Cebik Moxons, SDRplay and satellites

There was a pass of the AO-91 satellite over my location today and I listened to the ham radio operators operating through it.

I used my home-built Cebik Moxon aerials which are located in my attic and the nice SDRplay RSP2.Cebik Moxons in attic

Here’s a screenshot of SDRuno displaying the AO-91 signals for those of you who don’t think the Doppler effect is real.

Screen Shot Doppler Effect

You can easily see the received signal changing frequency as the satellite hurtles past.

This is what it sounds like. 

AO-92 12.09 Wednesday, May 8, 2019 20190508 1209.m4a

It was recorded using Audio Hijack Pro from a Microsoft Remote Desktop session of SDRuno on a Dell XP workstation. Apologies for the over-driven audio — I was concentrating on receiving rather than recording.

SDRplay Macs and Linux

The Good News

I bought an SDRplay RSP2 recently and have been enjoying using it a lot. The RSP2 has three antenna connections and covers from 1kHz to 2GHz. It is amazingly good value. SDRplay provide a nice receiver application called SDRuno. The SDRplay website has links to reviews of the RSPs and they must be pleased with them.RSP2

The Bad News

My shack is full of computers accumulated over the years. However, as far as Microsoft Windows machines are concerned I only have an ageing Dell Precision 380 running Windows XP and a VirtualBox VM on a Mac Mini running Windows 10. My W10 VM isn’t fast enough for SDRuno and the audio stutters. The XP machine is usable as long as the sample rate is kept low and you decimate a lot.

On Macs and Linux computers SDRplay only provide an API/HW Driver, although they do provide a full image to boot a Raspberry Pi from. The software for Mac and Linux is CubicSDR which just about does the job but does not have all the features of SDRuno. I prefer GQRX  and have managed to get it working on macOS but the Hi-Z antenna connexion only works intermittently. It’s all quite unsatisfactory. It’s frustrating to have such good hardware spoilt by the lack of easy-to-install software. You get sucked into handling a morass of libraries with differing versions and it’s time-consuming if not impossible to find out which versions you need to use.

The Raspberry Pi image works well as long as you connect an HDMI display. I prefer to run my Raspberry Pi through Microsoft Remote Desktop so I don’t have multiple monitors, keyboards and mice on my desk. But with Microsoft Remote Desktop CubicSDR does not display well at all.

I only hope that SDRplay comes out with SDRuno on other platforms other than Windows. 

In Other News…

So I’m currently using the Dell XP computer with SDRuno run through Microsoft Remote Desktop as the best solution for me, even though it isn’t supported. Don’t worry, my insecure Dell XP is only connected to my LAN and is not connected to the Internet.

The blue arrow points to the low sample rate and high decimation needed on the Dell XP.

SDRuno Screen Shot

But it works well enough, it just doesn’t use the capabilities of the RSP2.

SDRuno Maiin Panel Screen Shot

SDRplay Safety

 

I bought an SDRplay RSP2 SDR recently. I had been looking for a multimode VHF/UHF transceiver but couldn’t find one within my budget. The transceivers that get listed on eBay go for silly prices and I’m not sure I want to pay the asking price for a new model. 

So I’ve decided to concentrate on receiving signals from satellites rather than transmitting, hence the purchase of the RSP2.

One of the specs of the RSP2 is the maximum signal it will tolerate at its aerial inputs. It says 0dBm which by my calculations says the voltage at the input must be under 0.22 Vrms to be safe. This sent alarm bells ringing as my Cebik Moxon aerials share the same attic as my small transmit loop aerial.

So I made some measurements.

I measured the voltage seen in the shack from the Cebik Moxons when transmitting 10W to the magnetic loop. The measurements were on my HP 54615B scope with a 50Ω termination.

No RF 800µV
TS590S tune (10W) 191mV at 14.2 MHz
TS590S tune (10W) 288mV at 7.07 MHz

So my RSP2 is unsafe in its current location and configuration. I must not transmit using my HF rig if I have the RSP2 connected to my Cebik Moxon aerials.

My first thought was to put a high pass filter on the Cebik Moxons to block 6m and below. I’m sure this would work but there would be some insertion loss.

But I have an Moonraker M-100 masthead preamp which has a 24-2300 MHz filter. This wouldn’t block 10m and 6m, but I don’t transmit on those bands using the small transmit loop and if I did the radiated power would be quite small as the impedance mismatch would be high.

So I made some more measurements with the M-100 in the shack. I know these won’t give the same effective gain at the receiver as measurements made with the M-100 in the attic, but it’s close enough for a start. The dial on the M-100 is laughable as the gain you get at each position bears only a passing relationship to the actual gain. The most you can say is the more clockwise the dial the higher the gain. Hence the ‘on dial’ below.

• Zero gain (on dial)
No RF 18mV
10W 14.2MHz 32mV
10W 7.07MHz 39mV

• gain +10 (on dial)
No RF 18mV
10W 14.2MHz 25mV
10W 7.07MHz 53mV

• gain +20 (on dial)
No RF 19mV
10W 14.2MHz 559mV
10W 7.07MHz fuzzy on the scope so I assume the signal is distorted somehow

• gain at 12 o’clock
No RF 18mV
10W 14.2MHz 50mV
10W 7.07MHz 16mV

This looks promising as the preamp gain should be set to just retrieve the loss from the cable and this should end up being 12 o’clock or lower.

The next step is to install the M-100 in the attic and re-measure. I’ll probably use a bias-tee arrangement so I can power it from the shack.