>A bit of a thread has appeared recently on the Towertalk
>reflector about "QRO" antenna tuners; the discussion
>has touched on the subject topic,rollers, Delrin and ceramic
>cores. A few quotes from there:
>Jim, KH7M wrote,
>Inside an AT4K tuner by Palstar:
>"Roller Inductor: This is the surprise within my unit. The
>inductor is a tape wound obviously heavily silver-plated unit,
>using tape which measure 3/8" wide, edgewound, with a tape of
>0.062" thickness, The inductor, per the manual, is rated at 28
>uHy, 5 kV and 30 amps.
? E.F. Johnson and Multronics rate 1/2" wide ribbon at 15 RF
>A large, maybe 0.75" or so pinch "wheel"
>rolls around inside the tape or the air core unit as shown in one
>of a neat series of photos of the unit, facilities, and of Paul
>Hrevnek himself holding an AT4k, at
>(takes a couple of minutes to load pictures, then they appear
>sequentially one at a time).
>The inductor in my unit appears visually to be similar in
>construction and color to the unit Surplus Sales of Nebraska
>illustrates on the back cover of their catalog, 8; that is heavy,
>red plastic end plates, etc. However, surplus sales rates their
>inductor as 40 uHy, 12 1/2" long, but probably the same
>manufacturer, I would guess. Inductor in my unit is about 9
>As my unit was rcv'd, the bottom 14 turns of the inductor are
>shorted to ground; this can be removed if more L is needed down
>on 160 meters, but should be replaced, per Paul, when on 15
>meters, as the full 28 uHy coil has an internal resonance on 15
>meters. If, for some reason use of 15 meters is not anticipated,
>the jumper may be removed permanently.
>Appears completely unlike the apparently bare copper inductor in
>the AT4K photo appearing at the Palstar web site."
>Tom, W8JI, comments in response:
>"It looks like there is little difference in these tuners, based on
>your post. [KH7M: that is between the Palstar AT4K and the
>Ameritron ATR-30 units] That assumes there is nothing wrong
>with the components in the 4K, and the ATR 30 is built to spec.
>"I'm more comfortable talking facts than subjective opinions.
? During the grate-debate, Mr. Rauch rejected standard AC Circuit
Analysis of a vhf suppressor of his own design, when the result proved to
be somewhat unfavourable. .
>through the facts...
>> Capacitors: Input and Output 6kVm 350 pf, with an added 350 pf
>> 7.5 kV fixed doorknob C which can be switched in, in parallel with
>> the network output capacitor. This is said to be necessary when
>> on 160 or 80 meters, and the antenna impedance is below 25 or 30
>> ohms; switching in the added 350 pf will reduce the current
>> within the inductor, and thus losses and heating therein.
>"Untrue, as you will see. The change in heating is insignificant. As a
>matter of fact the capacitor most likely reduces power capability.
>"Let's look at the important components in three tuners, the
>components that routinely fail. Antenna switches, nuts, bolts and
>wires are the least of our worries. Consider a tuner with 350 pF
>max C, one with 500pF, and one with 350pF variable input and 350
>pF shunted across the output variable on low Z loads at 4000 watts.
>Capacitor voltage is peak (since that is the failure determinant) on
>the highest voltage capacitor and inductor current is RMS (since
>that is the failure determinant). All at 1.8 MHz since that is worse
>For 50 ohms load 1.8 MHz:
>350 pF /350 pF Vc 3300v / 11.6 uH 17.6 A
? How was this calculated?
>500 pF/ 500pF Vc 2325v / 8.4 uH 17.5 A
>For 20 ohms load 1.8 MHz:
>318 pF/ 500 pF Vc 3500v / 22.78 A
>350 pF / 548 pF Vc 3190v / 22.75 A
>"You can plainly see, even at 4000 watts, capacitor voltage is the
>least of our worries in either tuner. Current is the problem. The
>extra 350 pF doesn't do much, because the INPUT capacitor sets the
>"The doorknob capacitor actually becomes the component that limits
>power. At 20 ohms, it must handle just under 2/3 of the current in
>the output capacitor. That current is 14.12 amperes, so the doorknob
>must carry 9 amperes. The current rating of an x50 series doorknob is
>about 3.2 amperes at 1.8 MHz.
? Depends on the value. The 100pF unit is rated by ITT Jennings at
3.4A-rms for 1 MHz and 7.2A-rms for 10Mhz. My guess is that the current
carrying ability at 1.8MHz is c. 4.5A.
>An x57 series is 4.2 amperes, and an
>x59 series 7 amperes. Even the largest standard doorknob won't handle
>that current without heating.
? The highest-capacitance 50-series is 100pF. For padding a 350pF
variable, (3) 100pF units in parallel would be needed, yielding a total
capability of >13A. 13A into a 50-ohm load is 8.45kW.
>> As my unit was rcv'd, the bottom 14 turns of the inductor are
>> shorted to ground; this can be removed if more L is needed down
>> on 160 meters, but should be replaced, per Paul, when on 15
>> meters, as the full 28 uHy coil has an internal resonance on 15
>> If, for some reason use of 15 meters is not anticipated,
>> the jumper may be removed permanently.
>"Actually that resonance moves around. It starts out in lower VHF
>(by six meters) at mid-inductance setting and crosses ten meters and
>eventually moves down to 20 MHz as the roller is cranked to the ends.
