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[AMPS] don't choke on this

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Subject: [AMPS] don't choke on this
From: (John Lyles)
Date: Mon, 30 Mar 1998 17:10:39 -0700

Got some measurements of various RF chokes  - "plate chokes" - that we
measured with the HP 4193A vector impedance meter last winter. We only
measured at two frequencies of interest, and looked for the peaks and
recorded them. Above the first self resonance, the phase went negative,
indicating capacitive reactance. Then it flipped back positive at the next
resonance (which was a series resonance?), then back to negative at the
next parallel self resonance. I'm setting up for operating just above 160
meters, at 2.8 MHz, for a scientific machine.

What this all shows is that you have to cut, measure, cut, measure and cut,
to make a choke do as you want, and then you still cut some more when you
install it into your amplifier. Or hook it up and watch for smoke and
fireworks. You decide.

We tried a couple to use for screen chokes as our screen voltage is 1500-2000.

B&W 800
90 uH, 800 mA
5/8 inch diameter close wound
1830 Ohms inductive at 2.8 MHz
4820 Ohms inductive at 5.6 MHz
First Parallel resonance at 8.6 MHz (high Z)
Additional resonances at 24 Mhz (looks like a short), 27 Mhz moderate Z
peak again

90 uH 3 Amp choke (? from Peter Dahl Co)
1 inch diameter close wound
1743 Ohms at 2.8 Mhz
5700 Ohms at 5.6 Mhz
First resonance at 8.4 Mhz, then 22.5 (low Z), 24 MHz moderate Z again

* * * * * * * * * * * * * * * * * *

Following study for a plate choke for 16 KV plate DC voltage, 20 KV peak to
peak swing
All #16 AWG magnet wire, to handle the average plate current

1 inch diameter, 16 inches long, 16 TPI close wound
2350 Ohms inductive at 2.8 Mhz
45 K Ohms at 5.6 MHz, resonance at 5.7 MHz

1 inch diameter, 10.7 inches long, 16 TPI
3970 Ohms inductive at 5.6 Mhz

1.375 inch diameter, 10.7 inches long, 17 TPI
1957 Ohms inductive at 2.8 Mhz
8620 Ohms inductive at 5.6 Mhz
resonance at 7.2 MHz

1.75 inch diameter, 10.7 inches long, 16 TPI
5620 Ohms inductive at 2.8 Mhz
resonance at 4.5 MHz

1.75 inch diameter, 12 inches long, 16 TPI
7320 Ohms at 2.8 Mhz
7060 Ohms capacitive at 5.6 Mhz
resonance at 4 MHz

289 uH homemade, close wound, 12 inches long
1.75 inch diameter, 220 turns
20 K ohms at 2.8 MHz
First resonance at 3.2 MHz

190 uH homeade, 1.75 inch diameter, 192 turns, 12 inches long
8130 Ohms inductive at 2.8 MHz
Similar to above, resonance moved to 3.8 MHz

143 uH homemade, 1.75 inch diameter, 168 turns, 14 inches long
3700 Ohms inductive at 2.8 MHz
896 Ohms capacitive at 5.6 Mhz
resonance at 4.6 MHz

156 uH homeade, 3 inch diameter, 8 inches long
4030 Ohms inductive at 2.8 Mhz
resonance at 4.8 Mhz

* * * * * * * * * * * * * * * * * * * * * *
Studied two B&W Airdux inductors in series
2 inch OD, 8 Turns per inch
1206 Ohms inductive at 2.8 MHz
3820 Ohms inductive at 5.6 MHz
resonance at 8.4 Mhz

2 inch OD, 16 TPI
9620 Ohms inductive at 2.8 MHz
resonance at 3.6 MHz, greater than 200 K Ohms

Added two of the 8 TPI inductors in series, axes aligned
resonance now at 4.1 MHz, lower peak,series resonance at 8.5 MHz

Yes they added, and the resonance halved.

* * * * * * * * * * * * * * * * * * * * * *

For the amplifier, we chose two different plate chokes to try and keep the
shunt impedance high enough that we don't get a lot of losses and RF
current flowing. The 2.8 Mhz tuning will require RF choke D, and the 5.6
Mhz tuning will require choke B or C. Since I don't have to be able to tune
to one or the other in a short time, this is adequate, I can open the box
and replace the choke. To get optimal performace for both frequencies
(harmonically related) will be more difficult as large chokes such as these
have a lot of interturn capacitance. And I cannot afford fireworks, the
voltage is extreme in there.

I apologize to you ham amplifier designers as your frequency range is soooo
broad. Bound to hit a sneak resonance if you're not careful, not to mention
a lot of losses across your plate circuit. There have been a number of
segmented schemes which seem to work in a fashion. Seems to me that using a
GDO is like setting your timing in your automobile engine by blinking your
eyes fast. It sure helps to know if you've got a series or parallel
resonance nearby. You can short the leads and measure one, open them and
measure another. It's all very artistic. And a lot of trial and error. The
enclosure affects it too.

One old way to overcome the limitations of too low a Z and too low a
resonance is to use a banked winding. This is done by some of the pros in
large SW transmitters. Take a few turns, then back over a few, then wind a
few more, then step back on the form, then a few, then back over, and so
on. It's hard to picture in words, but the thing looks horrible. The effect
is to reduce the turn to turn capacitance, yet get a lot of inductance.
With reduced capacitance, the self resonance may be moved higher, and yet a
lot of inductive reactance realized (lot of L) over the desired range.  I'm
surprised that hams haven't tried it (or maybe they did.    Bill Orr, did

At the DC side of the choke, we didn't skimp on the bypass capacitor. It's
a big expensive ceramic thing rated for a lot of current with low
inductance. That way, if we get a harmonic banging through, it won't go
anywhere but to ground. I've seen them destroyed by using a bad choke, as
someone alluded to today on this forum.


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