More Antennas! Receiver Overload! Satellites!

In my never-ending quest to develop the perfect high-altitude ballooning chase car, I've been playing with preamplifiers mounted close to the receive antennas. This has some nice benefits in that the receive system noise figure can be very low (<1 dB), and the cable loss from a few metres of RG316 is lessened.

I've also added a new antenna - a commercial 70cm turnstile antenna purchased off eBay. With a few modifications and some bracket work, it now mounts nicely to the roof rails of the Rav4:

70cm Turnstile Antenna

This nicely fills in a null that's existed in my overall car 'antenna pattern' for a while - in the direction of 'up'. Using vertical antennas on both balloon payloads and cars means that when the balloon is at a high elevation relative to the receive antenna, the end-fire nulls of both antennas meet, resulting in poor signal. The turnstile antenna has a roughly hemispherical pattern which removes the null on the receive side, improving performance.

The pattern isn't perfectly hemispherical though (a result of the reflector). I've noticed a deep null at about 20 degrees elevation, and performance below 10 degrees when receiving vertically-polarised payloads is very poor.

So, obviously, I need more antennas!

I've always wanted to have the ability to switch antennas quickly while on a balloon hunt, and with the purchase of a MiniKits Relay kit and a bit of enclosure and bracket work (with help from Dennis VK5FDEN and Peter VK5KX), I can now switch between the new turnstile, and a vertical antenna.

The above diagram provides an overview of the new receive system. I now have two receive antennas (the turnstile, and a generic SO239 base), both with PSA4-5043 preamplifiers right next to the antenna feed-point:

More Antennas! (The antennas at the rear are for 2/70 comms, HF, and 4G)

A coaxial relay can be used to select which antenna is to be used, with the controls for this within the cars glovebox. A bias tee just after the switch provides power to the selected antenna preamp.

A splitter is used to drive two SDRs - one, an AirSpy is connected to the Car-PC (an i3 NUC), and is generally used with SDR Console v3 for general purpose receive activities, such as tracking RTTY payloads, or radiosondes. The second SDR, a NooElec RTLSDR (R820T2), is used for Wenet reception, or other Linux-based SDR RX activities. If nothing else is going on, I connect it up to a Raspberry Pi mounted in the back of the car and use it to automatically decode radiosonde launches.

The relay, bias-tee, splitter, and SDRs are all mounted in a semi-temporary manner to the cargo barrier in the rear of the car:

Switching, Splitting, and Receiving

From left-to-right, you can see the relay, bias-tee, splitter, AirSpy, and RTLSDR-in-box. The Raspberry Pi is mounted within an enclosure just visible at the top-right of the image.

Overload Issues...

The preamps being mounted right next to the antenna does pose a few problems. Whenever I key up on HF,  2m, or 70cm, the amplifiers are immediately driven into saturation, and there's bugger-all I can do about it.

With the PSA4-5043, this results in almost 100mW of power being produced by the preamp, going straight into my receivers. Ouch. A BAV99 dual-diode on the RF output of the Bias-Tee helps limit that to something a bit more manageable, which the SDRs own input limiting can handle.

Also, the preamps are wide-open. No input filtering. Yes - this results in a LOT of intermod in the presence of other strong transmitters. However, once out of the city area, they seem to work fairly well. Since install I've performed a few radiosonde hunts with good success.

I'm currently working on a re-designed preamp PCB with a bypass path. This will let me switch out the preamp when I'm closer to the transmitter, and will also allow me to use the antennas for transmit.

Satellites!

When I heard about the LilacSat-1 cube-sat and its Codec2 transponder, I knew I'd *have* to give it a go, if only to make sure David Rowe got a chance to talk through it (after all, he did write the codec!).

LilacSat-1 has a 2m FM uplink, but the downlink is 9600 baud BPSK on 70cm and multiplexer telemetry information with a 1300bit/s Codec2-encoded version of the received FM signal. The LilacSat devs provide their own GNURadio out-of-tree package to decode the downlink, but I ended up using Daniel Estevez's lower-latency decoder. I added a QT Constellation sink to his decoder so I could get an indication of receive quality.

A few hours messing about on my ThinkPad, and I had gpredict, gqrx, and Daniel's decoder all talking nicely to each other, and I was ready to try it out on a real signal! I organised to catch up with David for a coffee and some nerding out at an AREG working bee, which happened to be on right when a nice high-angle pass was going to happen.

Remember my discussion above about intermodulation issues? The working bee was right in the middle of Adelaide's eastern suburbs - lots of interference. However, I had a secret weapon:

No, the tea-towel is not the secret weapon.

Cavity filters are awesome. AREG, also being awesome, have a lot of cavity filters.

I tuned a band-pass cavity up for 436.510 MHz, the LilacSat-1 downlink frequency, and voila - no more issues with out-of-band interference. The insertion loss of the cavity was about 0.5 dB, and the 20dB bandwidth was about 10 MHz. Instead of using the turnstile on the bracket, I sat it on the roof and angled it closer towards the horizon, covering the sector of sky LilacSat-1 was expected to be in.

For uplink we used a Kenwood TM-D710G in another car about 20m away. We ran about 25W into a short Diamond dual-band antenna.

I hooked my ThinkPad into the RTLSDR, and once the sat got to about 20 degrees elevation, I could clearly see the downlink signal!

Main Screen Turn On!

Keying the uplink radio resulted in the downlink turning on, and after some fixing of settings, we could hear audio from the downlink. Andy VK5AKH was also at the working bee and managed to capture part of the pass on video:

The pass only lasted a few minutes, but it was great to see all the software working smoothly, and to see David use his own codec via a satellite!

LilacSat-1 also has an onboard camera which can be controlled via the 2M uplink, and will downlink images via the 9600 baud stream. It's currently not enabled as the cube-sat is currently oriented to optimise GPS reception. Once it's enabled, I look forward to getting out mobile and trying to capture an image of Australia from space!