RadioBanter - View Single Post - Ameco AC-1 problem

April 8th 04, 09:42 PM

in article , N2EY at
PAMNO wrote on 4/8/04 07:59:

In article , Uwe
writes:

Using the L/C meter I wound a proper coil, I checked the calibration of my
plate current meter, I did a more thorough check of the grid current (it is
between 1 and 2 mA) and so on and so forth.

I think you mean "screen current".

And I did connect a dummy load (even though they don't respond or send out
QSL cards when you tranmit into them).

Yup!

None of the thing did make any real difference and the dip, the elusive dip,
was in the order of magnitude of maybe 2 mA, nearly impossible to see on my
meter.

Then I changed the circuit around as you suggested, testing the RFC and I
got a dip the likes of which I had never seen. The meter went slowly from
about 30 mA to 50 mA and then dropped to about 25 mA, I couldn't miss it.

But what does it mean. I gather my RFC is not ok. What is wrong??

I used a Series 4590 high current filter inductor I had around, it has the
Digi Key number DN 4528.

The RFC you're using is not meant for the appliucation. It's intended for much
lower frequencies. You can't tell that just by looking at it.

RF choke design is a matter of compromises. For example, the use of a powdered
iron or ferrite core will raise the inductance. But that same core does not
work at all frequencies, and may saturate from DC current in the core.

The biggest problem is called "distributed capacitance". In order to get lots
of inductance, you put on lots of turns, closely spaced. But each turn has a
small amount of capacitance to the turns next to it. All these small
capacitances add up, and as the frequency is increased they become significant
to the total reactance of the choke. At one or more frequencies the choke will
actually resonate - these are called "self resonant" frequencies. At some
frequencies the choke may act like an inductor of much lower inductance, or
even like a capacitor, because of the self resonances. Self resonance in a
choke can be found with a suitable dip meter.

RF chokes that are meant for applications like the AC-1 are designed to have
self-resonant frequencies far from the amateur bands.

Happy about the dip but still not clear on the deeper reasons...

Hope this helps.

73 de Jim, N2EY

Yes this is all very helpful. Indeed I was thinking that it would have taken
me a very long time to figure this out by myself, if at all.

I put another choke in there, a Hammond 1535B, the self resonant min. frequ.
is 1.3Mhz.

I guess it does take some deeper inside what parameters to look for since
this one improves things slightly but not yet altogether (slightly more
pronounced dip).

Who carries the sort of chokes you were refering to?

73 Uwe