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26th August 2008 18:25

Alan Yates wrote...

G'day Stephen,

Yeah the VN10KM is quite a good device. The metal tab make it easy to bond to a heatsink. The 2N7000 is a bit cheaper and easier to find, but can't be driven quite as hard. I superglue mine head or face down on the board to give them a bit of heat sinking - seems to work OK.

Driving MOSFETs with 74HC-series logic is very easy, but I must admit I don't often take that approach. The Pierce oscillator here has only just enough drive for the job, at a higher frequency I don't think it would work. I ended up adding a gate pull-up resistor that nearly turns on the output devices with the oscillator stopped, this improves the output quite a bit.

1.5 Watts is about the limit without impedance transformation to load the drain at a lower impedance. At a lower impedance the current becomes too high for smaller FET devices, so you may as well go to larger FETs like the IRF510 which need much more drive because of their larger gate capacitance. I think that is probably where I'd insert either a 2N3053 or 74HCxx to supply the needed drive.

I've had some luck with class-E designs at 30 metres using a single 2N7000 delivering about 1.5 W with virtually no heating, but the output network design is much different.

Regards,

Alan.

26th August 2008 12:03

VK2BLQ wrote...

Hi Alan,

I put together a similar TX with a VN10KM(has a tab which is good for heatsink) as PA and 74HC04 as oscillator / driver. Also outputs around 1.5watts for 200 odd mA at 12V input. It seems to be a bit too power hungry for small batteries.

73

Stephen

20th July 2008 00:53

Alan Yates wrote...

Phil,

Thanks very much for the signal report, the first for the beacon from Victoria.

Regards,

Alan

19th July 2008 23:30

Phil vk3yb wrote...

Hi Alan,

Nice signal tonight 50km SE of Melbourne.

73 Phil vk3yb

14th July 2008 22:29

Alan Yates wrote...

Ian,

Thanks very much for the report.

It is interesting that all the reports I've got so far are from the north of my QTH.

Antenna this end is now a base-loaded 3 metre vertical on my balcony using the railing as a counterpoise.

Regards,

Alan

14th July 2008 21:49

Ian Miller VK2BJT wrote...

Hi Alan,

Heard your beacon running 3.5787MHz on 14 July at 8.20pm EAST. RST 579 in Newcastle. Interesting experiment. Received on dipole 40' high running NW-SE.

Best 73's,

Ian VK2BJT