Radio / SDR

Current radio propagation conditions:

Recently, dipped my toes back into Software Defined Radio. Using a DVB-T dongle I had lying around, I got to tuning some local FM radio stations using zadig and hdsdr software on the laptop, and it piqued my interest for sure..

I now have a decent quality RTL-SDR dongle (820T2 tuner) with the associated antenna kit in addition to three DVB dongles with FC0012 and E4000 tuners. I’ve been able to tune in a lot of different things.. From medium wave radio through the various HAM bands, through broadcast radio, through aircraft VHF, through maritime VHF, through DAB digital radio, through digital TV signals, through seeing my car unlock signals from the keyfob, to picking up and plotting ADSB signals from planes. A huge world of signals out there, and it’s really interesting to be able to see that much of the spectrum on the screen at any one time.

Currently I have Raspberry Pis serving radio signals across my network. One Raspberry Pi is serving FM signals via rtl_sdr from the RTL-SDR dongle, and another RPi is serving the HF spectrum via the SDRPlay RSP1a and rsp_tcp application. The Pi can supply ~2048 kHz of radio bandwidth across the network which is generally good enough. I do get more consistent behaviour when using 1Mhz instead.

To get decent HF signals I took a 10m length of parallel speaker cable, split the two conductors, and attached one end of each cable to the connectors on a co-ax connector, and connected both of those in a coupler. That gave me a centre-fed dipole, with fairly good connectivity end-to-end. That co-ax connector feeds the core of a 50m co-ax cable. The screen of the cable is earthed just outside the house and also just near the antenna. The core and sheath are connected indoors to the stubs of the dipole antenna supplied with the RTL-SDR dongle. This gives me a horizontal dipole antenna with a total length of ~20m. It’s currently extended across the end of the lawn, and gives some pretty good results.

I’ve been able to tune and decode some DRM (Digital Radio Mondiale) signals from the shortwave bands such as Radio Nigeria and Kuwait radio. Another DRM station is a radio station from Bulgaria. On ordinary AM, I’ve been able to receive BBC World Africa from Ascenscion Island, and Voice of America as transmitted from Botswana – a straight line of almost exactly 9000km, and very clear. I can also pick up DCF77 and MSF60, and I’ve picked up a Russian time signal that was clearly distorted by multipath routes. The RSP1a device is actually really good for browsing HF..

My current issue is to chase down why I have such high levels of RFI from 7Mhz downwards at my PC. If I connect my laptop and SDR device in the lawn, I have lovely clear and quiet conditions with a clean MW range. On the Pis or PCs in the room in the house, I have a huge amount of noise. I’m suspecting the UPS..

In the post I have a number of interesting toys on the way. I have some low noise amplifiers, an FM broadcast block, an amplified powered mini-whip antenna, and another RTL-SDR dongle with HF antenna kit. I also have an Airspy R2/Spyverter R2 combination inbound as well, as I recently received the RSP1a. Those receivers can show me 10Mhz of the spectrum at the same time, which is an upgrade from the current ~2Mhz that the RTL-based dongles can show.

I also recently got a KiwiSDR. This little device is based on a BeagleBoard Green, with an additional add-on board that takes an antenna connection and a GPS connection. It can show 4 users the full 30MHz shortwave spectrum at a time, with pretty good sound quality and a few interesting addon extensions. Very nice toy, and really easy to use.

A second KiwiSDR came into my possession, and I’ve updated the processing components from a Beaglebone Green to a Beaglebone AI, and that opened up either getting 14 wspr channels at the same time, or the ability to run a few concurrent DRM decoding sessions native in the browser.

All in all, it’s an interesting return to a hobby I’ve skirted around for years me. Maybe it’ll lead on to getting a HAM license?

Some useful links:

Antennas tried and built, and the results, unpowered then powered.

  • RTL-SDR standard dipole kit antenna: Works great on FM upwards. Nothing much below, though I can get tiny signals from ~10MHz up. Better for FM radio listing.
  • 5m dipole, wound onto RTL antenna stubs. Worked quite well, gave an appetite for proper antennas.
  • 20m centre-fed longwire. Not a great result for the wire, but a significant improvement on the dipole-wound.
  • 20m dipole. No balun, so not that good. Was updated to the next item.
  • 20m end fed randomwire, wire end to coax centre, direct to receiver. 2m above the lawn, N-S direction.
  • The above was improved with an earth connection to the coax braid at the antenna, then a NooElec 9:1 BalUn (converted to UnUn by breaking the centre tap earth connection underneath the circuit board. This is currently my lowest noise and highest strength signal.
  • 60m elevated loop, 6m to 1m height, but poor QRM and poor SNR. Probably as the baluns were outside the window and lots of wire directly beside the house.
  • 20m folded dipole. Strung along the hedge, with a ~510ohm resistor at the junction of the upper segment. Used with a 9:1 balun, low signal and high enough QRM
  • 30 sloper longwire (part of the 60m loop) from 6m to 1m, ~30m length or so. Another 9:1 unun, with an earth to the unun and another earth to the end of the feeding coax. Not too bad a performer, but not as good as the 20m randomwire.
  • ADS-B homemade colinear coaxial antenna. 8x 116mm segments of RG-6 alternating between core and shield, vertically mounted, performes really well with an LNA, giving a range of 250km for hearing planes.
  • Powered MFA-30 loop, Aliexpress special, attempted clone of the Wellbrooks. Performed moderately well as a loop, performed better for me as a 4m vertical dipole in the garden. Atlantic storms blew it down, so it’s currently a 4m amplified dipole on the ground. Better performance than I expected to be honest.
  • The other main antenna I have is an Active-Antenna.eu loop AAA-1c amplified loop. Currently configured as a pair of 1m dual heating pipe loops at right angles, and a horizontal flat-folded dipole. Installed in the attic in the apartment in Altstetten, performing very well for the location.
  • Miniwhip, China sourced, 12v powered. Very high noise levels. More experimentation needed to see if it can be improved.
  • Miniwhip, Chirio design. 5-12v powered. Performed very well indeed on battery power in the garden, not so good when mounted close to the house wall. More work required for that to perform better.

Transmit-capable antennas (SWR and resonance checked with NanoVNA-H only – no transmits possible until licensed):

  • I’ve got a QRPGuys randomwire with 9:1 UnUn, and that’s a pretty reasonable performer with 16.5m wire and 10m counterpoise. Will need a tuner to transmit efficiently though.
  • QRPGuys no-tune EFHW, with 49:1 UnUn. I’ve cut one wire antenna from really thin cable for 20m resonance, and I’ve recently cut another wire from cheap speaker cable that resonates on 40m. Needs a reasonable length of coax to act as a counterpoise though.
  • Inbound.. QRPGuys 40/30/20 trapped EFHW. Looking forwards to seeing this one build and work.
  • Also inbound is a tuner kit.
  • Desired: DX Commander’s All-Band 1/4w Vertical. 80m through 10m and higher, should be an interesting solution to getting on the air post-license.
  • Also, I’d like to try and build a proper all-band end-fed randomwire, It’ll be interesting to see if this is in fact possible or not.

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