[Amps] RF Chokes

[Will Matney]

Angel,

On the antenna side, there's only RF passing by and no DC in the choke to  
make it saturate easily. The DC current is what makes one saturate easy  
like in a filter choke in the PS. Even at that, the rod used in a rod type  
RF choke wont saturate as easy as a toroid as the magnetic path is broken.  
That's sismilar to a gap in an iron core choke but with a greater  
reluctance to the magnamotive force (MMF). Although, if enough AC current  
is applied to where the maximum flux density is reached, the core can  
still saturate. However that is thought of at design time in the maximum  
power rating. You want to size the RF choke so it wont open like a fuse if  
the HV blocking cap shorts so the line fuse will open. Most of the time,  
if it's rated for around 1 amp or so, the fuse will blow first without  
damaging the choke. I've seen some not use them, but that is some  
dangerous practice where somone could get electrocuted by a shorted  
coupling cap. The odds are low that one will short, but it's a possibility.

Best,

Will



On Mon, 31 Jan 2005 11:38:11 +0100, Angel Vilaseca <avilaseca at bluewin.ch>  
wrote:

> I am building a 3.5 to 30 MHz amp and I have been following this
> interesting thread.
>
> I have a question about the *other* choke in the output circuit, that is
> the one that is wired between the antenna terminal and ground. As I
> understand it, its role is to short-circuit HV to ground if case the
> tank circuit ever becomes hot with HV (plate cap failure). Its value
> should be about 2.5 mH 3 A.
>
> Most chokes of this value that are made available today on dealers
> catalogs have a ferrite core.
>
> My question is: can a ferrite choke be used, or should it have a
> non-magnetic core?
>
> If a ferrite core is used, could the ferrite saturate and generate
> harmonics?
>
> How large sould the ferrite core be and which mu should it have?
>
> Can a toroid core be used?
>
> It is interesting to see that lots of literature have been written about
> the plate choke and almost none about the output choke. Both chokes are
> in a circuit with high currents and voltages. Both are wired between a
> high RF point and RF ground. It seems that the worst problem with the
> plate choke - resonance - is simply not an issue with the output choke.
> Why?
>
> Also one would expect that the one between the highest impedance point
> and RF ground (the plate choke) should also have the largest reactance.
> Not so: abt. 200 uH for the plate choke, 2.5 mH for the output choke.
>
>
> Any ideas?
>
> Angel Vilaseca HB9SLV
>
>
> "R.Measures" wrote:
>>
>> On Jan 30, 2005, at 12:47 PM, Will Matney wrote:
>>
>> > David,
>> >
>> > There's not much to test that for it's self resonant spot. Take your
>> > coil and short out it leads.
>>
>> Experience has taught me to wire the HV choke in place before testing
>> for self-resonances.
>>
>> > Then use your dip meter and sweep all the frequencies it will be used
>> > at looking for a dip on the meter. If your not quite sure about the
>> > exact frequency at the dip, use a frequency counter with a pickup loop
>> > and place the dip meters coil beside it. It will show the exact
>> > frequency the dip meters set to (oscillating at). If you do find a hot
>> > spot, moving it to a unused band is accomplished by removing a few
>> > turns or even adding them. After that, just recheck it for where it
>> > moved to.
>> >
>> > Will
>> >
>> >
>> >
>> > On Sun, 30 Jan 2005 15:31:25 -0500, David C. Hallam
>> > <dhallam at rapidsys.com> wrote:
>> >
>> >> After reading the posts on the subject of RF chokes over last few
>> >> days, I
>> >> guess I am going to have to go back to the drawing board.  I am in
>> >> final
>> >> stages of completing a continuous tuning 3.5 to 30 MHz amplifier and
>> >> really
>> >> hadn't given the RF choke much thought.  Someplace (I don't remember
>> >> where)
>> >> I picked up a nice solenoid wound RF choke on a ceramic form that
>> >> "looked
>> >> right".  Now I guess I will have to do some serious testing before
>> >> applying
>> >> any power.
>> >>
>> >> David C. Hallam
>> >> KC2JD
>> >>
>> >>
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>>
>> Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
>>
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