EMRFD Message Archive 5605
Message Date From Subject 5605 2010-12-30 13:07:33 Wes "Polyvaricon" capacitor Hi all,
I did an experiment that turned out to be both interesting and perhaps useful for the folks in the greater QRP community. A popular capacitor type is the so called polyvaricon variable. I'm not sure if this is a proper term or just something that has evolved, but that is not the point. The capacitor is a variable element that is physically small and rugged. It is built much like a traditional air variable, but the plate spacing is smaller. The space between plates is then filled with a plastic. I have a batch of them that I purchased from DigiKey or Mouser back when they carried such things. I think that's the proper heritage, but don't hold me to it. The part in my junk stock was definitely unused.
Anyway, I was curious about the quality of these parts. Such things are popular as tuning elements in QRP transmatches. So a Q measurement was done. I've been on a kick recently to measure the Q of capacitors. Anyway, the Polyvaricon that I measured was especially bad. The result was a Q of about 500 at 10 MHz when the capacitor was adjusted for C of 100 pF. Q was even worse at 350 or 400 when the capacitor was fully meshed for C=270 pF. All of the details can be found in the "experiments" section of my web site at w7zoi.net.
Just to be sure that I was not making some colossal mistake, and I've been there before, I used the same VNA calibration and measured the Q of a good air variable immediately after I had measured the polyvaricon. The air variable was excellent at Qc=3100.
I will measure some additional versions of this capacitor type. If the rest are as bad as this, I'll be rebuilding some gear to replace the junk parts.
5606 2010-12-30 13:48:06 AD7ZU Re: "Polyvaricon" capacitor Just curious..
have you measured any of the air variables originally used in 5 tube AM radios?
usually a dual section 365pf in 1 section the other section is perhaps 200pf?
I have several old dual section air variables in the "not quite junk box" and
thought of using one in lieu of the polyvaricon that came with the kit. mostly
because the polyvaricon is just not as rugged for a transceiver kit I have on
the shelf. The thought being the old 365pf variable would be more stable and
less susceptible to shock .. it has a nice ball baring rotor mount and trimmers
for both sections. I also have a vernier knob that fits the shaft. I have not
considered any effect of a higher Q tuninc cap in the VFO.
5607 2010-12-30 14:08:27 ae5ew Re: "Polyvaricon" capacitor You were expecting some magic? AM radios are mostly junk. So I don't expect much from the parts used therein.
5608 2010-12-30 14:09:50 ae5ew Re: "Polyvaricon" capacitor If the air variable is in good physical condition, go for it!
5609 2010-12-30 15:25:28 Kerry Re: "Polyvaricon" capacitor G'day Wes.
You queried the name Polyvaricon in your very interesting paper on Q.
The Mitsumi company claims to have invented and named these devices;
I think the name is very descriptive of a polyethylene-insulated
variable condenser (lovely old word!).
Like hoover, aspirin, sellotape and many other words it has gone from a
brand name to a generic one.
There's an engineering lesson in your findings; polyvaricons are very
good devices for their intended use (small cheap broadcast radios).
I don't think that any part should be condemned for fulfilling its
It's only when we try to use a part for more-demanding service that it
was designed-for that we discover its limitations.
Sometimes, if the gods smile on us, we discover that a mundane part can
be used at a higher level than its design one; sadly the Polyvaricon is
not such a part.
[Non-text portions of this message have been removed]
5612 2010-12-30 17:03:48 kb1gmx Re: "Polyvaricon" capacitor Hi Wes,
I've never used them for that reason. They are good enough for AM
radio and crappy FM portables.
Their advantage is they are compact, disadvantage poor minimum C
and high loss in the plastic film used. Note, some are better than others but mica padder they are not.
I use them but in low Q tuned circuits and NEVER in VFOs.
5613 2010-12-30 17:13:47 kb1gmx Re: "Polyvaricon" capacitor The effect of Q in a VFO impacts noise and noise bandwidth of the oscillator.
In the case of Regenerative radios the losses in the plastic have a negative impact on the performance.
