2007-09-30
ARNSW Trash and Treasure
Peter VK2TPM once again was nice enough to supply me transport to the Dural site. The day promised to be quite fun, with not only the bi-monthly Trash and Treasure, but also a technical presentation to the Homebrew Group on RF filters, and of course the usual Homebrew group Show and Tell segment, this month largely composed of 80 metre challenge projects.
For once I largely ignored the Trash and Treasure... I think I'm burnt out on buying bits and pieces. With Rockby's recent weekly clearance sales I suspect I'll have enough parts to build radios for several life times. Storage is becoming a problem, my shack has two boxes full of unsorted bits and pieces from my Rockby orders. They must love me, I order something weekly, and I don't seem to stop until I've blown at least $100. That quest for just on more gadget or rare component is too strong!
Stephen VK2BLQ brought along his Ozi-pole antenna. The Ozi-pole is a Mid-North Coast Amateur Radio Club kit project. It is based on the Budd Brummond W3FF Buddipole concept.
It is an interesting design. The brass tap points make band changes very easy, but I do worry about its long-term weathing. It is not designed for permanent installation, its a portable antenna, but I suspect corrosion would become a problem anyway with those solder tags and aligator clips exposed to the elements.
It breaks down to something extremely compact and quite cheap to build. The use of telescopic whips for fine tuning is a nice touch. I think if I build my own I'd use much thicker wire for the main element, and perhaps winding wire for the coils. The wire provided is pretty thin and makes one wonder about its losses, especially in the coils.
At $99 it is pretty good value if you want to avoid all the fiddling around to optimise the dimensions. Personally I think for the price it should include components to build a balun at the feedpoint, it would only be a few dollars more. Peter VK2TPM has recently built one too and has offered to loan it to me for experiments/measurements. In particular I am very curious about how it offers such high feedpoint impedances for a compact antenna, especially on the lower bands. I've built something similar for 18 MHz as an experiment, which works quite well on RX, but until I complete my latest antenna bridge project I won't be able to measure its feedpoint impedance in the field.
Stephen also brought along his 80 Metre Homebrew Challenge transmitter, looking very pretty in its blue styled case. Stephen is a true believer in "radios that glow in the dark", to this end even this solid state rig has some LEDs inside arranged to make a soft glow through the cooling vents.
Definitely the most prolific builder of this group meeting, Stephen passed around the Marker Generator which is featured in the group's latest newsletter. Other items he showed us included a nicely constructed dipole balun case using hardware store PVC fittings and an experimenter's "blob board" prototyping jig he builds new projects on.
John VK2ASU brought along a nice kit transceiver he had just finished building from the UK. IIRC it was an 80 metre rig, but unfortunately I can't recall must ask him about it.
He also talked about his Homebrew Challenge transmitter which he had just got working in the wee-hours before this meet. He's a bit like me, his constructions tend to be tightly packed and his projects end up being mostly empty space!
Peter VK2EMU brought his Homebrew Challenge Transmitter too. We've all watched this rig evolve from an empty box to a working unit over the past 6 months of meetings. Peter never fails to bring it along for everyone to check out, very encouraging for those of us who are lagging behind (like myself!)
Peter VK2TPM talked about his neat little audiophile headphone amplifier. It uses the OPA2134 op-amp which features extremely low distortion and wide bandwidth. He built it into an altoids tin, the homebrewer's enclosure of choice. He also showed us all a photo of the AM waveform on his CRO from his homebrew challenge project. Like myself, his challenge TX is still in the QRP levels.
I demoed my Super-regenerative Tone Dip Meter and the nH Frequency Counter Adapter. Then it was time for the Technical Presentation of the day, RF Filters by Mark VK2XOF. Mark brought along his very nice HP Spectrum Analyser and Tracking Generator.
A UHF cavity was the first to be measured. It was a nice mini-keg sized Aluminium unit apparently with internal Gold plating:
Someone's homebrew LPF was next:
Then a helical VHF Airband filter. The filter is clearly very lossy and suffers from severe microphonics, tapping on the case makes the trace dance all over the place.
Quite a variety of filters were dismantled and displayed, swept and passed around for all to look at.
All in all a fun day!
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