RF Plate Choke
RF Plate Choke and Trap Testing and Operation
Related pages:
One of the biggest issues is trying to use too much inductance in a wide
operating frequency range plate choke. Physically large chokes over 50-100 uH
almost always develop low impedance resonances someplace in HF. Choke
physical size and choke inductance are the determinants. More inductance
moves unwanted resonances lower. More distributed capacitance to the outside
world does the same. It is the combination of shunt capacitance to the outside
world and choke inductance that creates the problem.
Chokes and large
inductors (including roller inductors in antenna tuners) should be tested or
measured for series resonances. Ideally test setups would emulate the final
choke, trap, or inductor stay capacitance caused by the final mounting and
surroundings.
There are two accurate methods to determine resonances of a plate choke,
inductor, or
trap. The first simple method uses minimal equipment and can give very accurate
relative go/no-go results.
Simple Test Method
This method uses a transmitter or transceiver covering the operating range, dummy load, T-connector,
and a series incandescent lamp
indicator.
1.) Minimum brightness trap (or parallel) resonance. Maximum brightness
unwanted choke series resonance.
2.) Use a calibrated RF impedance measuring device with transmission
line normalized out
1.) Trap test, try to keep the trap in open air and minimize stray
capacitance. Stray capacitance affects resonance.
2.) Choke test, try to space choke the same distance from ground plane as it
is from closest mounting box wall. Wall spacing affects resoance.
Note: It is only possible to get close without
special fixtures!
Skip to Testing
Chokes
Impedance Limits
An amplifier’s plate choke needs a certain minimum impedance throughout the
amplifier’s operating
frequency range. Let’s consider a typical pi-network amplifier.
The minimum required plate choke impedance varies with the operating impedance
of the output device, the choke’s Q or loss resistances, and compensating capacitance
available in the tank tuning capacitor. The RF plate choke must have several
thousands of ohms when the amplifier uses high voltage tubes. RF plate choke
impedance must be high enough to limit RF choke current to safe values. A
conservative RF current estimate would be E/Z=I where E is DC plate voltage and
Z is choke impedance. A 200 uH choke on 1.8 MHz only has about 2600 ohms
reactance, so a 3000 volt anode swing causes 1.15 amperes peak RF current. RF
choke dissipation would be choke Erms^2 / Rp. A 200uH choke with a Q of 40 would
dissipate something less than 2100^2 / 104000 = 42 watts.
42 watts is significant power, although perfectly acceptable in a single
layer ceramic form choke of fairly large dimension. The very high Q
requirements and the large RF currents illustrate why iron core or ferrite core
chokes are generally unacceptable at high anode voltages, and why the general
trend is to use physically large solenoid chokes on low loss high temperature
forms.
A second design issue appears. As frequency increases, choke impedance
increases. At some frequency choke impedance will peak. Significantly above peak
impedance, choke impedance will abruptly drop. This effect becomes a real
problem in 160-10 meter amplifiers. A choke adequate for 160 meters in high operating
voltage use develops
unwanted series resonances
someplace up above lower-to-middle HF.
Series Resonances
Series resonances produce very low choke impedance. This is accompanied by
very high RF voltages from the choke to the choke ends and to the outside world.
I’ve seen chokes arc across several inches of air in amplifiers operating at 6-8
kV supply voltage.
Series resonances are formed when the choke looks like back-to-back L-networks.
The choke winding forms a long series inductance and stray capacitance tunes the
winding. This of course could be viewed as a long helical transmission line of
very high impedance, but L networks provide a closer if not significantly less
complex analogy. Electrically, at the lowest frequency series-resonant point, the choke looks like this:
Fig 1
The SPICE sweep model below shows voltage at point A.
FigThis model very closely approximates an early Ameritron plate choke. This is why 26-27 MHz
operation generally destroys an Ameritron choke.
This model is with 200 volts peak-to-peak excitation, NOT the full voltage of the
amplifier. The waveform below shows RF current through the choke. DC current is
not shown, and would be superimposed on the RF current. 10 volts=1 ampere:
Ever wonder why an amplifier can arc and bang when operated near self-resonance
of the plate choke? The text above should explain it to you! Voltage at the
center of the choke can be so high the choke arcs for several inches. The choke
becomes a good Tesla coil, with peak RF voltage near the coil’s middle for the
first-order resonance.