My problem with using them is it's another plastic cap that can vary with temperature more than aluminum or brass separated by air. This impacts drift and general stability.
that and none of them I've played with had the mechanical repeatability of a decent air variable nor the durability.
Adding a shaft extension made it worse.
About the only place I'd use one is a compact tuner like the SLT,
then only at very low power and after looking through my collection of ARCO mica trimmers.
Then again if your going to use a varicap diode (also low Q) might as well use
5614 2010-12-30 17:48:46 AD7ZU Re: "Polyvaricon" capacitor That about does it...
Im going to buy a mini-box big enough to hold the old variable and hook up the
vernier. .. its about 1/3 the size of the entire transceiver board. I may even
blow some of the dust out of the plates and check it to verify no shorts.
5620 2010-12-31 08:47:01 James Duffey Re: "Polyvaricon" capacitor Wes - Very nice work. Thanks for sharing it with us.
I suspect that your Q measurements are accurate and pretty representative of polyvaricons in general. Several years ago there was a thread about this on QRP-L, mostly in conjunction with end-fed antennas. In that discussion someone said that the manufacturers spec was Q=500 at 10 MHz with 50 pF, so your measurements are consistent with that. I have not found any manufacturer's data on this though. W8JI had made some Q measurements on polyvaricon capacitors and a toroid inductor used in an end-fed antenna tuner. He got much lower values of Q, I think around 80 or so for the polyvaricon, but his measurements of inductor Q at the same time was off by a factor of 3, when compared to W1VT's toroid Q chart. Tom has since pulled that data from his web page.
All this aside, for all their warts, polyvaricons have nicely filled the gap created when the old air variables stopped being widely available and became expensive. I would avoid using them in applications that have large circulating currents though, like parallel tuned circuits for antenna tuners where losses will be exaggerated. But as long as one takes the losses into account, I suspect their use is OK in other less critical circuits. It is just that we assume the capacitor Q to be several thousand in most LC circuits, and when it is of the same order as the inductor, we get more loss than we expect. I wouldn't use one for a high performance receiver, but it is probably adequate for the ubiquitous NE602 based receivers we like to throw together. And the price is right, at least compared to $10 for a real air variable cap. :^)= - Duffey
Cedar Crest NM
5621 2010-12-31 10:59:35 Brooke Clarke Re: "Polyvaricon" capacitor Hi Wes:
You can not use a VNA to make Q measurements. The reason is that the
accuracy of a VNA measurement degrades as the impedance of the device
under test moves away from 50 Ohms and is very poor near open (>500
Ohms) or short (<5 Ohms) conditions. See:
also see the Impedance Measurement Handbook link at the bottom of the Z
The best way to measure Q is with a Q meter and the second best way is
to use a 4-terminal pair auto balancing bridge like the HP 4274 or HP
Brooke Clarke, N6GCE
5624 2010-12-31 11:40:37 Wes Re: "Polyvaricon" capacitor Hi Brooke,
Agreed. I rebuilt my "cal standards" and jumped through a few other hoops and never got any direct VNA measurements of Q that were at all pleasing. But I'm not trying to use the VNA for a direct measurement. Rather, the measurement scheme is that show in EMRFD Fig 7.66, but with a VNA as the signal source and detector. This provides resonator Q. The steps used to get viable capacitor Q from this are outlined in the web piece that was referenced.
I've seen your stuff on the web. Good info. Also, Agilent app notes 1369-6 and 1369-6 are great. N2PK steered me to them -- tnx Paul. The best schemes for measurement of Q would probably use the RF I-V test head presented in the Agilent notes. That might be a fun project -- it's
5632 2011-01-01 09:52:36 Wes Re: "Polyvaricon" capacitor Hi All,
Whoops. An alert reader pointed out a typo in my post of yesterday. The great Agilent app notes are 1369-6 and 1369-2. See the error below.
The first note was written early in the day, long before any New Years celebrati
5633 2011-01-02 05:39:17 Tim Re: "Polyvaricon" capacitor 5634 2011-01-02 07:37:10 ae5ew Re: "Polyvaricon" capacitor Building should be strongly encouraged. Experimentation. Creating.
