On Sun, 21 Sep 2003 23:45:09 -0400, "Tarmo Tammaru"
wrote:
Mike,
You seem to be talking about situations like old time AM car radios, where
the feedline was really used as a shielded wire, and not a transmission
line. They got away with it, because the feedline was very short, and added
less than 100 PF of capacity.
When used as a transmission line, the line is terminated in, or at least the
same order of magnitude impedance as the the coax. The coax has both
capacitance, and inductance. So, if you connect a 50 Ohm antenna to 100 feet
of RG58 coax, the impedance you see at the other end is 50 Ohms, and you
don't have to worry about the fact that there is also 2800 PF of
capacitance. A receiver input generally has a transformer, or other device
that transforms the 50 Ohms to hundreds, or a few thousand Ohms. In
connecting a transmission line to a parallel tuned LC circuit, you don't
connect the line to the top of the LC. Rather you connect it to a tap near
the bottom of the inductor, or you add a second winding to the inductor to
make it into a transformer.
Tam/WB2TT
FYI - I am a newbie SWL
OK, that answers my question. I now see for a transmission line the
coax also has inductance value which balanance out the extra
capacitance added by the 50 foot run of RG58 itself.
I was only looking at the capacitance value and it was driving me
crazy to understand why this wouldnt make tuning the circuit nearly
impossibe. I thought I was dealing with thousands of pf.
I am using a homebrew PI network tuner at the receiver end of my Sony
portable.
To get rid of household noise I want to move from an outside random
wire fed directly to the tuner to a coax line feeding feeding the wire
into the house , then to the tuner.
The schematic of my tuner looks like this:
http://www.qsl.net/dl2lux/fish/fishpi_e.html
Eingang = entrance, Ausgang =exit, Masse = ground
After translating those German words I just realized my transmission
line differs as it comes into the tuner and connects to the right side
capacitor movable vanes which is connected to the inductor tap switch.
So mine seems backwards in regards to the schematic. how much
difference would this make?
Mike