A simple question about antennas and lines

I have yet another total noob question that I can't seem to find the answer for online. Maybe it's because I'm not even sure how to phrase the question! 8-O As with all my noob questions, I'm not only looking for answers, but also for links to places where I can teach myself more about the topic, or even useful search terms.

As I understand it, when you make an antenna, not only the antenna but also the transmission line receive signals. I guess I've done a lot of playing around with antennas without even really thinking about it, but I'm not sure how... er... well, how the antenna works differently than the line.

Maybe an example will help: I found this site while researching my current project, which is the construction of a passive signal repeater to improve cell reception in a few places in my house. The simple version of that project I actually DO understand (more or less) and don't (yet) need any help with it from here, involves an external antenna, a transmission line, and another antenna inside.

Now, from everything that I understand, this passive repeater system would work a lot better than a simple line without antennas on it. But why? I am very new to electronics, with a background in history and philosophy... I can understand conceptual descriptions of physics, but honestly I have a pretty much middle-school level understanding of math. I'm working on that, but it's been slow going. That's a real impediment to understanding a lot of what I'm reading. So far in my tinkering I've relied as much on instinct as understanding. Instinct tells me, for example, that the antennas in my passive repeater system should emerge at a right angle to the transmission line, and in the case of the external antenna, perpendicular to the line of the signal... but I'm not sure I could explain why. And thinking about that, I realized that I had no idea why you need an antenna, and not just a line.

Hello,

Normally spoken a transmission line is to transport "energy" from one place to another (for example from the antenna to the receiver). So it normally does not radiate nor receive. This means the center conductor current equals the current at the inside of the screen, but with opposite phase. Outside the coaxial cable, the field of the center current cancels the field of the screen return current.

Now the difficult thing, a transmission line may radiate (or receive). An antenna may induce so called common mode current into the transmission line (coaxial cable). Quarter wave whips without ground plane, directly connected to a coaxial line do generate lots of common mode current. The common mode current runs on the outside of the screen and has no part that cancels the fields, so common mode current on coaxial lines do radiate.

A standard half wave dipole directly connected to a coaxial cable may provide good VSWR, but also some common mode current. The field radiated by this common mode current changes the radiation pattern of the dipole and my result in interference. A so-called balun (balanced to unbalanced transition) is normally used to connect an unbalanced transmission line (coaxial cable) to a balanced antenna (like a half wave dipole). Just a clamp-on ferrite (from suitable magnetic material) may serve as a balun. This is frequently used in the HF to low UHF frequency range.

There are antenna designs where a part of the coaxial cable's screen is used as radiator, or provides tha balun function. Generally spoken, it is not recommended to have common mode current on the feedline that connects the antenna to the transmitter/receiver.

If your math is not very good at this moment, try some radio amateur antenna literature. Once you get better understanding of antennas and math, antenna books from Joe Car and "Antennas" from Kraus can be very helpful.