Mounting the Choke
At upper frequency limits, a typical choke can have peak voltages several times
the operating dc voltage at some points along the windings. Stray capacitances
also tend to concentrate RF currents into small areas of the winding. Because
stray capacitance aggravates voltage and current stresses, and because stray
capacitance shifts series-resonances lower in frequency, a clear location for
the choke is most desirable. If possible, a choke should be 1/2 its winding
length, or four times the winding diameter, away from things that add
capacitance. This includes large dielectrics, which increase stray capacitances
from increased dielectric factor loading of electric fields.
How to Move the Series Resonance
Choke designs require a
certain minimum inductance, to ensure reasonable impedance near lower frequency
limits. If the choke is physically large, and if reactance is fairly large at
the lowest frequency, and if a wide frequency range is covered, unwanted
series-resonances can fall within desired upper operating ranges. This can
result in very high currents and voltages from normal fundamental RF excitation,
although it is sometimes blamed on a “parasitic” by less knowledgeable designers
or technicians. The solution is to move undesired series-resonances outside
desired frequency ranges.
Many publications, including the ARRL Handbook, and many personal opinions,
claim
choke winding gap designs are “magic”. At one time, the ARRL Handbook reported
there was no rational, logical, reasoning
behind removing sections of solenoid choke windings. This is not true, and
really only shows
those who make such statements do not understand why a choke has series
resonances, or how to move the resonances. If we understand why a choke
misbehaves on some frequencies, we will easily understand how to “correct”
problems in the RF choke by using gapped windings.
There actually is a method to designing choke winding gaps. Since the series-resonance
problem is rooted in the choke behaving like two (or multiples of two)
back-to-back L-networks, the solution is very obvious.
When moving unwanted series-resonances, the system’s necessary minimum
inductance often rules out significant inductance reductions. Turns must be
added or removed where they have the largest effect on series resonances, with
minimal reduction of lower frequency inductance. To do this, the designer must find the highest voltage
area of the winding at the problematic frequency, and reduce capacitance
at that physical point in the winding.
The
capacitance involved in the series resonance is generally on the order of a few picofarads,
or less. The designer generally does not want to locate the choke center near metal (or even
dielectrics other than air) because metallic masses, or even dielectrics other
than air, will move series resonances lower in frequency.
Generally, the designer wants to do everything possible to move series resonances upwards
in frequency, keeping as many high-order resonances as possible above the
highest operating frequency.
The most expedient way to move series resonances is to change the winding pitch
at the very center of the choke for lowest resonance, or in the electric field’s “hottest”
winding areas, in the case of
higher-order resonances. A typical solenoid choke of constant pitch has
capacitance controlling the lowest resonance located in the very middle of the winding
length. This is area where inductances, forming the two phantom back-to-back
L-networks, are evenly distributed in both directions. At this inductive center-point, just one or two picofarads of capacitance can move the
series resonance as much as 50% in frequency! The exact amount of movement
depends on winding pitch, form diameter, and the overall inductance of the
choke.
The gaps in the Ameritron chokes (or any gapped choke I design) are not placed by accident.
The winding gaps are placed by design.
The most effective way to move a resonance is to remove wire from the area where
voltage is at maximum.
- A full winding choke is tested through all desired frequency ranges
- If a series resonance appears inside a band, the highest voltage area is
located - Wire is removed from the highest voltage area to shift unwanted
resonance up - The choke is retested
The 1990’s Ameritron choke design appears on the far left. Starting with a
continuous winding core, the fully wound choke had resonances at 10 MHz and 20 MHz. Looking at
voltage hot spots, sections of windings were removed. The lower gap near the
choke middle moved the 10 MHz resonance
up to 12.5 MHz or so. This shifted the upper resonance from 20 MHz to near the
24.8 MHz band. The upper gap moved the resulting second-order series-resonance from
25 MHz up to 27
MHz. Without the upper gap, the second overtone resonance
is too close to 24.8 MHz. The gaps park unwanted series-resonances between 30 and 20 meters, and at the
lower end of 11 meters. This results in the highest possible inductance for 160
meters, while keeping harmful resonances away from normal operating frequencies.