>It never stays on one spot, and the band it "kills" really depends on
>the load impedance you are trying to match and the amount of C
>being used. A more accurate statement would be "it causes
>major problems somewhere above 18 MHz, depending on load and
>"More inductance will help match wider impedance ranges on 160,
? Higher impedances, yes. Lower impedances, no.
>but the power handling will be severely compromised because voltage
>ratings of the capacitors becomes an issue with more inductance.
>"Actually there are two limits to power handling. The ARRL Antenna
>book treats this in more detail, but misses one very important point.
>It considers the dissipation limit of the inductor as a constant,
>but that isn't true at all.
>"Our Handbooks, and many people, assume that as more C is added
>power handling goes up. That isn't necessarily true, since as shown
>above current in the inductor barely changes.
>"While reactance is reduced 28% in a maximum C change from 350 to
>500 pF (driving 50 ohms), current is reduced under 1%!!!! At some
>point more capacitance may cause roller failure, because heat is
>concentrated in a smaller area.
>"Rollers safely dissipate less power as they are cranked down to less
>and less inductance. That's because the heat, even though slightly
>less is generated, is now concentrated in a smaller and smaller area
>of the roller! Too much capacitance, or too little, can cause more
>heating. There is a "sweet spot" where power handling is maximized
>for a given set of components, and it isn't the spot software
>"Looking at the ratings of a roller, or even the size, can be very
>misleading. As a matter of fact, most ratings I've seen are simply
>pulled from a certain place near the back pocket of the person giving
>the ratings. A 30 ampere roller (at 5 MHz) is a BIG component. Try
>5/8 inch copper tubing and perhaps a foot in diameter, that'll get
>"Even calculations based on data like Q and impedances often fails to
>tell the story. The real test is to run power and measure the heat.
>"The ATR-30's "sweet spot" is 150 ohms. At 150 ohms on 1.8 MHz, it
>will handle 7.5 kW carrier with a 50% duty cycle. The only
>concern is the roller shaft heating, something soon to be moved up
>higher by a material change. The goal is a tuner that handles 3 KW
>continuous carrier on any mode on any band into the widest load
>"The penalty of a high power rating is a more restricted matching
>range, but proper choices of feedline lengths usually eliminate that
? For a multiband antenna fed with ladder-line, the tuner must be
capable of dealing with any eventuality.
>"If you look at the currents above, and consider the current drops at
>half the rate of RF power reduction, you'll see why tuners that use
>thin wire (#12 to #16) inductors fail on 160 meters (and 80 meters).
>Measures (and perhaps others) who reach empirical conclusions on a
>regular basis blame the Delrin, but the dissipation factor of the
>Delrin is not the real issue at all.
? Please prove that D-factor doesn't matter.
>The electric field is spread
>out over a wide area of the inductor. It isn't a capacitor, it's an
>inductor and the bulk of the electric field is outside the Delrin
>making dissipation factor virtually unimportant in this application.
? Nevertheless, even though you profess that it shouldn't Delrin
assuredly melts in antenna tuner applications. When I stick a neon bulb
into a tuner, it lights up plenty next to the roller-coil. If the
roller-coil is wound on a Delrin form, the Delrin is undoubtedly exposed
to plenty of RF voltage. When a Delrin coil form melts down, the
bubbles form throughout the Delrin, not just where the Delrin touches the
wire. I have seen the same effect with antenna end insulators that
melt. The melt-zone is not where the plastic touches the antenna wire,
rather it is a few cm from the contact point. .
>The roller operates at well under 3kV, not 30,000 volts like an
>antenna end insulator.
? In my 160m tuner, which feeds an end-fed 1/2-wave Hertz antenna, when
driving it with100V-peak to the 50-ohm input (100w), I measured
1100V-peak at the end of the 55uH coil. Using 1500w, the coil potential
is over 4000V.
>"The real problem is the small wire size, the very high currents, and
>the fact Delrin (and many other plastics) melt at low temperatures.
>The solid core also restricts airflow and reduces the ability of the
>component to dissipate heat. If the Delrin was replaced with
>Polyethylene, the power rating would remain nearly the same.
? There is a substantive difference in the D-factor between
Polyethelyene and Delrin.
>Only Ceramic would stand the heat and increase power ratings.
>"The XMatch uses a small gauge wire roller, but it has a ceramic
>form. The Q difference between that roller and the Delrin rollers is
>minor, but the ability of the ceramic to handle heat is a major
>factor in the XMatch's ability to handle higher power.
>"Truthfully, most things are sold on pure "fluff". Like antennas and
>SWR, the equipment that works best is the stuff you are happiest
>with. Examples of "fluff" abound....
? Obviously, the early MFJ "3kW" tuner was never tested at anything like
>> provides large tables of exactly what antenna R and +/- XL
>> can be tuned, and keep the losses less than 20 % of the power;
>"I would hope losses would ALWAYS be less than that! At 4000 watts
>20% loss would be 800 watts. I did some rough calculations, and the
>inductor would be nearly incandescent if loss were that high.
>it would be a dull red glow. Power loss is the least of our worries,
>component failure is the real worriment.
>"The Delrin roller fails at 70 watts dissipation. You don't have to
>lose very much power to make a component fail. Only 30 watts of heat
>causes the #8 wire inductor in my 4 square ATU to get hot."
? With #8, that would take a bunch of amps of RF.
>Thought the above might add some info to the conversation on
>going here about RI-40.50, Multronics, inductors and cores.
>73, Jim, KH7M
>FAQ on WWW: http://www.contesting.com/ampfaq.html
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R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
FAQ on WWW: http://www.contesting.com/ampfaq.html
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