Jim,
## I dont buy into the RF RMS vac / XL = RF current flow
through the choke. Even W8JI doesnt buy into it either. Its
more like RF RMS vac / Choke Z.
I fully agree with that! It's just that I was assuming that at least on
the lowest bands, which are more predictable, the Z will closely match
the XL.
On the higher bands anything can happen, due to parasitic effects (AKA
resonances).
I wanted to get a better feeling for those parasitics. So I dug out a
big old plate choke from my junk box, and measured its impedance.
The choke is tight-wound with 175 turns of #23 wire, on a 1.5 inch
porcelain tube. It has 1/4 inch wide metal clamps around the ends,
forming shorted turns touching the first and last wire turns. It is
supported by brackets and huge brass bolts that run deep into the
porcelain tube. Both the shorted turns and the brass bolts must be
reducing the inductance below the value it would be without them.
Calculation without considering the effects of these elements predicts
roughly 350 microhenry, while measurement done at very low frequency
gives only slightly more than half that much.
I'm not positively sure where this choke came from, but I think it's
from an antique AM broadcast transmitter of roughly 1kW, handmade in
Chile or Argentina, probably in the 1940's or early 1950's. It might
also have been from a marine transmitter, but that's less likely.
Impedance measurement gives the following:
1830kHz: 2100 ohm, 86 degrees
3600kHz: 4125 ohm, 89 degrees
7080kHz: 8000 ohm, 81 degrees
13110kHz: 1310 ohm, zero degrees (first series resonance)
14250kHz: 5700 ohm, -80 degrees (capacitive, above resonance)
28300kHz: 44000 ohm, 42 degrees
29990kHz: 45000 ohm, 23 degrees
My interpretation of this is: The choke behaves as a reasonably clean
inductor on the lower bands up to 40 meters, with its impedance closely
following its inductive reactance. The Q peaks around the 80 meter band.
At 13110kHz it has its first series resonance. At that frequency it has
a rather low impedance of only 1310 ohm, and what's more important, this
is purely resistive! So the loss (and heating) at this frequency would
be extreme, and the choke would burn up in a matter of seconds if
operating at that frequency.
Above that frequency the impedance becomes capacitive and rises again.
Slightly above 30MHz apparently there is a parallel resonance, but I
can't check the exact frequency because I was using my HF radio as a
signal generator, and it doesn't transmit above 30MHz.
It looks like this choke would be usable in a ham amplifier, but the
operator must never try to use the amp near 13MHz, or Very Bad Things
will happen. Bad luck for "freebanders".
And the chokes Z will be a lot higher than its XL.
Not on the low bands, according to my measurements. Z closely follows XL
there. And going higher, first Z goes DOWN, way below XL!
I tested that ameritron choke..which is wound with 27 gauge
wire, with turns butt tight,,, except for the 2 x gaps.... and it
runs stone cold at 1.5 A CCS..with my small lab supply. Any more,
and it heats up up..and quick like.
The quick heating has to do both with the losses rising with the square
of the current, and with the temperature coefficient of the conductivity
of copper.
But I would like to insists on the point that RF causes a lot more loss
than DC, on the worst bands, in a typical amplifier application. tetsing
with DC alone is pointless.
> JI depicts some sky high Z
numbers, etc, when he tested that choke with his VNA.... on all 9 x
bands.
Would like to see that. Is it piblished somewhere?
Some bands were better than others. But imo, without the
extra C that is normally used, the VNA results are invalid.
Jim, any capacitance you connect EXTERNALLY to a choke does not change
the choke's impedance, AT ALL, NOT A BIT. It DOES change the impedance
of the complete circuit, of course, but not that of the individual
choke. So the choke's current to voltage ratio (its impedance) on a
given frequency will remain the same, no matter what you connect to kit.
What CAN change things is if you mount the choke so close to the chassis
or other stuff that stray capacitance to the choke's windings becomes
important, or if the choke couples to the tank coil and picks up a lot
of its magnetic field.
VNA test results should be correct, as long as the VNA works right,
isn't taken outside its calibrated range, and the user knows how to use
it. When I'm in doubt with VNA results, I prefer to go to the bare
bones, using a signal generator, a scope, suitable test probes, and my
trusty old pocket calculator.
