On 15 Sep 2005 01:01:40 -0700, wrote:

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

After looking at the data sheets and the apnote it confirms what I've
said.

The APT/ARF488 is a RF device and not close to the IRF parts. One
difference easily noted is the lack of the avalanche diode present
in the IRF parts. It's appearance spec wise is that of a low cost
flavor of currently know RF types without the high cost package.
I may add the ARF488 is unobtainium, apparently they have
discontinued it and gone to newer versions.

The apnote confirms the need for elaborate input match for a rather
low input impedence that is highly reactive. The 50Mhz amplifier
is not broadband and for 6m would be classified as very narrowband
with it's specs covering on the 50-51mhz range. Their use of
striplines to affect input match and output match contribute
to the narrowband behavour. It is a high power amp at 300W with
an IMD3 of -25dbm which is only ok, not great. You also need an
85V supply and have to deal with cooling both the supply and
the amp.

You could try an IRF parts in that circuit but read the apnote well
as it explains the problems and techniques applied to overcome
those problems at 50mhz. However the values required ot match
the IRF vs the ARF parts may vary greater than the design limits
resulting in poor performance.


Allison
KB1GMX

Hello,

first of all, I just reread my original post and I've written ARF488
instead
of the correct ARF448, sorry for the mistake.

ha scritto:

On 15 Sep 2005 01:01:40 -0700, wrote:



After looking at the data sheets and the apnote it confirms what I've
said.

The APT/ARF488 is a RF device and not close to the IRF parts. One
difference easily noted is the lack of the avalanche diode present
in the IRF parts. It's appearance spec wise is that of a low cost
flavor of currently know RF types without the high cost package.
I may add the ARF488 is unobtainium, apparently they have
discontinued it and gone to newer versions.

yes, indeed it's very difficult to find these mosfet today.


The apnote confirms the need for elaborate input match for a rather
low input impedence that is highly reactive. The 50Mhz amplifier
is not broadband and for 6m would be classified as very narrowband
with it's specs covering on the 50-51mhz range. Their use of
striplines to affect input match and output match contribute
to the narrowband behavour. It is a high power amp at 300W with
an IMD3 of -25dbm which is only ok, not great. You also need an
85V supply and have to deal with cooling both the supply and
the amp.

well, the real problem is difficulty in matching and poor IMD3,
we still are restricted to the 50-51 MHz part of the band and
I don't remember to ever have heard anyone over 50.300 MHz here
in EU.


You could try an IRF parts in that circuit but read the apnote well
as it explains the problems and techniques applied to overcome
those problems at 50mhz. However the values required ot match
the IRF vs the ARF parts may vary greater than the design limits
resulting in poor performance.

indeed, and the diode in the IRF part could be a problem, right?

I'll probably start to search a good bipolar transistor for this
project.

thanks

Francesco IS0FKQ