Walking over to my c-pole I felt the common mode choke was boiling hot. So I changed antennas and continued my operation.
Back home I began my investigation in what went wrong. Immediately there were two suspects: the antenna (high swr for some reason) and the choke (too low choking impedance).
The antenna was fine when I used it the first time and I did not change anything in the mean time. So the choke seemed a more logical candidate. The purpose of the choke is to minimise the amount of common mode current that you will generate with an unbalanced antenna like the C-pole. If the choking impedance is too low however there will still be a considerable common mode current left that will generate heat in the choke.
|W2DU style choke with ferroxcubes beads
|Removing turns of RG58 from two stacked FT240 toroids
On my first C-pole I used a different current choke, designed on the basis of the excellent information by the late G3TXQ. I used two FT240-43 ferrite toroids stacked with a couple of turns of RG58. It is bulkier and heaver though than a W2DU choke, that is why I changed it.
That toroid based choke also became very hot on my first c-pole but I later discovered the antenna itself had an issue - causing high swr - causing high voltages over the choke.
So I returned to this old choke - abandoned but not scrapped. I measured it and it had an interesting profile. It was particularly useful in the 80m band range (5k Ohm) but certainly not bad in the 40m range (2.5k Ohm).
|Measurement #1 of the choking impedance and transmission loss on my balun
I decided to take it apart and remove one of the RG58 turns. As expected the choking maximum moved up. The profile now suited a 60m antenna perfectly while impedance was higher in the 40m band. After removing two more turns I got a maximum in the 40m band. The choking impedance is now more than 5k Ohm there. That should do the trick.
|Measurement #2 after removing 3 turns of the coax
When I have the time I will go out /P with my C-pole to check it without a choke - just to be sure it still is resonant in the 40m band - and then add my old toroid based choke.