Antenna

la7no · Posted: Sat Jul 05, 2014 10:17 pm Post subject: Feeder SWR & power loss

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?

bigSteve · Antenna Wizard Joined: 14 Mar 2009 Posts: 265

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.

la7no · Posted: Sun Jul 06, 2014 7:27 pm Post subject:

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?

bigSteve · Antenna Wizard Joined: 14 Mar 2009 Posts: 265

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.

la7no · Posted: Tue Jul 08, 2014 7:03 am Post subject:

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?

la7no · Posted: Tue Jul 08, 2014 10:09 pm Post subject:

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.

bigSteve · Antenna Wizard Joined: 14 Mar 2009 Posts: 265

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?

la7no · Posted: Wed Jul 09, 2014 6:21 am Post subject:

E Kafeman · Posted: Thu Jul 10, 2014 2:37 pm Post subject:

mkiv · Antenna-Theory.com Newbie Joined: 09 Jul 2014 Posts: 4

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.

mkiv · Antenna-Theory.com Newbie Joined: 09 Jul 2014 Posts: 4

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.

la7no · Posted: Fri Jul 11, 2014 4:04 pm Post subject:

E Kafeman · Posted: Fri Jul 11, 2014 5:14 pm Post subject:

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.

la7no · Posted: Sun Jul 13, 2014 2:42 pm Post subject:

la7no · Posted: Mon Jul 14, 2014 7:08 am Post subject:

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