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Re: NanoVNA Under The Covers

 

No the return loss of open and short is theoretically 12dB. The RLB has a voltage factor of 8 and the transmission has a factor of 2 .?Sent from my Samsung Galaxy smartphone.

-------- Original message --------From: Jeff Anderson <jca1955@...> Date: 05/08/2019 21:26 (GMT+01:00) To: [email protected] Subject: Re: [nanovna-users] NanoVNA Under The Covers On Mon, Aug? 5, 2019 at 10:06 AM, tuckvk3cca wrote:>> You measure an open load, you measure a shorted load both of which should give> 12dB return loss>I'm probably missing something here, but shouldn't opens and shorts give 0 dB of return loss, not 12?? After all, their Gamma ought to be equal to 1.- Jeff, k6jca
Re: NanoVNA Under The Covers

 

You are correct. In theory, a 6 dB attenuator will give a 12 dB return loss, which another way to test accuracy of the instrument.
Stuart K6YAZ

-----Original Message-----
From: Jeff Anderson <jca1955@...>
To: nanovna-users <[email protected]>
Sent: Mon, Aug 5, 2019 12:26 pm
Subject: Re: [nanovna-users] NanoVNA Under The Covers

On Mon, Aug? 5, 2019 at 10:06 AM, tuckvk3cca wrote:


You measure an open load, you measure a shorted load both of which should give
12dB return loss
I'm probably missing something here, but shouldn't opens and shorts give 0 dB of return loss, not 12?? After all, their Gamma ought to be equal to 1.

- Jeff, k6jca
Re: NanoVNA Under The Covers

 

On Mon, Aug 5, 2019 at 10:06 AM, tuckvk3cca wrote:


You measure an open load, you measure a shorted load both of which should give
12dB return loss
I'm probably missing something here, but shouldn't opens and shorts give 0 dB of return loss, not 12? After all, their Gamma ought to be equal to 1.

- Jeff, k6jca
Re: NanoVNA Under The Covers

 

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

-----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!
Re: NanoVNA Under The Covers

 

Tuck

I am not sure if you are misunderstanding (I don't think so) or simply miss-stating the concept. A load, any load, does not have an inherent return loss. It only has a return loss as measured relative to a reference. Typically the reference is understood to be 50 ohms. Minute differences in the resistance and reactance of the reference load can lead to huge differences in return loss, especially when you are talking RL on the order of 40 or more dB.

The numbers that I have shown are not the result of design or calibration routine errors but, in fact, differences in the characteristics of the loads themselves.

As a verification I have re-run the tests I made prior with the nanoVNA on my spectrum analyzer/tracking generator/RF bridge combination. It has an inherent dynamic range of 100 dB at a resolution bandwidth of 1 kHz..... 130 dB if I engage the preamp. I again used each of the three loads as a reference on the bridge with the other two measured for return loss in succession. Here is a comparison table of the nanoVNA as compared to the professional setup.

As you can see, the results are remarkably close. Especially at return losses less than 40 dB. I have no concerns whatsoever about relying on the nanoVNA results for hobby and home lab use.

Warren Allgyer
WA8TOD

Screen Shot 2019-08-05 at 2.58.05 PM.png

Re: "Hand capacitance"

 

An antenna without a counterpoise (ground or ground-plane) is just a piece of wire. If you are testing a "rubber duckie" I would recommend mounting it onto a piece of metal; even a piece of aluminum foil would give a truer reading. A whip antenna mounted on a hand-held radio is a poor compromise, but at least the radio and your hand acts as a sort of ground.When I worked for Kenwood USA and Standard Communications we did some antenna testing of this sort.

Stuart K6YAZLos Angeles, USA

-----Original Message-----
From: Frank S <ka2fwc@...>
To: nanovna-users <[email protected]>
Sent: Mon, Aug 5, 2019 9:33 am
Subject: Re: [nanovna-users] "Hand capacitance"

You can try a short coax loaded with low freq ferrites that fit tight on
the coax. It helps (somewhat). Cal at the end of the coax

On 8/5/2019 12:13 PM, Mike Brown wrote:
I saw similar effects myself when testing the antenna from a handheld
transceiver. As Peter says it is to be expected as, in its intended use,
the human operator together with the conductive part of the rig form the
ground plane that is required by that type of simple vertical antenna.

