Bill,
I thought as much...will search for a better part.? I sure wish
someone would publish the mods
for 2M!!!
73
Glen K4KV
On 8/9/2018 09:28, J68HZ wrote:
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I
built my U3A for two meters about 2 years ago now and I
opted to use a replacement to the BS170 that has a much
higher Ft¡ I don¡¯t recall what I used off-hand but it puts
out much more power than the BS170.? As I recall the
equivalent Ft for the BS170 is about 100MHz (although I
could be wrong because this is all by memory).? Even a
2N3866 silicon NPN has an Ft of 800MHz¡ and you can scan the
parts catalog for a MOSFET that has an Ft in the 500+MHz
range that would be a better choice.? Nothing against the
BS170¡ it¡¯s a great little switch, but it just gives up the
ghost a bit above 100MHz¡
?
?
Dr.
William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ
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Hans,
Wow, that is pretty low output!? I think the VHF fet in the
5W PA needs 50mw for
adequate drive.
Well, thanks for the reply, but what about the +V for the U3S
supply, should I use +12V ???
I already figured the components on the U3S for 2M were not
adequate.? I changed the inductor
feeding the BS170 drain to around 450nH, then changed the
caps involved to 0.001uf instead of
0.1uf.? A friend told me he made up a U3S for another guy on
2M.? He never mentioned any mods to
be done.
Regarding the scope, well it is a 100Mhz Rigol, but shows a
pretty decent signal on 2M.? I was able to
see some clipping when adjusting the bias, so backed off
until it looked clean.? Sounds good on the 2M
rx...
?
73
Glen K4KV
?
?
On 8/9/2018 01:41, Hans Summers wrote:
Hi Glen
It's been some years since I tried
a U3S on 2m. It was before the change to the bifilar
output transformer. It could be worth trying it also
with a single winding inductor.
My power output was 14mW (zero idle
current adjustment) and could be pushed to 17mW with
higher gate bias.
As Alan said - do you believe your
scope and probes? If a 'scope is what you used? You'd
need a pretty good setup to get any decent accuracy on
2m. In my case I used a spectrum analyzer for the
measurement and I was reasonably confident in it.
?
>
I am having trouble getting power out on 2M (using
it as a beacon).
>
Glen,
I'm not aware much has been said about 2m operation.
> I see 200mv on the collector of the BS170 fet
with 5V on the drain.
>
200mv on the gate? I'd expect it should be higher
but capacitance must
be high for 2m. Are your scope and probes good for
2m?
> I peaked the output adjusting the bias, for a
clean looking signal on
> the scope.
>
I thought the bias was adjusted to the point that
the FET just begins to
take current. You do not say where you look on your
scope but it is
squarish waves around the FET. Not until after the
LPF will it look clean.
Sorry, that's about all I can say.
73 Alan? G4ZFQ
> Am I supposed to use +12V on the U3S PA+ line?
>
> The 5W PA does amplify the signal, but I fear
the drive power is too low.
>
> I do have the VHF fet in the 5W PA board.
?
