The main problem stems from the 160 meters operation!
For the higher bands, chokes are usually in the range of 68 to 100
microhenries. These are easily wound in a solenoid manner and if you space
wind the last 20 turns or so, chances are you get a choke without resonances
in the ham bands. On the other hand for 1.8 MHz operation the coil
inductance goes up in the millihenry range: a lot of turns, crammed
together. This is prone to parasitic resonances and usually you will find
that the better amplifiers DO NOT include the top band, or else, they have
some queer arrangement, like two chokes in series or some oddly wound choke.
I remember very well the one made by Barker and Williamson , wound in
sections, each on of a different number of turns. At some time Collins used
them in military radios that covered 2 to 30 MHz continuously. I don't know
whether someone is making them today. This was on of the main reasons that
in military equipment (high power!) they dispensed with the choke and moved
over to series feeding of the high voltage supply. The GRC 106 was one good
example. It employed series feed of the 2400 VDC supply and took the power
out through a link on the coil.
Alex 4Z5KS
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