No hand effects have been noticeable when I have tested HF, VHF & UHF
verticals (all with adequate radial systems) or with VHF/UHF yagis.

There seems to be no need to do anything with the s21 port but it would
probably be good practice to put a 50ohm load on it.

Regards

On Mon, 5 Aug 2019, 15:15 , <spaceopqa@...> wrote:

Hello Peter,

On the device itself.

So I will elaborate a little more, as I would like to use one nanovna at
the field, as a simple SWR measurement tool, and of course operate it
handheld. Maybe it will not occur in a proper HF antenna?

Would you isolate the S21 port, or simply the "open" calibration tool will
suffice?
Re: NanoVNA Under The Covers

 

Yes Warren, what you say is exactly what is done on any return loss bridge. You measure an open load, you measure a shorted load both of which should give 12dB return loss but in practice they differ because of a small O/S error. Then you measure a matched load. This should be infinity but it is not because of the real world components. If you have a power meter like me that can get down to 70 to 80dB then you can measure 60dB or more. The difference is the directivity and gives effectively the dynamic range for the instrument. Actually there is one other parameter called the isolation which I forgot how it is measured. The fact that we use 3 such calibrating loads is exactly because we want to extract these 3 parameters that characterises the bridge and therefore the rest of the instrument at each frequency. I am not disagreeing that a different load can be used as a standard and you will get different results. However a proper calibration as I have done a dozen times should not and had not given such a vast discrepancy between similar loads from 38dB to 70dB as you have shown. That is the issue. Is this a flaw of the software or the design of the bridge or the calibration routine in the nanovna?Sent from my Samsung Galaxy smartphone.

-------- Original message --------From: Warren Allgyer <allgyer@...> Date: 05/08/2019 17:17 (GMT+01:00) To: [email protected] Subject: Re: [nanovna-users] NanoVNA Under The Covers 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 AllgyerWA8TODPS: "Pete" thinks he is still the lap dog he was as a 10 pound puppy. At 105 lbs he is a lap full!
Re: "Hand capacitance"

 

You can try a short coax loaded with low freq ferrites that fit tight on the coax. It helps (somewhat). Cal at the end of the coax

On 8/5/2019 12:13 PM, Mike Brown wrote:
I saw similar effects myself when testing the antenna from a handheld
transceiver. As Peter says it is to be expected as, in its intended use,
the human operator together with the conductive part of the rig form the
ground plane that is required by that type of simple vertical antenna.

No hand effects have been noticeable when I have tested HF, VHF & UHF
verticals (all with adequate radial systems) or with VHF/UHF yagis.

There seems to be no need to do anything with the s21 port but it would
probably be good practice to put a 50ohm load on it.

Regards

On Mon, 5 Aug 2019, 15:15 , <spaceopqa@...> wrote:

Hello Peter,

On the device itself.

So I will elaborate a little more, as I would like to use one nanovna at
the field, as a simple SWR measurement tool, and of course operate it
handheld. Maybe it will not occur in a proper HF antenna?

Would you isolate the S21 port, or simply the "open" calibration tool will
suffice?
Re: "Hand capacitance"

 

I saw similar effects myself when testing the antenna from a handheld
transceiver. As Peter says it is to be expected as, in its intended use,
the human operator together with the conductive part of the rig form the
ground plane that is required by that type of simple vertical antenna.

No hand effects have been noticeable when I have tested HF, VHF & UHF
verticals (all with adequate radial systems) or with VHF/UHF yagis.

There seems to be no need to do anything with the s21 port but it would
probably be good practice to put a 50ohm load on it.

Regards

On Mon, 5 Aug 2019, 15:15 , <spaceopqa@...> wrote:

Hello Peter,

On the device itself.

So I will elaborate a little more, as I would like to use one nanovna at
the field, as a simple SWR measurement tool, and of course operate it
handheld. Maybe it will not occur in a proper HF antenna?

