Kim, assuming the 65 +j445 is a good number, adding -445 ohms (198 pF) in
series with the feed should result in 65 +j0. This in turn would yield a vswr
of
1.3.
So, a good first step would be adding the capacitance and confirming the
expected reduction in vswr as a result. From that point there are a number of
matching schemes to reduce the impedance from 65 to 50 ohms. A simple network
for example would be a shunt C across the feed line and a series L between the
feedline and shunt feed wire.
In actual practice, the series L can be created by increasing the value of
the series C enough to end up with 65 +j25 or 30. This is in fact exactly what
the Omega match does and works just fine in the case you described.
You can expect a fairly narrow vswr curve due to the Q of the shunt feed and
to a less extent by the Q of the omega capacitor.
If you are willing to add a motor to the series C you can resonate the
system remotely from your shack, allowing operation across the whole band.
73,
Gerald K5GW
In a message dated 11/17/2007 9:24:51 P.M. Central Standard Time,
cw_de_n5op@sbcglobal.net writes:
I'm sure shunt feeding towers has been beaten to death numerous
times, but I have a specific question.
I'm shunt feeding my 48 ft tower at 1.8 MHz, with various antennas on
an 8 ft mast above the feed point. I measured the impedance with a
noise bridge and found it to be Xr = 65 ohms X = +445 ohms (inductive).
The resistive part of the feed point impedance will probably get
closer to 50 ohms as I add radials (there are none right now -- only
three ground rods and the connection to the service ground and shack
ground so all grounds are strapped together).
I initially I tried feeding it through a 500 pF vacuum variable, and
I also tried varying the spacing of the shunt from the tower, all
while watching the SWR. I couldn't find any combination that worked.
Tomorrow, I'll look at how the impedance changes as I change the
capacitance. I also have a 1000 pF vacuum variable that I've not
tried yet. I've seen examples that use a 500 pF variable in series
with the shunt and 1000 pF variable between the shunt and ground, but
I haven't tried this, yet.
Am I on the right track? What other tack should I take?else should I
try? I have a good sized B&W coil should work for an L network.
Should I simply try that?
Kim Elmore, N5OP
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