SWR and Radiated Energy

[Bob Guercio]

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

 

[Resqueline]

Welcome to the forum Bob!

I'm not an authority on this but it's my understanding that the energy reflected from the antenna is more or less absorbed (& turned into heat) by the output stage, certainly not reflected 100% back again.
The transmitter can be said to consist of an ideal generator and a 50 ohm output impedance in series. A signal returned into this by a 50 ohm line would be totally absorbed by the output impedance, just like it would be 100% absorbed by an ideal 50 ohm antenna. It doesn't matter if there's a zero impedance generator in series.

 

[Bob Guercio]

Welcome to the forum Bob!

I'm not an authority on this but it's my understanding that the energy reflected from the antenna is more or less absorbed (& turned into heat) by the output stage, certainly not reflected 100% back again.
The transmitter can be said to consist of an ideal generator and a 50 ohm output impedance in series. A signal returned into this by a 50 ohm line would be totally absorbed by the output impedance, just like it would be 100% absorbed by an ideal 50 ohm antenna. It doesn't matter if there's a zero impedance generator in series.

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

 

[Bob Guercio]

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

 

[Resqueline]

Yes this phenomenon certainly seems confusing to many. If I understand correctly what I've read, it seems that a former author for the ARRL handbook on this subject is in disagreement with the present ARRL view.

If the output impedance of a transmitter is not resistive, but purely reactive, it would seem natural to me that standing waves would be re-reflected and not absorbed. Caps & coils can't absorb any energy, only "transform"/reflect it. Only the output transistor could absorb anything. But I'm not enough experienced with transmitters to have any standpoint on how they really behave in practice.
It would be interesting though to look at a transmitter under different conditions with a thermal camera.

Part of the SWR & tuner action can be compared with mains phase compensation. Say you have a room with (old) fluorescent lamps. Say they draw 16 amps, but you need to run them on a 10 amp circuit. So you add capacitors in parallell with them. The caps are dimensioned to draw X amps, and so you get the total draw down to 9 amps. But still there's a 16 amp current going between the caps and the lamps.

An antenna tuner would not only correct the phase relationship but transform the impedance as well. I would rather place this tuner close to the antenna instead of the transmitter to avoid possibly high standing currents or voltages in the feed line.

 

[davenn]

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

Yes that is correct .... which is why most transmitters on amateur radio gear has SWR protection.. That is .... as an increase in reflected power is detected the drive to the final
stage of the transmitter is decreased thus decreasing the output of the transmitter.

I really have to comment about ATU units ... as I did recently in another forum......

You have to remember that an ATU only fools the transmitter into thinking that its
looking into a good load and therefore deliver full power. BUT!!! this DOES NOT mean that you are radiating any more power WHY.... because
the antenna is still NOT RESONANT therefore it is NOT an efficient radiator at the
frequency that you are trying to use it on and it CANNOT radiate all the energy being delivered to it.
The hi SWR... that is ...
bad mismatch caused by a non resonant antenna is still there it is just being
hidden from the transmitter by the ATU.

there is ONLY ONE way to efficiently radiate all the transmitter energy (minus the coax,
connector, heating etc losses) and that is to have an antenna cut resonant to the freq of operation.
Personally I dislike ATU's they are a necessary evil to get some energy into a hunk of wire on multiple freqs and keep the transmitter relatively happy (fooled)
If you have to use an ATU then you have to accept the losses of efficiency associated
with the setup.

Dave
VK2TDN