[TowerTalk] Importance of feed line length

[Paul Christensen]

> "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 on-line 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 600-ohms to 
minimize loss over that long run.  Let's further assume that the inverted 
vee has a feed-point 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 feed-point 
impedance will repeat at the input end of the line.  The SWR at any point of 
the 600-ohm 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 
600-ohm line, let's just use some unknown but convenient length of 50-ohm 
coax to get it connected at the transceiver in the shack.

Something really unique happens:  The impedance at the point where the 
600-ohm line meets the 50-ohm coax is exactly 50-ohms 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