> "That is what I have. One 15kV 200pf door knob cap in series with the
> shunt."
> > ----------
> > Plus one other component -- the 9:1 un-un.
The 200 pF doorknob, assuming the R at that point where it is in
series is 450 ohms (allowing you to cancel reactance with that
capacitor), carries 1.8 amperes. The reactance of the capacitor is
442 ohms, voltage across the capacitor is 1.8*442=760 VRMS, or
1075 v peak.
Q of the shunt system at that point is essentially 1, so in your
case it looks like a good result. It won't always be a good result,
but in your case it is.
> > If you move the tap point down from the 450-ohm point to the 50-ohm
> > point, you'll be able to get rid of the un-un and the required gamma
> > capacity will be about 600 pF, three times what you're using now. Tom
> > basically said to use the fewest number of components to reduce losses
> > and that the greatest amount of capacity is the most efficient.
He already has a good combo as far as how the system is right at
the capacitor. Assuming your numbers are correct there would be
147 ohms Xc and 5.5 amperes through the capacitor. Voltage
across the cap would be 5.5*147= 808 volts and Q at that point in
the feed system would be almost 3.
Remember the qualifications I included. I said for a given system, a
change in gamma effective diameter to increase capacitance
required, efficiency is improved and Q lowered.
Let's say we keep the feed system at 50 ohms by moving the tap
point (so we still only need one capacitor), and increasing shunt
diameter until we need 2000 pF of capacitance, Xc is now 44
ohms. Q is less than one, and since the resistance at that point is
50 ohms (same current) voltage is 242v RMS across the capacitor.
So you see, for a constant value of R anything you do to reduce
the reactance reduces Q and improves efficiency (because VAR
power and distributed losses are less).
By far most of the losses (ignoring ground system losses) are in
the conductors in the gamma and the tower. That system also has
"Q" of it's own that figures into the system Q. The results above are
only valid for what is happen right at the capacitor. The Q of the
transmission line making up the gamma and the tower increases
with higher gamma impedance
> But, will it provide a better pattern or ERP? That's the issue. I am
> certain I could figure out 50 different ways to get a match. Does it
> improve my SNR?
SNR ratio, assuming you system does NOT have common-mode
problems with noise, is unaffected by any of these changes. You
can feed an antenna any way you like as long as it is done
correctly without common-mode currents on the feedline, and S/N
ratio is unchanged as long as pattern is unchanged.
Noise is an electromagnetic wave just like a signal, and you can
not "dc ground it out" nor can you "shield it out". The only
exception is where noise is generated right in or very near the
antenna, like corona discharge in the antenna itself.
If your system has common mode-noise problems, improving the
ground system would reduce noise. So would adding a choke on
the feedline (like a sting of beads).
My only point is, for a given system, anything you do to reduce the
number of components and reduce the reactance required to
cancel the feed system reactance will improve efficiency and
increase bandwidth. In other words, if you have a 120 foot tower
and you run a wire to the top for an omega match, the Q of that
system will be higher and losses will be higher than running a
gamma to the correct point for 50 ohms resistance with a simple
single series capacitor cancelling reactance. The thicker you make
that gamma (while still maintaining a tap point for 50 ohms), the
more series C you will use and the lower Q and less loss you will
have.
But all of this is splitting hairs. The bulk of loss will be in the
ground system in most installations. Bandwidth is a good reason
to mess with the feed system, efficiency or "noise" is not.
73, Tom W8JI
w8ji@contesting.com
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