[Amps] RF Chokes

Mon Jan 31 11:12:16 EST 2005

On Mon, 31 Jan 2005 07:53:24 -0800, R.Measures <r at somis.org> wrote:

>
> On Jan 31, 2005, at 2:38 AM, Angel Vilaseca 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?
>
> Yes.  Mu = 125 ferrite rod material begins to saturate at c. 13MHz, so  
> it's a risky business.  Powdered-iron has the advantage of not  
> saturating.
>>
>> How large sould the ferrite core be and which mu should it have?
>
> For less than 13MHz, Mu = 125 is okay.
>>
>> Can a toroid core be used?
>
> Sure, but split-winding should be used to reduce distributed-C and  
> silicone rubber tape should be used to wrap the core.    However, a  
> single-layer solenoid choke is a natural for a HV-RFC.
>>
>> 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?
>
> Since there is only about 400V-peak across the output choke, much less L  
> is needed and less L means fewer resonance problems.  Also, since  
> resistors are virtually resonance-free, a better means of discharging  
> static on the coaxial feedline is a high-ohm MOF resistor across the  
> Load-C.  100k-ohm, 3w works well for 1500w amplifiers.  3 in series are  
> good for another 10db of output.
>>
>> 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.
>
> 2.5mH for the output choke is a laugher.  20uH is plenty - even for  
> 1.8MHz.  (XL = 500-ohms)

A 22 uH air coil choke can be wound on a 1/2" diameter form with a length  
of about 2" using about 22 - 26 gage wire. If I recall, it takes about 90  
turns, but don't hold me to that.

>>
>>
>> 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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>>>>
>>>>
>>>>
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>>>
>>> Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
>>>
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>>
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>
> Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
>
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Will

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