Hi Jim,
It looks like there is little difference in these tuners, based on
your post. 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
case.
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
adjustments".
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
predicts.
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
problem.
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 X-match 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 X-match'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.
73, Tom W8JI
w8ji@contesting.com
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