Re: New to group and thoughts on Return Loss and Loss
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I am on holiday so did follow the various threads recently.?A bridge or Directional coupler is the heart of any VNA. Strictly speaking if you are serious, the bridge or coupler ought to be characterised carefully before you make any accurate measurements. A standard Return loss bridge is a three post device. in others like the nanovna it is in fact a 4 port device. In, out, return mag and input phase. So in principle you should require S11, S12... A 4x4 matrix with 16 components. Only a masochist would want to go about doing all these measurements. FOR most common purposes it is also not necessary. Nevertheless the information may be necessary depending on what you are doing.?That 12dB inherent bridge loss for open/short loads is related to the above 16 parameters. It is not nonsense. Theoretically you can work out that it's 12.014.. dB but as frequency increases it's value increases, it never decreases. That is why we zero it out, but there is a price. Assuming your detector can get down to 92dB (dynanic range) then at dc you will have a directivity of 80dB but as? frequency increases this drops. The directivity is the maximum range, the largest RL value you can measure with the instrument, provided it is properly calibrated.? In some cases if you want to do S11, S21 in line measurements this calibration is non trivial. at his is why professional talk about 12 points and 16 point calibration procedures etc.?For causal work a 3 point is quite adequate, in fact the minvna uses only 1 point calibration and I am ok with it for casual antenna work.Incidentally a RL of 70dB corresponds to 1.00063 vswr or a reflection coeff magnitude of 0.00031.? If anybody claims that the nanovna can measure these values accurately at even 10MHz, as Dr Kirby has objected,? then I would like to see some hard evidence.Hope this helps with the discussion. I now rather look forward to my Prague appointment for the Mozart: Don Giovanni opera, in the Opera House where he first debut it to the world in 1787.Sent from my Samsung Galaxy smartphone.
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-------- Original message --------From: Warren Allgyer <allgyer@...> Date: 22/08/2019 01:00 (GMT+01:00) To: [email protected] Subject: Re: [nanovna-users] New to group and thoughts on Return Loss and Loss When using a spectrum analyzer/tracking generator/RF bridge to make S11 measurements, part of the setup is to normalize the results to zero with an open at the DUT port. This the the analog of the ¡°O¡± calibration of the VNA.A reading of ¡°-12 dB¡± or, in my case, -19 dB¡¡ is simply the inherent loss of the bridge and it must be normalized out if you need a direct reading of RL or used as an offset for the uncalibrated reading.Return losses of DUT at the far end of a coax line require calibration at that end, not at the device itself.WA8TODOn Aug 21, 2019, at 4:23 PM, Stuart Landau via Groups.Io <stuartl73@...> wrote:Return loss is a very old term used in the telephone industry for a very long time. There is confusion about the sign, but the negative has been used as long at the term return loss has been in use. Most of us understand the meaning and implications.The 12 dB nonsense came up in a 100% reflection measurement because of losses in the RL bridge, but it's meaningless.The reference for either an open or short is a return loss of zero. The bridge losses should only reduce the dynamic range of a measurement, not the accuracy.However, if you have any transmission loss between the RL measuring device and the 100% reflection, it will give you a better return loss than zero.If a coax has a 3dB loss for instance, it will show (in theory) a 6 dB return loss because the RF will travel twice through a 3dB loss (the coax).Stuart K6YAZLos Angeles, USA-----Original Message-----From: Hans J Albertsson <hans.j.albertsson@...>To: nanovna-users <[email protected]>Sent: Wed, Aug 21, 2019 11:33 amSubject: Re: [nanovna-users] New to group and thoughts on Return Loss and LossA loss is a negative gain.So, saying a return loss is negative simply means you are expressing thatloss as a formal gain.Don't overdo the besserwisserness of your insights.Den ons 21 aug. 2019 20:07Ron Spencer via Groups.Io <ron.spencer@...> skrev:> Interesting topics being discussed.> > > > I was surprised to read that the RL from a short or open is 12.XXXdB. That> just isn't true, ever (assuming, of course, lossless feedline/ connectors).> Its 0. Always will be. Energy sent down the feedline to either the short or> open has "no where" to go so is 100% reflected.> > > > I also read loss is always negative? When calculating power out of a> system sure. But when expressing it (such and such feedline has XdB of> loss) its always a positive number. Negative loss implies gain.> > > > I see this often so I can readily understand how folks can get this wrong.> Professionals get it wrong. I saw one commercial (not for ham radio)> amplifier maker have a graph of return loss labeled in -Db.> > > > All that said, I find the nanovna a very useful device. Much easier to> bring to the tower base than my HP 8753. I look forward to using it more.> > Sent using > > > >
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