Saturday, May 23, 2020

Virtual audio cable for macOS

Forgive me if this is old news but on Windows I've long used the excellent and free VB-Cable from VB-Audio Software for piping audio from SDR# into WSJT-X.

I mostly use macOS and Linux but for some reason SDR# seems to be the SDR software I like the most and it only runs on Windows.

A 2012 MacBook Pro is used to dual boot Windows for running this stuff as you see above right.

Today I noticed that there is a version for macOS and so far it looks good. (The software is free but "donation ware" and I've send them US$10 in appreciation of the macOS version).

A new device simply appears as an input and output device and settings are available in the Audio MIDI control panel.

I'm aware of Rogue Amoeba's Loopback but it is US$109 and seems more complex than is needed for this basic task. Their other software is excellent though.

Tuesday, May 19, 2020

What WSPR signal to noise ratio is needed for sideband to be intelligible?

The end-fed antenna is working pretty well on 40m and I'd love to have a contact with Sydney friends. John, VK2ASU, kindly agreed to run a WSPR beacon so I could find the best times to receive him.

Over a 24 hour period, with him transmitting just 10mW, the best SNR was -21dB here.

My question, to the brains trust hopefully reading, is what WSPR signal to noise ratio would be enough such that a higher power sideband call would be audible?

I have read that a signal to noise ratio of 6dB is needed to be able to copy SSB.

John is transmitting 10mW which is 10dBm but he could run 100W on sideband or 50dBm. An extra 40dB.

If the bandwidths were the same then -21dB + 40dB = 19dB of signal to noise but while SSB is about 2.5kHz wide, WSPR is much narrower, perhaps only a few Hz?

Here is the SNR between VK2ASU and VK2TPM (portable VK3).

Spots from all stations to me looks like this:

Any insights or pointers gratefully received.

Kevin, VK2KB, commented: "My understanding is the noise power doubles every time the bandwidth is doubled while the power level of the received signal stays constant. I've verified this on my SA by changing the bandwidth of the SA response and noting the noise floor, you can see the floor decrease by 3db every time I halve the  SA  window but the amplitude of the signal I'm watching stays the same.  With WSPR you need to know what the effective bandwidth is. I had a look at this site:

They say the effective bit rate is about 1.5 baud  and its possible to recover a wspr signal at -34 db  on a 2500Hz bandwith rx.

Say that you are using an effective bandwidth of 1.5 Hz to recover the digital signal the ratio of 1.5/2500 =  .0006 or -32db  Because the wspr signal is coherent I think they get a few more dbs in the demodulation process.

The difficulty with this is however the power bandwidth of the wspr signal vs the power bandwidth of the ssb signal.  Anyway something to spend time on."

Saturday, May 16, 2020

Townhouse antennas

My first antenna in the rental townhouse was a dipole for 20m which has been working pretty well. The challenge here is that the windows seal tightly and it's not possible to run coax through them so I had the balun inside and the two legs of the dipole stuffed through the window seal.

One leg ran straight out and the other around the corner to the left. The KX3 tuner is excellent and could match this quite well.

Today I visited Jaycar and purchased some iron powder toroids, some wire and a soldering iron (all my electronics is still in storage). I constructed a QRP 9:1 Unun. (Three turns to 9 turns).

The end fed wire goes out through the window, around the corner, over a citrus tree and is held on the side of the clothes line. This now tunes up well on 20m, 30m and 40m.

Compared to the dipole WSPR reception has worse signal to noise but the addition of extra bands, particularly 40m, makes it a much better option. I'm receiving well and transmissions on 40m are getting out quite well.

I did notice that a ceiling fan turned itself on after I'd been transmitting and I'm worried about affecting TV reception so I'll be receiving mostly until I test that. Amazing what WSPR can do even in very challenging circumstances.

It's Sunday morning and I've been able to listen to the 40m broadcast from South Australia and also from Dural near Sydney. Here's a snippet. It does fade up and down but is mostly audible.

Saturday, May 09, 2020

Moved to Melbourne

We've now moved in to a townhouse in Alphington which is north east of Melbourne. It's much smaller than our home in Sydney and even though we still have stuff in storage it has been a bit of a struggle to downsize enough to fit in.

The area is nice and I've enjoyed walking along the river which is nearby and through some lovely park land.

In the photo above you might be able to make out the yellow wire which is one leg of a dipole for 20m. It's a bit of a challenge to run cable to the antenna as the sliding glass windows seal tightly. Currently I have a balun inside and run the two legs of the dipole out through the window. It tunes up quite well but I'm sure the trees absorb quite a bit of the transmitted signal. WSPR works ok and I'm hearing stations around the world to some extent.

Unfortunately the power lines for the block attach to the corner of the room I'm using for my office so there's quite a lot of noise.

There's lots of FT8 activity visible but so far no contacts. I called CQ on PSK31 but nothing heard just yet.


The unit has Telstra cable NBN connected and I've arranged a transfer from Sydney. Even though I can see that the cable is active they want to deliver a new wireless router before we can use it. It's been mobile data for a few days but I plugged our Sydney router in and it works! This is handy as yesterday we were informed that delivery of the new router has been delayed beyond the promised date of next Tuesday.

Vale Tim Mills VK2ZTM

Sad news that ARNSW volunteer Tim Mills died this week. He's been a dedicated part of the NSW amateur radio community who I've seen every time I've been at Dural and often heard on the Sunday broadcast. Many years ago he was one of my first contacts when I put up a new antenna at Killarney Heights in Sydney.

Tim was always cheerful and helpful. He worked behind the scenes to keep everything running smoothly. I didn't know him much beyond saying hello and chatting about the sausage sizzle and other trivia but I know he was a big part of the organisation.

It's my birthday today and it's a big one as I'm now eligible for a seniors card. Only a few of my friends have kicked on but I guess it's something that will become more frequent as the years pass.

Australian COVIDSafe app source code

The Australian Government has released the source code to the COVIDSafe app for both Android and iOS. I have downloaded the iOS code and have been reviewing it.

The source code in Swift looks modern and well written. There are few comments but names are well chosen. (I suspect many comments and attribution has been removed).

There has been some criticism about how the app needs to be running in the foreground on iOS to beacon and find other users but this is not a complaint about the app but rather a deliberate limitation that Apple enforces. iOS can respond to beacons without an app running but this is done at the operating system level and it looks like that capability will be added in the next update.

The app uses a CoreData database to store timestamped records of beacons it has detected along with their signal strength and transmit power which can be used to estimate distance.

Communication with the Amazon server API is protected with SSL and certificate pinning, so you shouldn't be able to intercept them.

I think it is great that the Australian Government and DTA have released the source. (Of course, there's no guarantee that the code in the app you install is the same as the source code we have seen but I feel more secure that it probably is).

If you haven't installed the app, please do, but note that when you are near a group of people you'll need to run the app and leave it in the foreground for the duration of your encounter.