[Amps] RF Chokes

[R.Measures]

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)
>
>
> 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