Stable VFO with BF245

[BLF278 unlimited]

Hi there
I want to let you know about this project here:
http://www.jogis-roehrenbude.de/Sender/ ... stufig.htm
it uses a BF245 With a varicap Diode as Oscillator.
I built some VFOs with the BF245 the seem to be very stable if i let them war up for at least 20 minutes.
I know that its cheap to build VFOs with 74 logic devices but i tend to like this kind of low tech.
i already drove a output stage with an RD15 at 500mW Input and 15 Watts of output with this circuit just by using some Resistors.

please Let me know what you think of it, and if you only use PLL for tuning frequency, or do you also like this kind of stuff.

Also i want to thank Albert H, i never realised how freaki′n cool FETs are.

[Albert H]

FETs are wonderful devices!

I don't like etched VFO coils for two reasons - they're microphonic and they have lower Q than a conventional coil. However - if your VFO is working well for you, that's great.

I used to use a dual-gate FET oscillator:

40wvfo.png

The switch above the second transistor was the RF off / on relay contact, operated by the link receiver RSSI output. The oscillator ran all the time as it was most stable that way, and the PA fired up when the link was received. The VFO was modulated with the white noise from the link receiver when not linked, so it wasn't obvious that there was anything there. We used several dozen of these rigs when we were having them taken at the rate of one or two a day! The dual-gate FET was sometimes a 40673, a BF981, or BF900. The output devices were as shown or MRF238 or 2N6084. The driver could be an MRF237, 2N6080, R5174, TP1028. It all depended what we could scrounge up from the various component suppliers around the UK!

They were on PCBs and built on to pieces of extruded aluminium normally used for office room dividers, and had a PCB material "lid". The aerial feeder, audio input lead and power wires were soldered straight to the board! The power supply was based on a cheap car battery charger (from Halfords), and the link receivers were cheap Japanese band FM receivers which could be tweaked down to around 65 MHz. The receivers had an LED "tuned" indicator, and this was enough to drive a little reed relay and switch the output rig. These were remarkably stable and clean, and we could build several of them in an evening!

[BLF278 unlimited]

Thanks again Nice circuit.
As i said im not very into those FET transmitters, i never designed an transmitter based on an FET for Output Stage, well except for my little 4Mhz Transmitter that i built around an IRF510. Is there literature around about designing FET-Transmitter stages?
Can I replace A BJT with a FET with same specifications? For example: can i use an RD15 instead of MRF238.
My recent RD15 amp was developed, just by tuning for maximum smoke As Ray would say .

[Albert H]

No - it's not as simple as just substituting a FET for a BJT transistor! The designs of the stages will be quite different. A good starting point for a PA design is the datasheet for the RD15HVF FET. The circuit given there can be scaled to Band II, and you'll find that it works well.

[BLF278 unlimited]

Yeah that was pretty much what i did when i developed the RD15 amp, except for leaving out the filters having the risk to may cause crashes to some airplanes...
(Im just joking almost all of the HF Power goes in an oil can load)
Im pretty sad since i recently bought a book with more than 2000! Circuits and more than 200 of them are 80, 40, and 2 meter band transmitters or amplifiers. But although the book was published in 2009, most HF transistors used there, are extremely rare types. Even some of the ICs
Im not an Engineer so im able to design something from scratch, altough im able to build most circuits out there
I mean in some cases its easier to use f***n vacuum tubes, because they are not that rare.
ok enough of rage today.

[radium98]

hi can you put and show the rd15 amplifier to get inspired thanks

[BLF278 unlimited]

Here.




A Note for everyone newbie (like me ) who may wants to build this circuit:
for serious transmitting you need a proper filter. constructing a filter is, without good HF equipment (like me) specially without frequency analyser almost impossible, if you want to to broadcast anyway, you can use the filter described under schematics.

by the way has anyone here tried out putting an homemade ATU in front of the antenna? would be nice to have some information about this, because i may build my own.
In normal SWR meters there are often ATUs to match antenna, but i dont get how they work (i ve got an SWR-meter for up to 100Mhz)

[BLF278 unlimited]

F*ck.

[BLF278 unlimited]

...

[radium98]

? schematic pcb photos of building PA ?