Would you isolate the S21 port, or simply the "open" calibration tool will
suffice?
Re: 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!
Re: NanoVNA Under The Covers

 

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!
Re: NanoVNA Under The Covers

 

This seems like taking an arbitrary resistor from the bin and calibrating your ohmmeter to it. From that point you only know how other resistors compare.

This also reminds me of some of the time-nuts discussions where they debate whether a millihertz matters in a 10 MHz reference oscillator. Try bringing that into the real world where the last 5 or 6 places can¡¯t even be reliably set.

Can someone please tell me to what levels of return loss it matters in different applications? For example, setting an antenna tuner, designing a RF amp, low level reception?

Is an an expensive and challenging to maintain lab VNA going to have any practical advantage over these $50 units? And under what circumstances and applications?


Peter

On Aug 5, 2019, at 10:12 AM, tuckvk3cca <tuckvk3cca@...> wrote:

Thank you Warren. This is no argument over a pin head. Do you believe any of your loads are 50dB or better or even 38dB or better. How do these figures degrade with Frequency?Sent from my Samsung Galaxy smartphone.
-------- Original message --------From: Warren Allgyer <allgyer@...> Date: 05/08/2019 13:46 (GMT+01:00) To: [email protected] Subject: Re: [nanovna-users] NanoVNA Under The Covers I am not sure if I am adding to or further confusing this discussion which has descended into, in my opinion, an argument over the number of angels on a pin head.First, when a three point calibration is done on any VNA, the result will be to null the return from the load used in the calibration. When the calibration is finished the display will be showing the noise floor of the instrument, not the return loss from the calibration load. The only inference you can draw about the return loss of the load in this is that it is below the noise floor of the instrument.As an illustration, I have attached a chart showing the results of three different 3 point calibrations. I used two random "50 ohm" terminations from my junk box and the calibration load supplied with the nanoVNA. After each of the three calibrations I recorded the indicated return loss for all three loads in succession.As you can see from the chart, the reading for any of the the three loads when used as the calibration load is the noise floor i.e. "<-70 dB". What can also be seen from the chart is the symmetry of the readings with different calibration loads. For example, when I calibrate with Load 1, Load 2 indicates -53.6 dB. When I calibrate with Load 2 then Load 1 indicates a similar -54.3 dB.The point is that, by definition, it is impossible to get an accurate measurement of the return loss of a calibration load on the instrument that was calibrated by that load. The instrument has been set by the calibration to assume that load is perfect and will return only its noise floor level.The nanoVNA cannot and should not be expected to match the performance of a $15,000 lab instrument. But it does a remarkable job for its cost. In my "lab" I do not care about the difference in return loss between -40.5 and -41.0 dB. What I care about is at what frequency the return loss is best and, at that frequency, whether or not the load is reactive or not. I also care about having an instrument that can show a filter profile and allow me to optimize it for the performance I want. I care about what the input to my linear amplifier looks like for return loss and impedance. For these things I find the performance of the nanoVNA to be more than adequate and the value to be tremendous!
Re: "Hand capacitance"

 

It should be much better in the field on a proper HF antenna, both because of the counterpoise and also the lower frequencies involved.

Well at least your HF antenna should have a proper counterpoise, or your HF transmitter will be ¡°hot¡± when you transmit.


Peter

On Aug 5, 2019, at 10:15 AM, spaceopqa@... wrote:

Hello Peter,

On the device itself.

So I will elaborate a little more, as I would like to use one nanovna at the field, as a simple SWR measurement tool, and of course operate it handheld. Maybe it will not occur in a proper HF antenna?

Would you isolate the S21 port, or simply the "open" calibration tool will suffice?
Re: "Hand capacitance"

 

Hello Peter,

On the device itself.

So I will elaborate a little more, as I would like to use one nanovna at the field, as a simple SWR measurement tool, and of course operate it handheld. Maybe it will not occur in a proper HF antenna?

Would you isolate the S21 port, or simply the "open" calibration tool will suffice?
Re: NanoVNA Under The Covers

 

Thank you Warren. This is no argument over a pin head. Do you believe any of your loads are 50dB or better or even 38dB or better. How do these figures degrade with Frequency?Sent from my Samsung Galaxy smartphone.

