As has been stated before, the importance of SWR or return loss for most practical users is not how little power is reflected (within reason) but at what frequency is the antenna actually resonant. Once you have been involved in this for a while, you realize how the the antenna environment influences the resonant frequency and SWR.
Stuart K6YAZLos Angeles, USA
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-----Original Message-----
From: Warren Allgyer <allgyer@...>
To: nanovna-users <
[email protected]>
Sent: Mon, Aug 5, 2019 8:32 am
Subject: Re: [nanovna-users] NanoVNA Under The Covers
And, just to ground this discussion a bit, for practical purposes in the professional RF and video worlds any return loss in excess of 30 dB is considered excellent. A return loss of 30 dB for example translates to a VSWR of 1.07:1.
On Aug 5, 2019, at 11:17 AM, Warren Allgyer via Groups.Io <allgyer@...> wrote:
Hi Tuck(?)
Whether a load shows a return loss of 50 dB, 30 dB or 70 dB depends upon what load was used to calibrate the VNA. As you can see from the chart, all three loads exhibited better than 70 dB when they were used as the calibration source. On the other hand, these very same loads all exhibited in the 30's when checked after calibration with the other load.
The nanoVNA divides the displayed frequency range into 101 "bins". The normal return loss for an open, short, and a nominal load are all known factors. So when you tell the VNA you are using a short it checks each of the 101 bins and determines what correction factor is needed to make that value 'normal". This is done three times, once each for open, short, and nominal load. Then those bin by bin correction factors are applied to any measurement you make on the theory that the calibration procedure has "nulled' out any inaccuracies.
So there is no defined return loss inherent to a particular load. There is only loss as measured against a calibration standard. Anything can be a calibration 'standard'.... but if you use one that is reactive or of a non-standard resistance, the VNA assumes it is correct and reports all subsequent measurements against that standard.
This principle, by the way, can be used to your advantage. Say you have devices to be measured at the far end of a piece of coax. If you apply the calibration standards at the far end of the coax instead of at the instrument you will effectively "null out" the coax and the values reported will be as if the far end device was connected directly to the VNA. It is a very useful technique.
This VNA is an incredible value. I am thrilled with mine!
Warren Allgyer
WA8TOD
PS: "Pete" thinks he is still the lap dog he was as a 10 pound puppy. At 105 lbs he is a lap full!
