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@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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