26 Jul 2008
I have now breadboarded the 10m DC receiver for this rig using a single balanced mixer with 2 diodes followed by two stages of audio. It works well and was pulling in plenty of stations in the IOTA contest this weekend on both SSB and CW without any sign of AM breakthrough. I think the secret is plenty of LO injection to switch the well balanced diodes hard.
21 Jul 2008
For some time I have been toying with the idea of building a simple but effective 1-2W DSB transceiver for 28MHz. Well, this weekend I made a start by breadboarding the transmitter up to, but not yet including the final PA stage. The 2N3904 has been used throughout plus a couple of 1N4148 diodes (which seemed well matched) for the TX balanced mixer. Output from the balanced mixer was sub milliwatt so a couple of linear stages lift the output to around 50-100mW pep. With a single 2N3866 PA stage this should take me up to the 1-2W pep level.
Next stage to breadboard is a mixer-VFO. I am thinking of a 20MHz xtal oscillator (because I have a crystal) and an 8MHz VFO. Ideally I'd like a higher xtal oscillator so the VFO is lower in frequency and more stable.
The RX was breadboarded last week and uses a single balanced diode mixer followed by 2 stages of audio. This works really well with no sign of AM breakthrough at all.
15 Jul 2008
This last week I have been building again. Not much, just another DC transceiver built ugly style. The rig covers 14048-14070 VXO controlled and puts out about 500mW using the OXO design. It uses an SBL1 mixer on RX followed by 3 stages of AF gain. Only station worked so far is M0BXT but hoping for some Europeans maybe at the weekend. It suffers a bit from AM breakthrough and the 2N3904 transistors used in the AF sections are too noisy really. This is really not a perfect radio, more a work in progress.
Why the name? The Pipit was a similar (better!) radio design I put in SPRAT many years back for 15m.
2 Jul 2008
After some time without use, I resurrected the little Micro80 4 transistor transceiver for 80m last weekend. To increase the power a bit I've now got provision to run it from an external gell cell battery at 12v, so giving me about 2dB more RF out compared with the 9V internal pack. Using it with the bench PSU was impossible because of 50Hz hum pick-up. I also managed to improve the AM breakthrough by around 10dB at least by putting a 4n7 cap across the base-emitter junction of the first stage of the darlington in the RX AF. This makes it a usable radio on 80m at night now as it is not obliterated by the BC stations around 4MHz. Success soon followed with a solid QSO with G3XIZ in Biggleswade around 30 miles away late Sunday afternoon.