Reflections

[Bob Guercio]

Hi All,

In a lossless system where there is a mismatch at the antenna/transmission line junction, the rf energy is reflected back and forth between the antenna and the transmitter. At each reflection, a bit more of the energy gets radiated until all 100% of it is radiated. There is absolutely no loss of rf energy in a perfectly lossless system despite the fact that the SWR on the transmission line is not 1:1.

It is for this reason that with a low loss transmission line, such as ladder line, a high SWR reading can be tolerated as long as a tuner is used. The 100% rereflection is then accomplished at the tuner/transmission line interface.

Could someone please explain to me what is happening at the transmitter/transmission line junction or at the tuner/transmission line junction to cause this to happen? How does the transmitter or tuner do this?

Thank you,
Bob Guercio

 

[Miles Prower]

Different impedances between the T-line and the source / load impedance. Since the two different impedances will establish a different ratio of voltage to current, the incident signal will split into two different components.

 

[Bob Guercio]

Different impedances between the T-line and the source / load impedance. Since the two different impedances will establish a different ratio of voltage to current, the incident signal will split into two different components.

Hi Miles,

Could you please elaborate on this?

Thank you,
Bob

 

[ke5frf]

This is a very "loaded" question, pardon the pun

OK, let me do my best to explain. Ladder line is a "balanced" transmission line. Depending on the dielectric property of the insulator and the spacing between the conductors, it presents an impedance that is different from the resonant impedance of your antenna element. Ladder line is typically in the hundreds of Ohms. It's been a while since I built a dipole with balanced feedline, but 300-450 Ohms sounds about right.

When you feed a resonant antenna with coaxial (unbalanced) transmission line, all you really need is a "BALUN" at the feedpoint...this is because the antenna is a balanced dipole and the feedline is unbalanced..requiring a transformer to accomodate the unbalanced to balanced transition. This is debateable BTW, as many people build resonant dipoles with 50 Ohm coax without baluns and they work just fine.

At any rate, your balanced transmission line is theoretically a better method transferring power to the antenna feedpoint because it is balanced, and the cool thing about balanced conductors like this is that common-mode currents tend to be nulled. However, if a conductive material or structure is in proximity it can cause an imbalance. Any imbalance will cause coupling and this advantage is no longer seen.

Balanced feedlines are typically used with DOUBLETS, that is, dipoles that are used on multiple bands. There is a reason that amateur radio bands, for instance, are multiple wavelengths of each other. A doublet cut for the longest wavelength band intended will work well, with a tuner, on the other amateur bands that are higher frequency but wavelength divisible.

If you are building a single band antenna, the advantage just mentioned will not be realized with balanced line. You are better off in reality with 50 ohm coax because tuners create a certain amount of heat loss. With a resonant antenna and 50 ohm transmission line the tuner is not needed. Now, you don't have to worry about coupling the antenna and causing the common mode currents and radiating feedline.

The antenna tuner is a capacitive-inductive network with, at least, a variable capacitor...the impedance mismatch of the feedline and antenna feedpoint can be "corrected" with the tuner...if the reactance of the antenna is more inductive than capacitive, the capacitor can be tuned to add capacitive reactance and peak the power transfer, much like a resonant parallel tank circuit. If the antenna is more capacitive than inductive, the capacitance of the tuner can be reduced to the same effect.

Again, the best feedline for a resonant dipole is coax and the best solution for a multiband doublet is open wire or balanced feedline with a tuner.

 

[ke5frf]

BTW,

A correction to a mistaken premise in your OP.

There is "no such thing" as a lossless transmission line. Perhaps you already know this, but there was an indication that you expect 100% of the signal to be eventually radiated after the back and forth reflections of the standing waves. This is not going to be the case because ALL transmission lines have a series resistance that will convert some of the power to heat. Also, the tuner, while an effective method of transfering power to a mismatched load impedance, is not perfect either and therefore the more severe the mismatch, the higher the voltages present, and the more opportunity for loss in the form of heat in the tuner.

Also, consider that in a tuned antenna circuit, the voltage and current are not in phase. The nature of balanced transmission line is to not shift the out of phase relationship between voltage and current. This is how maximum power transfer is achieved. But this only works in practical use with a tuner. When the voltage and current shift from resonance and there is an overlap in their phase, this is where resistive components of the antenna system will convert RF power to heat.

BTW, Antennas are one of the more complex electrical systems to understand and even more difficult to explain. I have posted W4RNL's webiste before when referencing antenna theory. Google L Cebik or W4RNL and, though he is recently deceased, his website can still be accessed with a short registration procedure. He was THE BEST, IMHO, antenna expert in the online amateur radio community and expressed his wealth of knowledge in some very good online tutorials. He does a better job of explaining things, and I'll be the first to admit the gaps in my knowledge hinder my explanations.

 

[Miles Prower]

Try this: T-Lines Free download.