"I've been doing research to determine how important feed line length is.
I've been reading up on this off and on for the last few weeks."
Justin,
Sounds like you're off the a great start. There are many online and print
publications that will help with these concepts. My favorites are the older
ARRL and Frank Jones Antenna Handbooks from the 1930s and '40s.
Minimizing feedline length reduces line loss in matched and unmatched
antenna systems. In unmatched systems where the line characteristic
impedance is not equal to the load or antenna impedance, then line length
becomes important for a number of different reasons including the design of
phased feed systems, for example, and trying to use the varying line length
(e.g., 1/4 wave line sections) as a matching transformer to convert the
impedance at one end of the line into a different impedance at the opposite
end.
In your 80m antenna example, say you were using a resonant inverted vee and
instead of coax, and you wanted to feed it with a long distance of line
using balanced feeders with a characteristic impedance of 600ohms to
minimize loss over that long run. Let's further assume that the inverted
vee has a feedpoint impedance of exactly 50 ohms resistive and no
reactance. The SWR on the line will be a whopping 12:1 but line loss is
low. If you use line lengths of any 1/2 wavelength multiple, the feedpoint
impedance will repeat at the input end of the line. The SWR at any point of
the 600ohm line section is still a constant 12:1. Let's call this 12:1
(600) since it represents the SWR on the balanced feeder section. Let's use
1.5 wavelengths of this balanced line to get that inverted vee way out back
in a field connected to your transceiver and then from the end of the
600ohm line, let's just use some unknown but convenient length of 50ohm
coax to get it connected at the transceiver in the shack.
Something really unique happens: The impedance at the point where the
600ohm line meets the 50ohm coax is exactly 50ohms resistive and the SWR
at any point on the coax is 1:1 (50). So, here's an example where line
length matters since the antenna impedance and the line characteristic
impedance are not equal by design. Such systems are common and I use one
just like this only, instead of optimizing balanced line length, a tuner is
used outside the house where the 600/50 ohm transition is made. As SWR on
a line increases, minimizing line loss becomes even more important than in
matched systems. The handbooks mentioned above, new or old, are full of
these types of examples.
Paul, W9AC
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