Sunday, June 29, 2014

Balun day at Amateur Radio NSW

Today was "balun day" which began with breakfast followed by a terrific talk from Mark, VK2XOF, about the theory and practice of building current baluns. The biggest mystery for me is what core to use.


After the theory, and lunch, we all received out balun kits from TET-Emtron. I went for the TB-11K 1:1 kit rated at 1kW peak. We then built our kits under the guidance of folks who had built them before.


Finally there was a spectrum analyser and antenna analyser on hand to measure the baluns. Some home constructed baluns were found to have high reflected power which was very interesting to see.


The method for assessing a balanced to unbalanced balun's operation is to attach a 50 ohm load on to the antenna terminals, feed it from an antenna analyser and connect one and then the other of the antenna terminals to the ground of the input and see if the impedance changes very much - if all is working well it should not change.

My sincere thanks to the ARNSW committee and all who clearly worked hard to make this day a great success. Thanks also to TET-Emtron for producing the fantastic kits and for their support.

Saturday, June 28, 2014

Windy in Sydney this afternoon


Up here on the 28th floor it was pretty windy today. The doppler wind radar looked pretty interesting. RNDSUP seems like a fun technology.

Friday, June 20, 2014

ABC Download This Show appearance - Chromecast, Upworthy, Unicode

We're videoing radio again this week. Here we are in the Radio National studio talking about technology.




Exciting news for people who like watching radio programs on video - Download This Show is now available on ABC iview. (Search for it or look in the iview channel).

Monday, June 16, 2014

DDS VFO purchase

I recently purchased an old crystal controlled Codan lunchbox radio and want to put it on 40 and 80m so I've found a rather nice DDS VFO on ebay for AU$45. This one has two PIC CPUs on the board, a nice backlit display and comes with a tuning knob but no info on wiring it up.

The same unit is in other ebay auctions and one of them has this "schematic" which will be a help, if it's correct.


My board came with some cables with sockets to match the connectors on the board and the rotary encoder. Here's other info, for future reference. (It all seems a little mysterious to me but hopefully will be clear when I play around with it).

Note that the wiring of the rotary encoder in the sketch above (from an eBay ad) is shown from the rear of the encoder. I wired it up in reverse and it works but the tuning goes clockwise for up in frequency.

My thanks to Stephen VK2BLQ for access to his workshop, here's my VFO all boxed up.


One criticism of these boards is the lack of mounting holes. I've stuffed the box with cloth to press it against the front and that works fine but is not ideal.

I've now added a 9V three terminal regulator so that the unit can run from my 12V bench supply.

All up this is a terrific VFO, pushing while turning the knob changes the digit that will be adjusted so it's quick to get around. Thanks to commenter Yiannis, the documentation and software source has been traced to Ham Radio India.

The objective of this device for me was to use it as a VFO for my Codan 6924 radio. There's a handy page outlining how to do this. This weekend I ran a cable to an empty crystal spot.


This makes the old radio look very fancy.


As I tune around I notice carriers sweeping by at some frequency ranges but not others. I took the DDS over to VK2BLQ's lab and it seems that for many frequency ranges the output drops dramatically. I'm not sure what's going on here but my guess is that it's designed to cover only certain bands and not others.