Monday, March 14, 2022

A two valve regenerative receiver for 40m from Stephen, VK2BLQ

Home brew master, Stephen, VK2BLQ has been at it again and has created a two valve regen a la 1967 All Wave Two, using two 6BL8 or 6U8 triode pentodes (heaters in series), and it still works with a 12 V HT supply instead of the 90 V used previously:

Stephen writes: 

"The receiver sort of works, but the 12 V supply limits the audio output, so is only good for headphones or feeding to an external amplifier. The stability on 40 m is Ok but overloads on strong signals, so I think an antenna pot might be better than the volume control, doing double duty, but it does combine the glamour of thermotron technology with the safety of low voltage.

For the makeshift vernier dial I used an old audio CD , which as fortune would permit, possesses the magic of Soldersmoke so is destined for greatness. Hacking an old variable pot and two grommets seems to function as a vernier drive.

Audio quality on the 39 m AM shortwave band seems to me more pleasant than one of the superhets and 40 m SSB and CW sounds "mellow". (Click to enlarge image).

I did make some circuit changes to suit the lower voltage: I changed the pot and resistors for the Regen control and I did use a 230v to 6v mains transformer for the speaker. Valves are great, the 6U8 worked with the same components even though I didn't have a 6GW8.

Next version gets the upgrades and a heterodyne converter, and if I can quote a line from the TV series "The Nevers":

"It's a prototype". 

If anyone actually has questions, I'm happy to help."

Here it is in action:

-- It's a fine looking prototype if you ask me. Thanks Bill for the link from Soldersmoke!

Friday, March 04, 2022

Raising an antenna support with a drone

This week I had visits from several hams including Dave, VK3ASE, who brought a long squid pole and an offer to help with antenna raising. Dallas, VK3EB, helped with the off centre fed dipole. 

I decided to try raising a line over a nice clear tree branch with a drone rather than the squid pole. The drone could lift the line without problem, the issue (which I anticipated), was landing again on the other side with the line now trailing above the drone.

Despite my best efforts to keep the line between the rotors eventually it tangled and the drone struggled for a while before I shut it off. This left the drone hanging from the line but undamaged. Dave recorded some video of the enterprise:

The supports for the ends of the dipole were in trees with substantial leaf cover so we used the squid pole for those.

My antenna tree supports an Off Centre Fed Dipole cut for 80m that does very well on 40 and 20m. Now it has a second support that currently has a dipole cut for 30m. The branch is nice and clear so the drone was easy to fly over it.

Wednesday, March 02, 2022

ABC Digital Radio Mondiale transmission on medium wave

The Bendigo Amateur Radio Club held a meeting where Brad Geier, VK3VLK, spoke about the great future of Digital Radio Mondiale (DRM) transmission. I didn't realise that the ABC has been running a transmitter on 747kHz from Wangaratta.

I can see the transmission quite clearly here at Drummond and also on the PkLoops SDR at Croydon.

In the past I've been able to receive DRM transmissions from Radio New Zealand International and they can sound very similar to FM quality despite weak signals and band noise.

The software needed to decode DRM is Dream. I couldn't get the latest version, 2.2 working but version 2.1.1 does work under windows. Another interesting lead is swradio-8 which I was able to build but not quite get going on Linux.

I'm using the IC-7300 as receiver by switching USB audio output to IF, choosing AM and winding the bandwidth out as much as possible.

My antenna is not very well suited to 747kHz so I'm not decoding audio just yet: