Monday, May 25, 2009
VHF - Aircraft RX
Why would you make an aircraft RX when you allready have 2 receivers that can do the same?
..because you can? Because it's cheaper? Because you want get rid of any commercial equipment that in a few years it's more expensive the repair than buying an new one? All of the above!
After searching the net I found an simple receiver that suited me, it's relatively cheap to make, "it should work" just by looking at the schematic from my experience and it uses components that in 200 years should be around :)
This is the design: http://py2ohh.qrp-br.net/trx/corujinhavhf/corujinhavhf.htm
After initial problems with the VFO (read: diferent components than the original design) I now have an more or less stable VFO. Also some problems with de audio circuit now sorted. It works but due the VFO nature it's dificult to get AM because of frequency drift. I am now studing the inclusion of a more stable and external VFO. I can get FM due the nature of the design but only stations on high power so theres is sone issue on the gain/bandpass of the bandpass and rf amplificator filter.
The ceramic filters (10.7 and 455) were removed fron the low budget frequency counter radio, so their bandwith is too large ence the AM receptions has no great gain/selectivity relation. The VFO varicap I used it's an BB109 (I think, since there's no marks) laying around in the shack.
The front panel has one rectangular hole that was to get the low budget frequency counter in FM mode but since I allready broke 2 I have to find other "cheaper" alternatives... There's the big (red) an small button for tune and fine tune. I allready left provision for the squelch circuit and pot.
I also tryed with the schematic (basicaly the same) from: http://www.py2mg.qsl.br/receptores.htm see http://www.py2mg.qsl.br/Arquivos_PDF/CONTVHF.pdf
There are some subtle diferences in the way biased is done on the TA7358 from the posivite line to the 2.7uH choke and on the TDA1072, I think this last schematic (from PY2MG) is near the datasheets for the above IC's.
The box if from an old computer power supply the was broken. These make fine boxes for small projects and are dirty cheap!