On early solenoid chokes, before we had WARC bands, resonances
were more easily parked in clear spots. There was no need to move higher
order resonances out of an “overtone” or harmonic relationship with the
lowest frequency
series resonance. The double gaps reflect a change in choke design from the
Heathkit and Ameritron chokes I designed before WARC bands existed.
Variety of winding styles from left to right:
3-unequal section tight wound. 245 µH
nominal with resonances at 11.7 and 16.3 MHz.
4-equal section tight wound. 229 µH
nominal with resonances at 13 and 16.8 MHz.
Single section space-wound. 117 µH
nominal with resonances at 19.5 and 27.3 MHz.
Single section space-wound iron core.
906 µH nominal with resonances at 13.5 and 24.1 MHz.
The iron core choke has low Q, and runs much hotter for a given RF current.
Series resonances are also very wide, the lower resonance rendering this choke
unusable between 13 and 14.5 MHz.
From the above data, we can reasonably conclude an evenly-spaced winding
produces the poorest multiple-band high-voltage choke. The choke can have a great deal more inductance by using
a tight
winding pitch, with harmful resonances moved by intentionally inserting large gaps
at appropriate places.
Testing the Choke
Open air test fixture
A standard transmitter, dummy load, and 12 volt lamp can be used to effectively
test an RF plate choke.
Electrically, keep the transmission line from the “T” connector to the choke and lamp very short.
Less than five electrical degrees (one-half foot at upper HF) is generally short
enough.
The other two transmission line lengths are not critical.
The best place to test any choke is in the actual operating location with the
choke cold end bypassed to ground normally. The the top end of the choke should
be disconnected from
the tank system and tubes, and the tank shorted or detuned so it does not act
like a suck out trap. The lamp would go between the disconnected top choke
end, and the TL supplying RF. The lead above the lamp to the TL can be somewhat
longer, even 5 inches is unlikely to have a large effect. The lead from the lamp
to the choke must be short.
Leads from the tap point on the transmission line must be short, as in the bench
setup below!
4-section choke under test for series resonance. The transmitter is set at 25
watts and the VFO swept up through the frequency range until the lamp glows.
Adjust power so the lamp lights, but does not burn out. In this case series
lower resonance was at 12.985 MHz. Moving up from lower HF, this is where the
lamp glows brightest.
Spinning the VFO up, we find another resonance around 16.8 MHz.
Finding the “hot” area:
The “hot spot” or “hot spots” can be located by sliding a well-insulated metal
tipped tool along the choke.
Adjust the radio’s frequency to find maximum lamp brightness. Without adjusting
the radio, move the insulated tool’s metal tip along the choke and watch for the
spot where the bulb dims the most. This is the “hot spot” where voltage peaks.
If you remove wire in this “hot area”, series resonance will shift upward the
maximum possible amount for the least change of overall inductance. To lower
self resonant frequency, either add dielectric (a thick coating of insulating
varnish) or rewind with closer turns spacing.
The higher frequency resonances will be in two or more places out near
choke ends. The lowest frequency resonance is always near choke center.
When you find a frequency with the largest hand effect near the center of the
choke, you can be pretty sure you have the lowest self-resonant frequency.
Network Analyzer Measurements
When used with proper fixtures, network analyzers provide the most accurate
measurements.
This method also works with traps, although traps should have several inches
spacing above the ground plane. Personally, I test traps mounted centered in a
three foot copper wall box that is open on one side.
My test fixtures have precision 50-ohm small loads. I can
stretch the clip lead out and preform an open, short, and load base calibration.
I use a variety of low density foam spacers to hold the choke (or trap). I can
easily move the connector to different places. This choke is 13
inches long!
Look at Z. Test results near series resonance are:
Minimum required Inductance and Choke Current
The minimum necessary choke inductance in an amplifier or other RF system is dependent on five things:
- RF voltage across the choke’s impedance
- Choke Q and ability to dissipate heat
- Bypass capacitor’s ability to handle current
- Q increase that can be tolerated in the PA anode system
- The extra plate tuning capacitance available
RF choke impedance varies widely with frequency. At low frequencies the choke
looks like an inductor either shunting (parallel equivalent) or in series
(series equivalent) with a resistance. Let’s look at a Heathkit and Ameritron
choke I designed in the late 1980’s.
