Sunday, July 30, 2023

Built VK3XU's "Simplex sidebander"

I'm on a bit of a retro transmitter trip at the moment. Armed with some low profile 3.6864 crystals I built the "Simplex Sidebander" described in the first volume of Drew Diamond, VK3XU's wonderful "Radio projects for the radio amateur".

The transmitter is simplicity itself. An NE602 with a crystal pulled up in frequency produces double sideband that goes through a 4 crystal bandpass filter. Three RF gain stages produce about 4W out. The final is a low cost IRF510. Mic amp is a 741. Here's how mine looks:

Despite trimming the oscillator crystal I couldn't get decent sounding sideband out of it. The reason is that the little crystal wouldn't pull high enough. Acting on the advice of SmartFriends™I replaced the single crystal, first with two and then three of the little ones and in the end with three larger crystals which seem to pull more. In the end I have a full size polyvaricon for tuning:

Drew warns that the whole thing needs to be in a shielded box and I can attest to the fact that when attached to my antenna it takes off magnificently. 

The exciter itself seems very narrow to me - here it is fed into a local SDR.

Off to the shops to buy a tin of biscuits... (to get a tin to put it in).

Boxed up and it no longer takes off when connected to the antenna.

I have a bit of instability as you can see in the dips in the AM test signal here:

Saturday, July 01, 2023

Built a low pass filter for using the Minion on 80m

I was horrified when I looked at the spectrum coming from the minion on 80m so clearly a low pass filter is needed. I used the notes from GQRP on low pass filters for my design and it looks good when swept.

My build is not very compact and indeed it's almost the same size as the whole transceiver.

I quite like these boxes, purchased on AliExpress and plan to make low pass filters for 40 and 20m as well.

The filter on a spectrum analyser shows that it's 3dB down at 5.16Mhz and the first harmonic, at about 7MHz would be down 25dB at least.