5635 2011-01-02 11:32:50 William Carver Re: "Polyvaricon" capacitor I've always known that "capacitor Q is so much higher than inductors
that you can neglect their Q". Yea....right.
As Tim pointed out, in his Q=5 to Q=10 tuner a Q=500 capacitor won't
make much difference and the polyethelene-insulated capacitor may be OK.
But keep in mind that same low-Q capacitor might not survive trying to
get five watts on 7 MHz into an 80m dipole.
Wes presented MEASURED DATA for a component, DATA that allows you to
put that capacitor in a SPICE model of your circuit and decide if it
might be suitable. Thanks Wes.
Will a nice Arco 315 trimmer from the junkbox tune my 80/75m class-E
amplifier tank? The ceramic-mica 1000-2000 pF Arco is a great looking
capacitor, but the peak RF current models at more than 20 amperes on
modulation peaks. I need to know its Q before I put a lot of other parts
at risk. OK Wes, now I need a Q number for this Arco 315.
Wes knows that's a rhetorical/grin request: I have a VNA and a Boonton
260A, and I will measure it myself. Nothing beats measured data when the
chips are down.
5636 2011-01-02 12:11:18 kb1gmx Re: "Polyvaricon" capacitor 5637 2011-01-02 13:46:58 w4zcb Re: "Polyvaricon" capacitor Bill, the shortcomings of the ARCO's, is that the mica is not
homogeneous. We used them in the broadbanded tube PA for the Signal
One (150 Watts out, Pi-L) and they would fail due to voltage. We
wound up disassembling every plate of those things and substituting
TWO sheets of mica between each pair of plates and cured the problem
the hard way. The empty spaces in the mica were never in the same
place in two sheets. Particularly with Sokals class E, I'd go forth
and do likewise.
> Will a nice Arco 315 trimmer from the junkbox tune my 80/75m class-E
> amplifier tank? The ceramic-mica 1000-2000 pF Arco is a great
> capacitor, but the peak RF current models at more than 20 amperes on
> modulation peaks. I need to know its Q before I put a lot of other
> at risk. OK Wes, now I need a Q number for this Arco 315.
> Wes knows that's a rhetorical/grin request: I have a VNA and a
> 260A, and I will measure it myself. Nothing beats measured data when
> chips are down.
> Bill W7AAZ
5638 2011-01-02 14:53:10 Tim Re: "Polyvaricon" capacitor 5639 2011-01-02 14:59:43 kb1gmx Re: "Polyvaricon" capacitor Signal One, mica trimmers failed on voltage..
Class E application, the questi
5640 2011-01-02 16:41:26 KK7B Re: "Polyvaricon" capacitor Didn't the Signal One use an 8072 conduction cooled tetrode as the final? Maybe running around 800 volts on the Plate? I used one for years on 2m as my linear amplifier--loved it. (Sold that along with most of the rest of my stati 5641 2011-01-02 18:17:58 billw77aaz Re: "Polyvaricon" capacitor If you have a simple parallel tuned circuit as a filter, misjudging the Q doesn't make a dramatic difference to either shape or gain.
When you want to "critically couple" TWO tuned circuits and get the right bandwidth, Q makes a big difference. Happily if you allow for variable coupling between the tuned circuits, and can tune their resonant frequencies, you can correct for a modest Q error and plod onward.
As you get more complicated.....say a five pole antenna filter for a RF-less 10m receiver with 5.2 MHz IF....you would be hard pressed to ever adjust things and compensate for a moderate Q error. Like the Three Bears, it's not having too long Q that's the problem, you also can be driven nuts by having more Q than you planned for.
You said "I understand the value in the numbers, but still beiieve it's possible that too much focus on specsmanship may turn some off of homebrewing." I agree to some extent. Excessive BS is a turnoff.
On the other hand, if something doesn't work right there HAS to be an explanation and FINDING THAT EXPLANATION is port of the learning experience. Best to both THINK and DO. Sometimes a difficult balance to find. Wes' mix of build-and-explain-concepts helps keep people going.