This choke has the following characteristics (from an Excel spreadsheet I used
in AL80B design work):
| F MHz | L or C | Xs | Rs | Xp | Rp | Q |
| 1.8 | 255 µH | 2550 | 22 | 2550 | 295590 | 116 |
| 3.5 | 275 µH | 6060 | 85 | 6061 | 432127 | 71 |
| 7 | .36 pF | -315000 | 3851 | -327203 | 26764200 | 82 |
| 10.15 | 2.3 pf | -6900 | 79 | -6901 | 602737 | 87 |
| 12.1 | 0 | 0 | 504 | 0 | 504 | 0 |
| 14 | .6 pF | -19200 | 380 | -19952 | 1008110 | 51 |
| 15.7 |
0 |
0 | 550 | 0 | 550 | 0 |
| 18.2 | .6 pF | -14300 | 220 | -14357 | 933205 | 65 |
| 21 | 1 pF | -7800 | 126 | -7846 | 486452 | 62 |
| 24.8 | 1.25 pF | -5240 | 87 | -5368 | 467250 | 60 |
| 28 | 1.37 pF | -4130 | 69 | -4229 | 291096 | 60 |
|
Rp = Rs + Xs2 / Rs |
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|
Xp = Xs + Rp2 / Xs |
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|
Note: L, C, and some other values will not be textbook perfect because they are subject to measurement tolerance and rounding errors. Values at or near parallel resonance (40 and 30 meters) may be subject to impedance measurement errors because measurements of extreme impedances is difficult. |
Choke Heating and RF Current
Choke RF dissipation can be determined by voltage across the choke and Rp of
the choke. The standard formula E^2/R applies. The above choke on 160 meter
looks like:
From the above table let’s use Rp (parallel equivalent choke
resistance).
Using E^2/Rp and assuming an RMS tank voltage of .6 times dc voltage, at 2800
volts dc supply we would have a maximum choke RF power dissipation of
(2800*.6)^2 / 295590 = 9.55 watts
The model to the left approximates these values, and is what we
would expect for a 1000 watt output class AB power amplifier with 2800 volts dc
on the anode.
We can see how critical choke Q becomes. We can reduce choke impedance if
choke Q is high, but we have to be mindful of low choke impedances with low Q.
In other words, choke Q becomes increasingly critical as choke impedance is
reduced. This, and saturation problems, are why magnetically-soft iron-core
chokes are generally a bad idea in high-impedance circuits of high-power
multiple band amplifiers.
Below are typical choke dissipations based on an amplifier plate voltage of
3000 volts for 160-10 meters. Spreadsheet like this show why operation on or
near series-resonant points is catastrophic! Also, heat dissipation is not
spread over the entire length of the choke. On higher frequencies, and
especially near series-resonance, heat (or loss) is concentrated in certain
areas of the winding. This is why calculations or series-resonance measurements
alone will not prove safe operation, although they clearly will indicate unsafe
operation.
Below is a copy of an excel spreadsheet showing worse case dissipation (Pd)
calculations used in my AL80B amplifier design:
| Freq | Rp | Q | Ep | Pd |
| 1.8 | 295590 | 116 | 3000 | 11.0 |
| 3.5 | 432127 | 71 | 3000 | 7.5 |
| 7.0 | 26764200 | 82 | 3000 | 0.1 |
| 10.15 | 602737 | 87 | 3000 | 5.4 |
| 12.1 | 504 |
0 |
3000 | 856 |
| 14 | 1008110 | 51 | 3000 | 3.2 |
| 15.7 | 550 |
0 |
3000 | 844 |
| 18.2 | 933205 | 65 | 3000 | 3.5 |
| 21 | 486452 | 62 | 3000 | 6.7 |
| 24.8 | 467250 | 87 | 3000 | 6.9 |
| 28 | 291096 | 69 | 3000 | 11.1 |
| Note:
Higher frequency heating, especially above 21 MHz, can Series resonant frequencies (in red) assume tube can |
since 1818Z Oct 10, 2010
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