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Re: 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.

WA8TOD

On 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 am
Subject: Re: [nanovna-users] New to group and thoughts on Return Loss and Loss

A loss is a negative gain.

So, saying a return loss is negative simply means you are expressing that
loss 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=
[email protected]> 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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