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September 15th 05, 09:01 AM

ha scritto:

On 14 Sep 2005 06:48:38 -0700, wrote:

Hello,

ha scritto:

On 14 Sep 2005 01:24:16 -0700, wrote:

Reading from the ARF488 datasheet (this mosfet is made for RF
amplification
up to 60 MHz or so) I see input capacitance of 1400 pf typical, and
150 pf output capacitance typical.

Yes that is true. However that input capacitance is close to what I'd
use to bypass a 50mhz circuit. It's near RF dead short unless you
try to absorb it into the feed network. Even then it's a very low Z.

Yes, I understand it. What isn't clear to me is why the ARF488 is
specified
for 60 MHz and has input capacitance of the one of an IRF840.
On QEX of may/jun 1999 you can find a 300W 50 MHz amplifier with
ARF488 push-pull.

I'll check that one out. I do not have QEX back to '99 so it may take
a while to get it.

I can send you both the QEX article and the ARF488 datasheet (also the
IRF840 one, but this is really easy to find in the internet). Just
drop me an email and I'll send them, my email address is the one I'm
writing from.

I suspect the ARF488 is a RF rated varient of the IRFs and may offer
other characteristice that are more complient at VHF. I cannot find
any data or even a reference to ARF488.

I am a 6m op that love to play on that band so curiousity always wins.

same here. I've found that my 10W CW aren't terribly effective even
with
the 8m boom yagi lately, so I'm thinking about something more powerful
while the sun is sleeping. Seems like the old days when I could hear
and work VK with a J-pole are gone.

One of those 4cx150s will do over 100W CW and the PS for that power
is small. Like you say something different is always a draw.

Also these tubes need forced air cooling and the power supply would
need
to live near the amplifier (unless I want to send a cable with a kV up
to the antenna pole), if the mosfet way fails I'll probably go with an
expensive
bipolar amplifier or put the amplifier in the shack.

73
Francesco IS0FKQ