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Feeder SWR & power loss
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la7no
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Joined: 05 Jul 2014
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Location: Bergen, Norway

PostPosted: Sat Jul 05, 2014 10:17 pm    Post subject: Feeder SWR & power loss Reply with quote

For a long time I've been reading everything I could find regarding antenna matching, feeders, SWR, reflections and power loss. What disturbs me is that there is a clear disagreement regarding quite fundamental things. In particular related to SWR and reflected power.

Some experts claim that power reflected from the antenna is lost. For example, with an SWR of 4, 36% of power delivered by the transmitter is reflected from the antenna, and therefore not transmitted.

Other experts (f.ex. Rothammel) are claiming that this power is simply re-reflected towards the antenna again. This will in turn will be reflected back towards the transmitter again, etc. etc. The losses are mostly caused by resistive and reactive losses in the feeder itself.

How can I find out who is right?
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bigSteve
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PostPosted: Sun Jul 06, 2014 6:41 pm    Post subject: Reply with quote

Whoever said "the power is re-reflected towards the antenna again" is full of crap. There is no "multiple reflections in time". The vector network analyzer that measures the reflection (s11, reflection coefficient, vswr, all equivalent) is measuirng a steady state reflection coefficient - there is no time component. Whatever "other expert books [Rothammel]" you are reading are total garbage.
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la7no
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PostPosted: Sun Jul 06, 2014 7:27 pm    Post subject: Reply with quote

Hello bigSteve,

Thanks for your reply and opinion.

Please have a look at these:

http://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf
http://zs6vd.bestwebs.co.za/SWR.pdf
http://www.hamuniverse.com/wc7iswr.html

Would you say that they have all got it wrong?
Or am I misunderstanding you?
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bigSteve
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PostPosted: Sun Jul 06, 2014 9:03 pm    Post subject: Reply with quote

I read the arrl.org section "Does higher SWR Lead to Lower Power Being Transmitted?" in which he argues that the power is eventually delivered to the antenna.

I've got to tell you, I interview antenna engineers a lot and have worked in the field for many years. If someone claimed that in an interview, I would immediately follow up with "ok great. So why do we bother with impedance matching?", i.e. why if the power will be delivered anyway, would we match the antenna to 50 Ohms?

The fact is, if I have 3 dB of mismatch loss (VSWR=6), and I tune it, my antenna efficiency or radiated power increases 3 dB. I can assure you from my own measured data, there is absoultely no ambiguity on that.

So...I'm completely at a loss for what they are talking about. For the radio to deliver power to a 50 Ohm system, it has an intrinsic 50 Ohm impedance. As a result, reflected power is absorbed within the transmitter.

Personally, I give the engineering in the article zero weight. If anyone can follow up with a reason why I'm wrong (or if you email the authors and ask) I'd really love to hear it.
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la7no
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PostPosted: Tue Jul 08, 2014 7:03 am    Post subject: Reply with quote

The assumption in the article is that an antenna tuner was being used, which is tuned to the lowest SWR available on the coax before the tuner. There is still a high SWR on the coax between the tuner and the antenna. The reflected power from the antenna is re-reflected at the antenna tuner, and goes back up to the antenna, and not down into the radio. Or?
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la7no
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PostPosted: Tue Jul 08, 2014 10:09 pm    Post subject: Reply with quote

Of course there is a time-component. From the moment the power is turned on until steady-state is achieved, time elapses.

If the SWR is tuned to be lower, the transmitter will see a load closer to its design specs. Consequently, it will be able to produce higher power, and the radiated power will increase too.
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bigSteve
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PostPosted: Wed Jul 09, 2014 3:52 am    Post subject: Reply with quote

I wonder how you can measure a "standing wave ratio" unless the reflections are already taken into account? Where do you think the standing waves come from if the measurement doesn't already take this into account?
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la7no
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PostPosted: Wed Jul 09, 2014 6:21 am    Post subject: Reply with quote

bigSteve wrote:
I wonder how you can measure a "standing wave ratio" unless the reflections are already taken into account? Where do you think the standing waves come from if the measurement doesn't already take this into account?


Sorry, I do not understand what you mean here.
Could you please elaborate a bit?
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E Kafeman
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Joined: 04 Feb 2013
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PostPosted: Thu Jul 10, 2014 2:37 pm    Post subject: Reply with quote

Quote:
The assumption in the article is that an antenna tuner was being used, which is tuned to the lowest SWR available on the coax before the tuner. There is still a high SWR on the coax between the tuner and the antenna.

It is a ham-myth. Antenna tuner, if it consist of passive linear reasonable lossless components is its behavior bidirectional. It will be exactly same VSWR in both directions.
Assume 100 Ohm impedance at tuner antenna input and 50 Ohm at radio side resulting in a VSWR 1:2. A good antenna tuner will then act as a ideal transformer with 1:2 ratio, resulting in VSWR 1:1 in both directions.
As a thinking experiment, reverse input/output connections at tuner while not changing 1:2 settings of tuner, and it will result in 1:4 VSWR in both directions.

