raypsi wrote:
All I ever see for Pi coils is cylindrically wound single layer. I
remember when I built my first 6146A output cw transmitter back 40 yrs
ago, I used a B&W pre made coil for the Pi.

Depending on the Q and plate resistance you see coils for multi band
use up to 33 uh, in transmitters.

The only place I see pancake coils used is in Tesla coils, one of
which I built with a pre made 18" tall secondary coil. I wound my own
primary pancake coil.

Done right a pancake coil takes up alot less room than a cylindrically
wound coil.

A six inch diameter pancake coil can have the same inductance as a
three inch diameter by six inch long coil with the same size wire.

Anybody know why I shouldn't use a pancake coil in my final? I just
bought a 4-1000A. from ebay.

73
n8zu
Pancake coils are usually wound with several sections, not one large
section. The problem with these chokes is that they may exhibit
resonance within a ham band, this would be bad as the coil would
'explode' as it absorbed the power. When we had only 5 bands,
80,40,20,15 and 10 meters it was easy to design such a choke keeping any
self resonance out of the ham bands. Now that we've added 60,30,17 and
12 meters the odd's that those old chokes don't self resonate in a ham
band is not the good anymore. A single layer choke will have only one
self resonate point, and it can be made to be above 30mhz (if it falls
at 40mhz the choke would also work on 6 meters). A pancake coil may
have several self resonance points.