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[AMPS] Roller Insuctors and Core Materials

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Subject: [AMPS] Roller Insuctors and Core Materials
From: (Jim Reid)
Date: Sat, 13 Feb 1999 11:49:15 -1000


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. 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
inches long.

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. Let's go 
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 

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 
system Q.

"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. 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.

> 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.

"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, 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 
you there. 

"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 
range possible.

"The penalty of a high power rating is a more restricted matching 
range, but proper choices of feedline lengths usually eliminate that 

"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. 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. 
The roller operates at well under 3kV, not 30,000 volts like an 
antenna end insulator. 

"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. 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....

> 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. At 10%, 
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."

Thought the above might add some info to the conversation on
going here about RI-40.50,  Multronics, inductors and cores.

73,  Jim,  KH7M

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