> It
would be interesting to place aprx 35.2 pf in parallel with the
ameritron choke.. then re-do the VNA test on 160M. 35.2 pf is what
would be required to cancel out the 225 uh chokes XL on 160m.
Measuring the parallel resonant circuit that way will show a very high
impedance for the complete circuit, but still the choke's impedance
hasn't changed.
## Put a clamp on RF ammeter at the cold end of the choke, or
better yet, for safety, put the clamp on RF ammeter at the cold end
of the bypass cap at the base of the choke. IE: between chassis and
cold end of bypass cap. Then measure it.
That's basically what I did, just that instead of a tube amp I used a 70
volt 50 ohm signal generator (a 100W HF transmitter loaded by a 50 ohm
dummy load). I used a scope to measure the voltage across the choke and
the current through the choke. The amplitudes and the phase ratio of them.
## With 7 kv under load of 3A, on a buddys 3x6 amp, the RF RMS
voltage is aprx 4.2 kv. He tried as little as just 45 uh, wound
with 22 gauge magnet wire, turns butt tight, with zero issues..and
thats on 160m over 4 x nights. I was expecting it to burst into
flames. Nothing happens.
He must have had that choke parallel-resonant on the band. How was that
choke built? With lots of interwinding capacitance? Anyway it's hard to
believe...
The only thing that happens is an extra
176 pf of tune cap C is required..... to cancel out the 508.5 ohms
of XL of the 45 uh plate choke..on 160m.
In that case it MUST have blown up. That would have been 8A of RF
current in that choke. It would have burned that wire. Except perhaps if
there was strong air flow. Was that the case?
> We don?t see 8.26 A of RF
current, that you would expect with 4.2 kv + 508.5 ohms of XL..on
160m.
You MUST have had that much current, unless either the choke was
internally parallel-resonant near the frequency, or its inductance for
some reason was much higher than you think.
## I got into this years ago with the late Rich measures. He
contends that choke RF current is RF RMS voltage / XL. Meanwhile
he also parallel resonates the oem 10 uh filament choke, used on a
heath SB-220, on his 160m mod, with an extra 900 pf cap across the
C2 cap used on his 160m PI tuned input. In the case of the 10 uh
bifilar fil choke, if a normal, say calculated values, using a Q
of 2-3 used for a 160M PI, tuned input is used, input swr is sky
high. An additional 900 pf required is to parallel resonate the
10uh bifilar fil choke. Ok, then input swr drops to 1:1 on
160m. That explains why C2 values end up being on the high side of
the calculated values..on 160m, when using various values of uh for
the bifilar choke on a GG triode. Bypass caps used at cold end of
bifilar, which puts the entire bifilar in parallel with the C2 cap
of the PI tuned input.
## I cant see a plate choke being any different. Plate choke is in
parallel with the C1 tune cap...and also the tubes anode to
grounded grid C.
> ## What am I missing here ? The required extra C on a Tune cap is
> always the correct amount to cancel out the chokes XL. If we are not
> parallel resonating the choke, what are we doing then ? Ditto with
> the bifilar and the 160m PI tuned input.
Jim, you fail to separate two things which are physically different: The
current in the choke, and the TOTAL current. That's is the choke's
current cancelled by the current through the resonating capacitance.
ONLY if this capacitance is internal to tyhe choke, and distributed
along it, will the actual choke current be reduced. Whenever the
resonating capacitance is external, or lumped, the current in the
choke's windings will remain high.
By adding the precise amount of external capacitance you can always make
the total current zero (for perfect components) or very low (for real,
lossy components), but this doesn't reduce the current flowing in the
choke. It only adds an equal current of opposite phase, flowing through
the capacitor.
It's perfectly right that you have to provide this additional tuning
capacitance, both in the plate circuit and in the filament/drive
circuit, to achieve proper circuit operation - but this doesn't reduce
the current in the chokes!
I wish we could get together, have a cup of tea, coffee, a glass of beer
or whatever, in the company of some sheets of paper and a pen. A few
drawings are worth more than a long-worded posting like this. A
schematic, some arrows indicating current flow in each component and
each connection, would make the issue crystal clear. But there is just
too much distance from my place to yours...
Manfred
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