In article , Uwe
writes:
in article , N2EY at
wrote on 4/6/04 12:29:
Uwe wrote in message
...
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.
22 microhenries? I get more like 8 microhenries using the formula
L = (a * a * n * n)/([9 * a] + [10 * b])
where
a = radius of coil in inches
b = length of winding in inches
n = number of turns
Well, the way I use the formula is 1.25*1.25*15*15/((9*1.25)+(10*0.45)) =
351.5/15.75 = 22.3
O.45 is the length of the 15 windings.
Do I not use the formula properly??
You used the coil *diameter* where you should have used the coil *radius*. A
coil with diameter of 1.25 inch has a radius of 0.625 inch.
Compute
0.625*0.625*15*15/((9*0.625)+(10*0.45)) =
and see what you get.
My meter is built into my bench power supply (thats why it reads up to
250mA), so I am measuring plate and screen current.
I put in a second meter which would only measure the plate current but its
reading is practicly identical to the first one, as if there was no grid
current.
That's odd.
My antenna is a dipole of about 75ft. length each side, connected with a 50
ohm coax, no balun or such things.
150 feet total length? That's not resonant on 40 meters, and your SWR with 50
ohm coax is probably quite high.
A half-wave 40 meter dipole is about 66-67 feet long (33 feet each side), and
will have a fairly low SWR on 40 meters when fed with 50 ohm coax. The next
length that will give a fairly low 40 meter SWR is about 205 feet overall (102
feet each side). Such a dipole is one-and-a-half waves long.
These are "ballpark" figures, not exact ones.
How high is your dipole?
I agree with Paul Morphy that a simple dummy load is best for testing. His
suggestion of paralleled noninductive resistors is excellent.
I will need a few days to try out some of the things you and also Paul
suggested and it might really be a good idea to get an SWR meter and a
tuner.
That will work, but first get the rig working correctly into a dummy load.
All in due time and I will surely get back to you.
If it takes me a while to respond, it's because I'm away from the computer.
Thanks for the help
You're welcome.
73 de Jim, N2EY