I bet the first 5-6 years only 10% of what I built worked. I thrashed my way through school(s) determined to understand how electronics works. I'm still learning. My success rate is much better than the initial 10%, but it's still not 100%.
EMRFD leans more toward the fine details but that is a niche that needs to be filled just as much as the simpler stuff. But as we homebrewers are spreading thinner and thinner, we need to give a helping hand not just to each other, but also to those who might still be trying to understand Ohms Law (real learning having been short circuited by memorizing the license test answers).
5642 2011-01-02 18:43:10 w4zcb Re: "Polyvaricon" capacitor Hi Rick, your memory isn't bad, the tube was the 8072, but the Anode
Voltage was 1500 Volts in the CX-7. GE or Motorola, I think GE, used
the tube in one of their 2 meter taxicab radios, FM via intense
multiplication. I believe they used something around 800 Volts.
Didn't the Signal One use an 8072 conduction cooled tetrode as the
final? Maybe running around 800 volts on the Plate? I used one for
years on 2m as my linear amplifier--loved it. (Sold that along with
most of the rest of my stati
5643 2011-01-02 20:38:00 KK7B Re: "Polyvaricon" capacitor Yea...my memory is pretty good for radios like the CX-7 that I only owned in my dreams, hi.
My 2m amp was a converted GE (I think--maybe RCA?) mobile radio PA, first running on 12 volts with the mobile DC-DC converter, then later I homebrewed a nice supply for it. Worked a lot of DX on 2 with that combination from Seattle. 8072 driven by an ICOM202 backed off to about 2 watts. Really super clean transmitter.
5644 2011-01-02 23:25:49 w4zcb Re: "Polyvaricon" capacitor You have to be right, I've only seen one of those mobile radios but
the 8072 was an RCA bottle, so likely their offering. They lost the
recipe for the tube for about 6 months and we went on assignment for
them. Darned near killed us. Eimac did the same thing when they moved
from Palo Alto to Salt Lake with the 8877. That one lasted most of a
>Yea...my memory is pretty good for radios like the CX-7 that I only
>owned >in my dreams, hi.
>My 2m amp was a converted GE (I think--maybe RCA?) mobile radio PA,
> >first running on 12 volts with the mobile DC-DC converter, then
>later I >homebrewed a nice supply for it. Worked a lot of DX on 2
>with that >combination from Seattle. 8072 driven by an ICOM202
>backed off to >about 2 watts. Really super clean transmitter.
5656 2011-01-03 22:30:11 William Carver Another trimmer number and a Class E question I measured the Arco 315 trimmer Q at 5 MHz. I set it at 1558 pF, close
to mid-range and put it in series with a toroid producing a measured
(loaded) Q of 294 and calculated the ESR of the capacitor which reduced
the measured Q to 163.8. The measured Rs=0.0879 ohms, Qc = 232.
I checked voltmeter and ammeter calibration of the Boonton 260A and
checked myself by putting a 0.24 ohm resistor in series, verifying the
measured and calculated Qs matched essentially perfectly. Wes, I'm
pretty confident you can add this data point to your capacitor Q data if
In a Q=5 class E series network the Arco trimmer would be in parallel
with a fixed 1500 pF capacitor so it is only carrying about half of the
RF current. That current will have a lot of harmonic content, not
perfectly sinusoidal, but I assumed so to estimate the capacitor
dissipation. On that basis at 120 watts its current will be 2.88 amps
and the power loss will be 1/4 watt. On modulation peaks it will go up,
but the duty cycle is low and the increase is not dramatic: it should
survive that. But on CW it should be possible to run it at full peak
power in which case the keydown power dissipation will be one watt.
The RF voltage will be about 62 volts with just carrier, and 124 volts
RMS, 175 volts peak at 100% modulation and CW. The sinusoidal assumption
is only part of the voltage: the pulse waveform on the MOSFET side of
the capacitor has an estimated peak of about 200 volts. So the capacitor
will be subjected to about 375 volts.
