SWR and Radiated Energy

Hi All,

I'm new to this forum so let me say hello and a word about myself.

I am a retired engineer and an active ham operator in the United States. I love electronics and thoroughly enjoy understanding the principals from a practical viewpoint, meaning the less math the better.

That said I do have a question.

It is my understanding that all the energy that gets reflected back down the transmission line from the antenna is reflected back up the line once it hits the transmitter output. It is also my understanding that this reflection is 100% and that if you had a perfect transmission line with no losses, 100% of the original energy put into a line would get to the antenna regardless of the SWR. In other words, absent losses, if 50% gets reflected back to the transmitter, this 50% finds its way to the antenna and due to the SWR being what it is, 50% of the 50% then gets reflected back to the transmitter. This back and forth goes on exponentially until all the power put out by the transmitter is radiated.

In other words, with a lossless line, the SWR has no impact upon the final power output to the antenna.

Please confirm that this is true and if so, I don't understand how you could have 100% reflection from the transmitter/transmission line interface. How could this be explained?

Thank you,

Bob, AC2Z

 

Hi Resqueline and thanks for the welcome.

My apologies but what I posted was not totally correct but I'm still confused about my understanding of the situation.

It is my understanding that there is 100% rereflection when there is an antenna tuner used between the transmitter and the antenna.

I have read this many times over the years but have never understood it.

Please see http://www.wc7i.com/reflection section.htm

Also see http://en.wikipedia.org/wiki/Antenna_tuner and I quote from this article

'It is a common misconception that high standing wave ratio (SWR) in and of itself causes loss. This is not the case. An antenna with a high SWR (e.g. 4:1) when properly configured with an ATU could have only a few percent additional loss compared to a perfectly resonant antenna. The ATU essentially redirects the reflected energy back along the feedline and antenna path. The additional losses come from the inherent losses within the feedline and antenna itself. SWR causes feed line losses to be multiplied. Low loss feedline would have minimal loss when tuned with an ATU whereas a "lossy" feedline/antenna combination of the same SWR could have significant loss."


This explains why ladder line with a tuner can be used with an antenna of virtually any SWR. The energy keeps going back and forth and due to the low losses of ladder line, most of the original energy gets radiated.

So I do believe there is 100% rereflection from the tuner but I can't understand how that could be!! How does the tuner do it?

Perhaps I'm not understanding this correctly but there must be something going on to give me this impression!

Thanks again,

Bob, AC2Z

 

Guys,

I'm sorry but I went back and read the first article again and it did mention 100% rereflection from the transmitter if no tuner is used.

The author goes on to say that a high SWR is very bad for the transmitter but it is not because of the absorption of the reflected energy. It could ruin the transmitter because the final amplifier of the transmitter is detuned and this causes it to draw to much current.

Bob (Very confused),

AC2Z