RadioBanter - View Single Post - Ameco AC-1 problem

April 6th 04, 02:49 AM

in article , N2EY at
PAMNO wrote on 4/5/04 18:59:

In article , Uwe
writes:

I will just have to fiddle a bit more
with the pi network (since at the B+ voltages suggested here my plate
current would be way high) and I will have to live with the chirp.

Are you getting a "dip" in plate current? If not, the coil is probably too
large or too small. Unless you get a real dip, the output network isn;t right.

I have used a very similar transmitter with 350 volts on the plate, and the
dip
is clean and pronounced.

Jim, the original docs I got for this tx call, at 40 m, for a 15 turn coil
on the coil form provided with the kit, which I hear was 1.25" diameter.
If I use the formula for air coils this turns out to be roughly a 22
microhenry coil.

The coil which works best with my tx is 8 turns on a 1,125" ceramic core.

To get guess work out of it I just bought and built a L/C meter and
measured my coil to have 2.7 microhenry. So I am way off, but it works, sort
of.

The air caps are 36 to 420pf at the plate and 15 to 728pf at the antenna, so
that seem right.

All this happens with B+200V and 35 mA plate current.

Older ARRL handbooks give typical values for pi network for 50 Ohm antenna
loads and my values are in range for the caps but my coil is too small.

The ouput voltage on my antenna measured with a scope is up to 75 volts peak
to peak, with a 50 Ohm load that would mean I get more out of the tx than I
put into it and I am not of the sort who says this might happen.

So my conclusion is, and tell me if this sounds right, that I have an
antenna which is far from 50 ohm resistive at 40m and that that makes
everything weird.
The dips in plate current are nearly imperceptible and they are not aided by
my 250mA full scale meter. They may be 2 or 3 mA.
I tune with the help of my scope.

73

Uwe