB for voice, we need to pay attention to the settings to ensure that the signal centre frequency is as expected. For clarity, here is a screenshot of FLdigi set up for 3.584 MHz, 1kHz up from the Olivia calling frequency of 3.583 centre frequency.

FLdigi settings for 3.584MHz Olivia signal centre frequency

The centre frequency is defined as the centre of the 500Hz wide Olivia 16/500 signal. It is generated by feeding a baseband audio signal centred on 1500Hz to the rig, with the dial frequency of the rig set to the suppressed USB carrier of 3.5825 MHz. The centre of the actual radiated signal is accordingly 3.5825+.0015 = 3.584 MHz.

Please ensure you have RXID set, we will use TXID on calling CQ at the start and any mode changes, which allows you to automatically set the mode from the signal. We will not send TXID on every subsequent over, to prevent making a slow mode even slower. The values highlighted with a RED rectangle should be the same, although PKTUSB may say something slightly different depending on your CAT control settings.

Overs are quite long with Olivia 16/500 so we need to run a structured net - we will invite check-ins, please send a short transmission with just your call on the invitation, and the control station will pass the channel to the next station in turn. Please return control to net control when you have done.

Please also have Contestia 16/500 and possible 8/500 available. If you haven’t masked unused modes in FLdigi you will have them available. They are similar to the corresponding Olivia modes but achieve an increase in speed in exchange for a reduces symbol set and EVERYTHING IN ALL CAPS.

Net Protocol

Olivia 16/500 is roughly 20wpm and Olivia 8/500 is about 30wpm, similar to BPSK31 but with a noticeable latency of a few seconds. This means the net will be slow. Please do NOT have TXID set, this adds a couple of seconds to your overs increasing the chance of a clash, leave RXID set as above to enable mode changes.

Please join the net sending just your call after net control invites check-ins. NC will acknowledge your call and will queue. NC will pass the net to you with

YOURCALL de NETCONTROL-CALLSIGN pse KN

and the channel is yours. For your first over please introduce yourself -

name
town/village
locator
tx power
antenna
background noise1

NC will try and capture the SNR of your signal and send it on the net later. When you are done send BTU NETCONTROL-CALLSIGN de YOURCALL and I will pass the net round to the next station. Please let us know if you are leaving the net, if you don’t respond then we’ll drop you from the list but by all means rejoin in a later check in.

The Olivia 16/500 data mode is slow. Once I have an idea of SNRs we may try to switch to 8/500 which is faster if the conditions support it.

What does the signal sound like?

This is the following text received off-air encoded in Olivia-16/500

CQ SouthWest NET CQ SouthWest NET CQ SouthWest NET
de G7LEE G7LEE G7LEE

callsign on its own

This is MX0IOA without TXID

Even just the callsign on its own without the RSID takes 9 seconds, including the preamble and postamble

You should be able to play these into FLdigi and get a decode, then the first sample will set your modem to OLIVIA-16/500

I get s/n: 7.6 dB on feeding the signal from the browser directly into FLDigi using virtual audio cable, and s/n: 5.7 dB by playing the signal out from the browser through speakers and setting FLDigi to listen through a dynamic microphone in the room. As you can see, both give a 100% decode. The garbage characters before the signal are there because squelch is off.

FLdigi receiving a replay from these off-air captures via VB-CABLE and via an open microphone, respectively

Farnham SDR recording at 132 km

This signal was received off-air at 10pm 5 Dec 2020 at the Farnham SDR which is located 132km east of Glastonbury.

It gives a s/n of -4 to -10dB depending on QSB. It decodes fine

CT

TEST de G7LEE G7LEE
VVV VVV VVV
TEST de G7LEE G7LEE
%

TX pwr is < 10W using a FT897 into a full-wave 80m horizontal loop ~5-7m agl

The same signal path on PSK-31 gives serviceable copy but needs twice the power and there is some data corruption.

Signal Reports

It will be interesting to see what sort of decode SNR values we get between stations, SNR is in the box highlighted in GREEN, but is ONLY valid while a signal is being received, so if you can make a note of that (and the originating station) that will be useful.

I will try and pull some of the information together, so if you could have the following information to hand:

Rig: FT 897
Ant: 80m full-wave horizontal loop
Pwr: 10W
Loc: Glastonbury IO81pd

a note of the SNR observed while receiving the signal will be valuable

eg

MX0IOA s/n 10dB de G7LEE

Creating a FLDigi macro

<CALL> your <INFO1>

allows you to insert this in the TX queue, provided you press the macro while the incoming signal is present, and have double-clicked the other station’s callsign to transfer it to the CALL box.

Note that there is quite a bit of latency - several seconds, between seeing the Olivia signal on the waterfall and getting a signal decode. Because it can decode signals that may be hardly visible, it is worth setting the squelch in FLdigi off or as low as possible. This latency makes the mode feel slower, and when checking in to the net please send a short initial call, to minimise the risk of doubling. Also please bear with me, I have never run a data net of any significant size ;)

calling frequencies/CoA in this region

We need to be mindful of other QSOs at this time because of the wide skip catchment area. At the moment we have no data about NVIS at this frequency, we need other users from beyond groundwave reach. RSGB lists 3.58 to 3.59 as a slot for 500Hz wide modes, and 3.59 to 3.6 as another slot - there is not much room to move in these 10kHz slots.

For general awareness, I have picked this selection of calling frequencies/centre of activity in the 3.58 to 3.59 area from Olivia groups.io site and also from SV1GRB The segment 3.57 to 3.58 is allocated to 200Hz wide modes, including some of the WSJT-X modes including FT8 if you are wondering what the frenetic activity lower down is.

Centre Frequency mode
3.570 JT65
3.573 FT8
3.575 FT4
3.578 JS8CALL
3.580 psk
3.5814 MFSK (Olivia etc)
3.583 Olivia (Olivia May 2020 suggested call)
3.590 RTTY

Additional information

FLDigi datamodes software
FLDigi Olivia mode help page
RSGB 80m band plan
Olivia datamode groups.io page
Farnham SDR on this frequency

Last Tuesday’s 80m experiment

  1. It will be interesting to see how the net performs with varying levels of background noise, this is the S meter reading with no signal, assuming no RF preamplifier engaged.