EMRFD Message Archive 7960
Message Date From Subject 7960 2012-12-05 06:26:07 jor LC antenna tuner coil question
I have the following question,
For a LC antenna tuner I want to use an Amidon ringkern and tap the windings with a switch. When I tap the windings and left the unused windings floating the Q factor will be maintained and the loss will be low. Bud when I short the unused windings the Q will go down and the loss increase as expected.
In literature we see both schematics.
What is the experience with a " magnetic closed ringkern coil" The best open unused windings ? a higher voltage at the end of the coil, any experience please
73" Joris PE1KTH8022 2013-01-03 07:36:41 drmail377 Re: LC antenna tuner coil question
Ringkern ~ Toroid (German/English)... Is this question about switching a single toroidal inductor tap (one of perhaps many) to ground compared with switching combinations of taps to ground (to achieve incremental inductance values)? If-so, I have wondered about this myself. A schematic or reference to what your design would help. If you are switching combinations of taps, like some automatic antenna tuners do using RF relays (to reduce parasitic effects - important) then your questi8023 2013-01-03 08:59:22 William Carver Re: LC antenna tuner coil question
coupling between turns. Shorting a turn doesn't just reduce the total
inductance by removing that turn from the circuit, a shorted turn also
produces a dramatic reduction in the inductance of the unshorted
portion.
In this respect it is like having a mains AC transformer with a shorted
turn: that transformer impedance will drop, draw lots of current and go
up in smoke!
If there is RF current flowing in the unshorted portion of the coil, the
current in the shorted turn(s), AND THE SWITCH PERFORMING THE SHORTING
FUNCTION, will be very high. Sometimes you might use that effect for
some purpose but not in a tuner: you would not want to short the unused
turns, leave them open.
The unconnected turns will have capacitance, and will have voltage from
end to end just like an unconnected transformer winding. And at RF the
windings will be self-resonant at some frequency. Unused turns, actually
all turns, should be very well insulated (inside Teflon sleeving is good
for higher power) and the switch must have good insulation to prevent
arcing under some condition of tap position, load impedance, and
frequency.
Bill W7AAZ
On Thu, 2013-01-03 at 15:36 +0000, drmail377 wrote:
>
> Hi Joris,
>
> Ringkern ~ Toroid (German/English)... Is this question about switching
> a single toroidal inductor tap (one of perhaps many) to ground
> compared with switching combinations of taps to ground (to achieve
> incremental inductance values)? If-so, I have wondered about this
> myself. A schematic or reference to what your design would help. If
> you are switching combinations of taps, like some automatic antenna
> tuners do using RF relays (to reduce parasitic effects - important)
> then your questi8034 2013-01-05 07:10:35 davidpnewkirk Re: LC antenna tuner coil question 8040 2013-01-05 12:29:32 William Carver Re: LC antenna tuner coil question
of yore could absorb some power, as well as altering the inductance,
changing when you screwed the top cover over the tank circuit. Those
guys DID understand what was going on and their rules of thumb about
spacing of air coils from metal were not totally arbitrarily.
IF your shorting switch and the turns you're shorting have zero
resistance, then the "circulating" current in the shorted turn produces
no I2R heating and you'll get an inductance change without any power
loss. But zero resistance simply is not reality and the circulating
current in a shorted turn can be very high. In a homebrew 8877 amplifier
I have a motor-driven plate bandswitch that stopped with the wiper
shorting adjacent coil taps, shorting the turns between those two taps.
I may still have a snapshot of the Radio Switch model 85 wafer: the
wiper and two contacts melted into one big blob of metal thanks to the
current circulating in that loop.
The brass slug in a WWII adjustable VHF coil is another example of this
shorted-turn theme.
The "variometers" where a turn (or turns) could be oriented to be in
phase, adding to total inductance, or out of phase, reducing total
inductance, or anywhere in between. That is really NEAT and not terribly
lossy if the wiper contacts have low resistance (it is NOT a shorted
turn, so the RF currents are the coil current, not extreme like a
shorted). Too bad so hard to make.
In that regard toroids are a marvel. It's still hard to believe how low
the coupling is between a toroid and another toroid almost touching it.
Tha 8877 amplifier has big multicore toroidal inductances for 160m and
80m, the airwound tapped coil that melted RS 85 contacts was only for
40m-10m.
W7AAZ