Balanced feed line - why is it better?

#1 · November 12, 2022, 11:05 am

Quote from YIJ on November 12, 2022, 11:05 am
Interesting knowledge dump I came across recently. Why is ladderline / openwire/ whatever you want to call it, better?

https://ham.stackexchange.com/questions/7333/what-is-the-actual-loss-in-a-feed-line-with-high-swr

In a quick nutshell, despite having high SWR, and even with an unresonant antenna, after a few reflections ALL the power radiates out the antenna with a small allocation to loss as compared to a HUGE loss with coax. The coax outer braid is more or less porous and is not a perfect shield:

Example SWR of 2.3 on ladderline.

https://ham.stackexchange.com/questions/5411/there-is-no-reverse-flow-of-energy-in-a-mismatched-transmission-line-is-this-co

"Wherever you have a mismatch on a transmission line, there is a reflection (at least partial) back to the source. A percentage of power is "actually" reflected back. It can be separated from the transmitted power using a suitable device like a hybrid. When the reflected wave comes back to the source, it can be absorbed if it is terminated at Zo (eg 50 ohms). A typical transmitter does not provide a 50 ohm load to returning power, however, so there is a (partial) reflection of energy back in the forward direction. There is an infinite series of partial reflections back and forth, fading away as power is transferred to the load or lost in the transmission line. In the end, a fraction of the transmitter's power is ultimately transferred to the antenna. The fraction not transferred ends up partly as heat due to transmission line loss and partly as heat dissipation in the transmitter."

"

"After "bouncing" back and forth the part of the signal that will be transmitted will probably be out of phase with the signal sent by the transmitter."

This seems to be the crux of the question. The phase shift in the RF carrier caused by the reflected wave delay(s) is irrelevant because the length of the transmission line can vary the phase shift by any amount including 360 degrees if the feedline is one wavelength long. There is no HF amateur mode that I know of that depends on the phase of the RF signal for data transmission. It would be nice to be able to send a data bit with every HF RF cycle but Mother Nature seems to have other ideas.

So the real question is: What effect does the round trips made by the reflected waves on the transmission line have on the (audio) frequency phase of the modulation of the radiated waves?

With each round trip to the tuner and back to the antenna feedpoint, the magnitude of the reflected wave decreases until it has negligible effect. So let's assume after being re-reflected 20 times, the re-reflected wave has no effect. Let's assume that 20 re-reflections take four microseconds at the speed of light in the medium. Does it make sense that if an individual reflected wave is damped and disappears (losses and radiation) after four microseconds that it cannot possibly have much effect on a modulating signal with a cycle time of 1000 microseconds (e.g. 1000 Hz)?

Since we don't depend upon HF/RF phase shifts to convey any information, worrying about HF/RF phase shifts is a waste of time. Since the delay in the re-reflected waves traveling at the speed of light in the medium is negligible compared to the cycle time of our modulating waves, worrying about audio modulation frequency phase shifts is a waste of time at HF/RF (except maybe for software designers).

However, at UHF and higher frequencies, re-reflections can be a problem."

Digest at your leisure.

Interesting knowledge dump I came across recently. Why is ladderline / openwire/ whatever you want to call it, better?

https://ham.stackexchange.com/questions/7333/what-is-the-actual-loss-in-a-feed-line-with-high-swr

In a quick nutshell, despite having high SWR, and even with an unresonant antenna, after a few reflections ALL the power radiates out the antenna with a small allocation to loss as compared to a HUGE loss with coax. The coax outer braid is more or less porous and is not a perfect shield:

Example SWR of 2.3 on ladderline.

https://ham.stackexchange.com/questions/5411/there-is-no-reverse-flow-of-energy-in-a-mismatched-transmission-line-is-this-co

"Wherever you have a mismatch on a transmission line, there is a reflection (at least partial) back to the source. A percentage of power is "actually" reflected back. It can be separated from the transmitted power using a suitable device like a hybrid. When the reflected wave comes back to the source, it can be absorbed if it is terminated at Zo (eg 50 ohms). A typical transmitter does not provide a 50 ohm load to returning power, however, so there is a (partial) reflection of energy back in the forward direction. There is an infinite series of partial reflections back and forth, fading away as power is transferred to the load or lost in the transmission line. In the end, a fraction of the transmitter's power is ultimately transferred to the antenna. The fraction not transferred ends up partly as heat due to transmission line loss and partly as heat dissipation in the transmitter."

"After "bouncing" back and forth the part of the signal that will be transmitted will probably be out of phase with the signal sent by the transmitter."

This seems to be the crux of the question. The phase shift in the RF carrier caused by the reflected wave delay(s) is irrelevant because the length of the transmission line can vary the phase shift by any amount including 360 degrees if the feedline is one wavelength long. There is no HF amateur mode that I know of that depends on the phase of the RF signal for data transmission. It would be nice to be able to send a data bit with every HF RF cycle but Mother Nature seems to have other ideas.

So the real question is: What effect does the round trips made by the reflected waves on the transmission line have on the (audio) frequency phase of the modulation of the radiated waves?

With each round trip to the tuner and back to the antenna feedpoint, the magnitude of the reflected wave decreases until it has negligible effect. So let's assume after being re-reflected 20 times, the re-reflected wave has no effect. Let's assume that 20 re-reflections take four microseconds at the speed of light in the medium. Does it make sense that if an individual reflected wave is damped and disappears (losses and radiation) after four microseconds that it cannot possibly have much effect on a modulating signal with a cycle time of 1000 microseconds (e.g. 1000 Hz)?

Since we don't depend upon HF/RF phase shifts to convey any information, worrying about HF/RF phase shifts is a waste of time. Since the delay in the re-reflected waves traveling at the speed of light in the medium is negligible compared to the cycle time of our modulating waves, worrying about audio modulation frequency phase shifts is a waste of time at HF/RF (except maybe for software designers).

However, at UHF and higher frequencies, re-reflections can be a problem."

Digest at your leisure.

From the hills of Laguna, the one and only operator of maximum pleasurable listening experiences.