I’ve had a bit of time to try out a few different antenna ideas, and it’s been a very interesting learning curve so far.
Self-build antenna fun:
The first antenna attempt was a simple short randomwire of a few meters whiule sitting on the lawn with one of the laptops and the RTL-SDR dongle. I was able to get some SW stations from Saudi and Eastern Europe with that.
Second antenna build attempt was that I set up about 20m of insulated copper wire across the lawn, and fed a coax cable to the receivers directly from the centre point. This gave reasonable results, but definitely left a lot to be desired. I tried better grounding of the coax sheath, and then I realised that I should try a proper centre fed dipole with one side earthed instead of both sides going to the signal feed. This improved things a little bit, but was still a little underwhelming, most likely due to the horizontal orientation and the 2m of distance from the ground.
I rejigged the antenna in the third antenna attempt to become a proper ~20m long randomwire, end-fed to the signal feed in the coax, and earthing the coax cable at that point as well. This gave a good step up in sensitivity and signal strength. Then, after a long-awaited Ebay delivery of a 9:1 NooElec balun, things got quite interesting. I modified the balun to become a proper unun after breaking the centre tap to ground connection, and I installed the unun at the receiver. This is the current best state of the longwire, performance descriptions to follow.
I had ordered a 4:1 voltage balun as an experiment. With 10m of cable each side, I had tried to have a 20m dipole with a proper correctly matching balun in DX operation. However, the experiemnt was not a success as I found that there was no LW or MW, and the signal/noise levels as seen by the receivers was quite low – lower than the pre-balun randomwire. This antenna has since been disassembled, but I’ll hold on to that balun for a possible future transmission antenna.
After the purchase of ~200m of 0.5 square stranded cable, I decided to try a long loop around the perimeter of the property I’m currently living in. I ran ~80m of cable along the tops of the bushes and walls around the edge of the back lawn (maybe 2-3m average height), raising up to 6m height by using the soil pipe vent stack and a gutter downpipe as supports for the wire. I can’t yet put the wires muvh higher due to possible neighbour issues. Both ends of the wire loop were run into the radio room and attached to a 1:1 balun, and a length of coax then running to the receivers. The noise level is generally much higher than I had anticpated, and the signals are present at least. It’s a fairly good option at this stage if I had no other option. The balun in use has been shown to give some operation down to the LF area at least.
I started out with an AiExpress-sourced miniwhip, needing a 12v source, and 239 type connectors. I had a slew of 12v power supplies to hand, and found that the RFI generated by most of them was utterly abysmal and made attempting to use them for VLF through the lower end of SW almost impossible. . I did find the best of them, and it gave a clean enough power feed, and I did get to use that miniwhip to try to listen to things. Noise levels were still into the -60dBm with little in the way of useful SNR, and the signals were definitely not clear and listenable. I may yet disassemble this one and replace a few components to get a useful antenna.
Then, I ordered a Chirio miniwhip with options of either 9v PP3 power or 5v bias-t power. This was a thinner antenna, and had ~6m of coax already installed with an SMA connector on the end. This antenna gave much better signal levels and a lower noise floor than the Chinese miniwhip, but still had a lot of crackling. Adding a better earth connection to the SMA connector on the cheapy Chinese upconvertor dropped the noisefloor by some 10dBm or so. I’ve currently got the antenna powered from the USB socket on the Pi3 that is hosting the FC0012 dongle and the rtl_tcp server that is serving this across my network.
Antenna performances – as like-for-like as I can get them.
The miniwhip has a noise floor currently of some -103dBm. With the RTL-SDR at auto gain and using 2048kHz bandwidth, Shannon Volmet at 5505 has a signal level of -80dBm., giving an SNR of 23dBm. The sound of the signal is not great, but definitely understandable.
The horizontal loop with the 1:1 balun, with the Airspy, 2.5MHz bandwidth, linear gain at 15, zero visual gain, has the noise floor at -82dBm and the same Shannon Volmet signal at -71dBm, for an SNR of 11 dBm. The sound quality is poor, and not very understandable at all unfortunately.
The randomwire with the 9:1 unun, with the RSP1a using 2048kHz bandwidth, RF gain at 9, IF at auto, and visual at zero, has a noise floor at about -134dBm and a Shannon Volmet signal of -103dBm for an SNR of 31 dBm. This signal sound is definitely the best of the three antennas currently in use.
I have a handful of RTL2832U dongles that I used when I was getting started in the SDR world. I’ve some FC0013 tuners, as well as an E4000 tuner. I bought two RTL-SDR 820t2 dongles with the antenna kits, and those were an absolutely fantastic introduction into the SDR world. The direct sampling mode is reasonable, though the lack of gain control is a bit of a pain. I’ve got one of the dongles set up with an upconverter on the Chirio miniwhip, and serving to my network.
I later bought an SDRPlay RSP1a, and an Airspy/Spyverter combination. The RSP1a is pretty much the best bang for the buck SDR system that I’ve come across. VLF through 2Ghz without a break, no external upconversion required, usb-powered, and a lovely clean signal output. These are all very useful characteristics. The Airspy combo also has its place, it’s worth noting. Linux and RPi support is better, the hardware is physically smaller, and there are a few additional programs that are very useful such as the fast-sweep spectrum analyser and the ADSB client. Both of these receivers are currently plugged into the Dell Precision laptop, and being successfully served to my network via the SDR Console V3 server.
When I’m working locally on the Dell Core I5 laptop, I prefer to use the Airspy combo with the 10Mhz bandwidth. If I’m listening to one particular signal. I’ll pick it out with SDR# and using the decimation feature, to get the best possible SNR for that signal. The 10Mhz is very useful for the general browsing of signals and seeing which one I want to home in on.
I’ve spent some time looking at the KiwiSDR as a useful addition to my radio listening system. The 30Mhz bandwith visibility, the perfectly networked interface, and the extra plugins currently available, added to the four separate receiver channels possible, are all very enticing for my use case.
I also have my eyes on a better LW/MW antenna than I currentl have access to. I’m carefully eyeing up the likes of a Wellbrook ALA1530LN loop, or an Active-Antennas AAA-1 set as a quad loop, or even a Bonito Megadipole type of thing. I’m spending quite a bit of time looking round at the KiwiSDRs that are publicly available, to see how different setups can perform.