Based on W4ZCB's comments on breakdown problems in the Signal One, this
could be asking for trouble. Combined with 1 watt power loss on CW I'm a
little leery of using it in my solid state "Viking II". The radio club
boat anchor guys would never let me forget if it dies on the Sunday
morning AM get together.
An alternative is a 3000 pF 1 KV fixed mica capacitor and relay
adjustment of the series inductance (I've seen Collins do this in
avionic HF tuners). I have no experience. How sensitive are class E
amplifiers to reactive loads? Over 3.5-4.0 MHz three relays to provide
1-2-4 binary steps in a Q=5 network should put the load within five
degrees from exact resonance. Anyone with class-E experience have an
opinion whether that's good enough, or do I need to be much closer to
5657 2011-01-04 04:20:00 Alan Melia Re: Another trimmer number and a Class E question Bill you state that the trimmer will be carrying lots of harmonic current.
Surely this is not the case if you have a genuine Class E stage?? Nat Sokal
quotes at 7% THD. A lot of harmonics means low efficiency. The output of the
Class E stages I have built have been relatively clean when the stage is
adjusted properly. It is assumed (incorrectly) by many because the active
device is used in a square-wave switching mode that the output must have
lots of harmonics. I believe this is only the case with a sub-optimum tuning
and low efficiency as a result.
5660 2011-01-04 06:35:21 Tim Re: Another trimmer number and a Class E question A couple comments with my experience in Class E series output networks:
1. RF Voltages developed across the series capacitor will be surprisingly high to someone used to parallel low-pass networks at similar power levels. If you can isolate and ground the shaft, and do so reliably, this would be a good thing. If you use an output transformer as part of the output network so you can ground the capacitor frame and shaft (a good idea) and think the shaft is being grounded by the capacitor frame, you might take into account that this is not perfectly reliable by itself.
You obviously know about the voltage issue (you call out some numbers) but I want to point out, that I say point #1 not from a component-rating specmanship standpoint but from a prevent-RF-burns standpoint. Mmmm.... smells like fried chicken. Wait, that's me!!!! I thought that shaft was grounded!
Yes, an RF burn smells exactly like fried chicken. It's curiously different than more traditional thermal burns (e.g. picking up the wrong end of a soldering iron!)
2. Air wound solenoidal output inductors work out pretty well and sidestep the magnetic flux limitations of smallish toroids. Although I've seen Sokal's papers where he uses these monster sized (I mean, one was the size of a big coffee mug!) mix -6 iron powder toroids at HF and he seems to have success there.
3. Class E output series networks is a place where those crufty old not-good-enough-for-a-VFO-cap air variables from a century ago really fit in perfectly with solid state homebrewing. I personally would not use a "trimmer cap", even a hefty one, for this purpose, but that might be because of my large selecti
5661 2011-01-04 06:59:30 Tim Re: Another trimmer number and a Class E question I can tell from your past articles that you like refined hi-tech solutions but an alligator clip and adequate winding spacing, or close winding spacing perturbed to make it easy to put an alligator clip 5662 2011-01-04 07:05:46 Chris Trask Re: Another trimmer number and a Class E question >Sokal
> Bill you state that the trimmer will be carrying lots of harmonic current.
> Surely this is not the case if you have a genuine Class E stage?? Nat
> quotes at 7% THD. A lot of harmonics means low efficiency. The output ofthe
> Class E stages I have built have been relatively clean when the stage istuning
> adjusted properly. It is assumed (incorrectly) by many because the active
> device is used in a square-wave switching mode that the output must have
> lots of harmonics. I believe this is only the case with a sub-optimum
> and low efficiency as a result.That's true if and only if (iff) you have a resonant trap for all
harmonics, which is not practical. Most switching amplifiers are
third-order or fifth-order at best:
N7ZWY / WDX3HLB
Senior Member IEEE
5665 2011-01-04 11:05:49 William Carver Re: Another trimmer number and a Class E question Yes, I DO like refined solutions. I "try" to keep my mind under
control but prefer to minimize the number of attempts to asymptotically
reach perfection (I know, the number is infinite either way). The '42
Radio Handbook and '56 ARRL handbook are referred to more than
I have a span of alternatives but don't view this project fitting on any
of the wooden breadboards in my inventory. I am trying to fit all the RF
portion inside an existing 3" high junkbox chassis without getting into
a lot of metalwork to enclose a big variable.
