On Aug 26, 2005, at 12:48 AM, Alex Eban wrote:
> 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.
I tested this idea by winding two chokes, One had a continuous
single-layer, and one had various gaps as per the Handbook. The
inductances were about the same and, to my surprise, so were the
resonances. This led me to conclude that end-to-end coils do not
decouple unless the gaps are quite substantial.
-- For maximal decoupling, chokes need to mounted at right-angles.
-- For maximal coupling, chokes need to be mounted end-to-end.
> 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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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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