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@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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