There's some conflict between the physical size and capacitance of the
variables and much higher Q of the network. I don't want it to be too
"peaky" and was thinking a Q of 10 or less OK. Another approach to the
Q/capacitor size issue is an RF transformer as Dave used in his article.
He says mox power/efficiency coincided when the transformer and MOSFET
capacitances were self-resonant at the operating frequency.
RF burns do smell funny. Unlike thermal burns which are on the surface,
they can go deep. I pulled a 6146 plate cap while the key was down and
the vertical cooked meat took many months to finally heal.
5670 2011-01-04 16:57:20 Chris Trask Re: Another trimmer number and a Class E question >I'm in the same league when it comes to refined solutions. It would be
> Yes, I DO like refined solutions. I "try" to keep my mind under
> control but prefer to minimize the number of attempts to asymptotically
> reach perfection (I know, the number is infinite either way). The '42
> Radio Handbook and '56 ARRL handbook are referred to more than
nice to engage people who are truly versed in FPGA and DDS so that I could
further refine the solutions I've put together for things like adjacent
signal interference, low spurious and harmonic DDS, etc.
N7ZWY / WDX3HLB
Senior Member IEEE
5672 2011-01-04 21:09:22 William Carver Mea Culpa for "Another trimmer number" I never make misteaks.
Well, maybe once in a while.
The Q meter measures the RF voltage across its own internal capacitor.
When the DUT is a capacitor in series with a test coil a voltage divider
is formed by the DUT and the Q meter tuning capacitor. The indicated Q
is the voltage across only the tuning capacitor. Duh. I didn't take that
The actual Q of the Arco trimmer is much higher than I reported.
My "calibration" using a resistor in series with the test coil confirmed
the measurement concept but since it doesn't form a capacitive divider
it does not confirm the trimmer measurement was correctly performed.
I've already confessed to Wes. Since I didn't record the tuning
capacitor values, I must repeat the tests and recalculate the trimmer Q.
5673 2011-01-04 23:55:08 William Carver Mea Culpa for "Another trimmer number" in a PM Wes privately chiding me about insufficient measurement
information in yesterdays post giving a low Q number to the Arco 315
trimmer from my junkbox. Independent of that I had measured some dipped
mica capacitors and became uncomfortable with low Q numbers for parts I
knew should be better. Then, as explained in my earlier confession post,
I realized that the DUT capacitor was forming a voltage divider with the
Q meter tuning capacitor: the Q meter reading was voltage across the
tuning capacitor rather than the series combination of DUT and tuning
I can't change how the Q meter is wired, but knowing the two capacitor
values I can extrapolate the Q voltmeter rading to what the voltage must
be at the series combination.
Wes' desire to see exactly what I'd done previously, combined with a
need to do more than just saying "I screwed up" and popping out a new
number, compel me to lay it all out this time. Unless this kind of
detail is interesting this might well be seen as trivia. Unless you are
inclined to slog through this looking for another misteak, just hit the
I measured the Arco 315 trimmer again and a new DM19 capacitor marked
The Arco 315 measured 1553 pF on LC Meter II, 1559.8 pF (0.44% higher)
on hp 4271B bridge
"1023" DM19 measured 1028 pF on LC Meter II, 1032 pF (0.39% higher) on
LC Meter II is within specification, but about 0.4% lower than the
(presumeably) more accurate hp
The T103-2 inductor is 2.785 uH according to the LC Meter II. I
resonated it on the Q meter, then measured the Q meter variable with the
LC Meter II as 300.8 pF. That should resonate at 5.5 MHz with 301.6 pF
so the agreement is about 1/4%. The inductive reactance is 96.24 ohms at
When resonated the Boonton capacitance dial said 294 pF, low by 2.6%.