If amount of input power relative power before and after antenna tuner differ in a reasonable lossless antenna tuner, is it against energy conservation laws.
Due to this will amount of forward power be the same before and after antenna tuner but same rules apply for reverse unbalanced reflection, it will be same VSWR before and after the tuner.

Quote:
The reflected power from the antenna is re-reflected at the antenna tuner,

Compare with a water hose having a pressure reducing valve arrangement half-ways and you is holding the hose end spraying water in the air. Probably will some water not be sprayed away and instead be reflected along outside of hose.
Million-dollar question is if amount of this reflected water will change before and after it have passed outside of the valve housing.
Reflected water will neither effect water passing inside of the valve, as no path exist.
For the electrical case, an unshielded antenna tuner with tuning components that also acts as antennas, poor coaxial terminations and very lossy coaxial cable can make total situation a bit more complex to summarize, but that is all.

System reflection coefficient is relative easy to prove that it is only depending on source and load impedances ratio seen from same measurement point. Actual impedance ratio measurement can be done with a VNA measuring impedance in one direction while having other directions disconnected, and then shift, measuring the other direction and finally divide both results.
Practical measurement can be done by injecting a short signal and then listen for reflecting amplitude and phase. There is no buildup-period as VNA not need to inject anything while listening for reflecting result. Some VNA's can however work in fully bidirectional, non gated, mode but it will not change measured result.
Same system VSWR will be measured if measuring near antenna, in the middle or near transmitter/receiver.
If coaxial-cable is less ideal can its parameters effect of total system performance be analyzed by S-parameter measurements with a VNA.
Agree with bigSteve, servers that are spreading "re-reflection ham facts" should be burnt down.
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mkiv
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PostPosted: Fri Jul 11, 2014 9:01 am    Post subject: Reply with quote

If the transmitter's impedance and cable impedance are pretty close (eg 50 ohms) then there shouldn't be much power re-reflected towards the antenna. The transmitter could have an isolater to absorb this reflected energy.
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mkiv
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PostPosted: Fri Jul 11, 2014 9:02 am    Post subject: Reply with quote

If the transmitter's impedance and cable impedance are pretty close (eg 50 ohms) then there shouldn't be much power re-reflected towards the antenna. The transmitter could have an isolater to absorb this reflected energy.
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la7no
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PostPosted: Fri Jul 11, 2014 4:04 pm    Post subject: Reply with quote

Quote:
Antenna tuner, if it consist of passive linear reasonable lossless components is its behavior bidirectional. It will be exactly same VSWR in both directions.


My setup is an Elecraft K3 with built-in 100W PA and tuner. I also have a LP-100A Digital Vector RF Wattmeter connected between the K3 output and the coax feeder. The antenna is a HyEndFed 5-band horizontal.

When I on 60M press 'Tune' on the K3, I could get a 1:1 SWR. However, the LP-100A could indicate a 1:3.5 SWR. Since the antenna is not working well on 60M, I find the 1:3,5 reasonable.

So, in this case, I assume the SWR between PA and tuner is 'perfect', but the SWR between tuner and feeder is not so good.

How can this be explained if the tuner is bidirectional?
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E Kafeman
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PostPosted: Fri Jul 11, 2014 5:14 pm    Post subject: Reply with quote

Have no knowledge about your hardware but typical measurement of system VSWR is done by measuring source impedance R+jX and then measuring load impedance. For 1:1 VSWR should it then be a conjugate match, RS=RL and XL=-XS.
Guess your LP-100A, if not calibrated for actual radio impedance, assumes an reference impedance 50+j0 Ohm and due to that not shows correct VSWR as there not is 50 Ohm neither as load or source impedance.
Shift input/output of the LP-100A and it do probably indicate that radio+tuner is mistuned with a VSWR of 1:3.5 but if you can read complex impedance values do it shows a not to bad conjugate match compared to measured antenna impedance values.
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la7no
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PostPosted: Sun Jul 13, 2014 2:42 pm    Post subject: Reply with quote

bigSteve wrote:
The fact is, if I have 3 dB of mismatch loss (VSWR=6), and I tune it, my antenna efficiency or radiated power increases 3 dB. I can assure you from my own measured data, there is absoultely no ambiguity on that.


The antenna efficiency does not change with feed line VSWR. What changes is the transmission line mismatch loss, not antenna efficiency. Those are two different things. If you had a lossless line and a transmitter that could drive *any* load, you would see no change in radiated power vs antenna to line VSWR.

The lower efficiency you are referring to is really transmission line loss due to high VSWR. If the transmission line was lossless, the only reason to match an antenna is maximum power transfer out of the transmitter. With a lossy transmission line, matching to the line also reduces line losses.
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la7no
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PostPosted: Mon Jul 14, 2014 7:08 am    Post subject: Reply with quote

mkiv wrote:
If the transmitter's impedance and cable impedance are pretty close (eg 50 ohms) then there shouldn't be much power re-reflected towards the antenna. The transmitter could have an isolater to absorb this reflected energy.


If the both transmitter output and cable input has the same impedance, then max efficiency would be 50%. And half the power would be dissipated in the transmitter. Does not sound good. Or?

Why would we like the transmitter to absorb reflected power? Would it not be better if it was re-reflected towards the antenna?
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