That's pretty good for such an old analog instrument, and within
specifications, but subsequent tuning capacitor values were also
measured with the LC Meter II rather than using the Boonton dial
The inductor Q is higher than 250 so the Q meter current was reduced to
the "X1.2" mark. The inductor then measured exactly full scale, Q=250.
That is multiplied by 1.2 to give a Q of the coil (loaded by the .02 ohm
source resistance of the Q meter) of 300. The ESR of the coil plus
the .02 ohms of the Q meter itself is then 96.24 ohms divided by 300, or
Each subsequent resistance calculation always includes the .02 ohms of
the Q meter plus the resistance of the coil. Since the trimmer Q will be
computed by subtracting two resistances, each of which includes the coil
and Q meter resistance, there's no need to deal with them separately.
The Arco trimmer was put in series with the coil.
The Q meter was adjusted to resonance, measured capacitance 368 pF. The
indicated Q was 199.
The "199" reading is taken from a capacitive divider with 368 pF to
ground and 1560 pF to the actual voltage at the top of the coil. So
multiplying the 199 reading by (1560+368)/1560 produces a reading of
245.9 for the two capacitors in series. This is multiplied by 1.2
because the current level is still at "X1.2". So the indicated Q after
adding the trimmer is 295.1
The resistance of the RLC mesh that now includes the trimmer is
96.24/295.1 = 0.3261 ohms. Subtracting the 0.320 ohms WITHOUT the
trimmer shows an increase of 6.13 milliohms which is attributed to the
The reactance of the 1560 pF trimmer at 5.5 MHz is 18.55 ohms so the
trimmer Q = 18.55/.00613 = 3,026
computing Q from the small difference of two large resistances
inherently increases any small error in either resistance to a much
larger error in Q, in this case about 50 times larger. I tried to reduce
errors by measuring the Q meter capacitor rather than relying on its
analog dial, and retaining a constant current setting on the Q meter.
But even with measurement precautions "Q = 3,026" should probably be
taken as "at least a couple of thousand".
That's a big difference from the Q of a few hundred I posted yesterday.
The big stack of plates and mica should be able to handle the 1/4 watt
of loss produced by 5 amps of RF current.
The RF plus drain voltage concern still exists. W4ZCBs Signal One
experience implies there could be a surprise when 100% amplitude
modulation doubles all the voltages.
Regards - Bill - W7AAZ
5675 2011-01-05 04:26:28 Harold Smith Re: Mea Culpa for "Another trimmer number" Wow. That is a very informative, useful post. Thanks.
de KE6TI, Harold
5676 2011-01-05 05:11:09 Tim Re: Another trimmer number and a Class E question I'm guessing you are aiming at the 100-200W PEP level from the voltages and currents mentioned elsewhere. Putting that into a 3" high chassis with air wound inductors and air variable caps for 80M could be done but I'm guessing that your result will look more like Sokal's high density class-E 13.56MHz designs.
e.g. he gets a kilowatt out of 6.75" x 3.1" x 2.7". His series output network uses porcelain ceramic SMT caps and a -6 mix giant toroid stack with 3/8" silver plated copper strap windings. Sokal's 13.56MHz designs were probably (I don't have any proof but the pictures match my guess) tuned up by soldering in additional SMT capacitors to match things up to a perfect resistive load. In the ham world we expect to be able to tune out at least some reactance in the antenna system and also tune over a chunk of a ham band, but maybe realistically that tuning could be considered part of the antenna matching network, and not part of the amp?
e.g. I look at NM0S's published designs and I don't see him using variable caps or alligator clip taps on the inductors like I do :-).
And it's been ages since I've seen plate and load tuning knobs on the front of a ham rig. I still like knobs though!
In my limited experience with class E at power levels of one-tenth of a kilowatt, I was most surprised by the copper losses in the series output inductor with circulating currents of just a few amps. I mean, the math works out correctly, I was just expecting things to be more comparable to the inductors in a CLC pi network by gut intuiti
5725 2011-01-13 11:13:52 jocjo_john Re: "Polyvaricon" capacitor