-------- Original message --------From: Warren Allgyer <allgyer@...> Date: 05/08/2019 13:46 (GMT+01:00) To: [email protected] Subject: Re: [nanovna-users] NanoVNA Under The Covers I am not sure if I am adding to or further confusing this discussion which has descended into, in my opinion, an argument over the number of angels on a pin head.First, when a three point calibration is done on any VNA, the result will be to null the return from the load used in the calibration. When the calibration is finished the display will be showing the noise floor of the instrument, not the return loss from the calibration load. The only inference you can draw about the return loss of the load in this is that it is below the noise floor of the instrument.As an illustration, I have attached a chart showing the results of three different 3 point calibrations. I used two random "50 ohm" terminations from my junk box and the calibration load supplied with the nanoVNA. After each of the three calibrations I recorded the indicated return loss for all three loads in succession.As you can see from the chart, the reading for any of the the three loads when used as the calibration load is the noise floor i.e. "<-70 dB". What can also be seen from the chart is the symmetry of the readings with different calibration loads. For example, when I calibrate with Load 1, Load 2 indicates -53.6 dB. When I calibrate with Load 2 then Load 1 indicates a similar -54.3 dB.The point is that, by definition, it is impossible to get an accurate measurement of the return loss of a calibration load on the instrument that was calibrated by that load. The instrument has been set by the calibration to assume that load is perfect and will return only its noise floor level.The nanoVNA cannot and should not be expected to match the performance of a $15,000 lab instrument. But it does a remarkable job for its cost. In my "lab" I do not care about the difference in return loss between -40.5 and -41.0 dB. What I care about is at what frequency the return loss is best and, at that frequency, whether or not the load is reactive or not. I also care about having an instrument that can show a filter profile and allow me to optimize it for the performance I want. I care about what the input to my linear amplifier looks like for return loss and impedance. For these things I find the performance of the nanoVNA to be more than adequate and the value to be tremendous!
Re: "Hand capacitance"

 

Are you seeing the effects of your hand on the instrument or on the antenna? Remember also that those antennas are dependent on a counterpoise, usually the HT and your body, so you will see significant difference capacitively coupling your hand to the Nano. You even see thus difference on a full size VNA, so the test setup for those small antennas really matters.


Peter

On Aug 5, 2019, at 9:54 AM, spaceopqa@... wrote:

Hello team,

While I was measuring an handheld VU antenna, I noticed that the nanovna was suffering of "Hand capacitance" influence.

My unit is an original one, shielded. What can I do to improve it? Have you noticed the same? What about calibration, do you think your hand will influence it?

Cheers!

LL
Re: NanoVNA Under The Covers

 

I don't? think this vna calibration just arbitrarily sets the 50ohm dummy to the noise floor. If it is based on a 3 load measurement then it must use Thomas Baier' s DG8SAQ procedure which is to find 3 parameters a, b,c in his article which will be a sort of curve fitting for S11 and S12. In some of the cheaper vna' s like the mini or the F4GOH version there is only one calibration, by the open load.My view is that like all instruments, it is only as good as the return loss bridge sitting inside the box, not the noise floor or number routines or other ekwctronicd. I have built several RLBs myself from HF to 1 GHz. Without elaborate shielding at HF and with the right fetrite cores you can get 60dB which drops to about 20dB at 1GHz. I do not imagine the SA602 bridge is better than this. There are minicircuit directional couplers that can do better but the cost is not worth the gain.?Sent from my Samsung Galaxy smartphone.

-------- Original message --------From: Warren Allgyer <allgyer@...> Date: 05/08/2019 13:46 (GMT+01:00) To: [email protected] Subject: Re: [nanovna-users] NanoVNA Under The Covers I am not sure if I am adding to or further confusing this discussion which has descended into, in my opinion, an argument over the number of angels on a pin head.First, when a three point calibration is done on any VNA, the result will be to null the return from the load used in the calibration. When the calibration is finished the display will be showing the noise floor of the instrument, not the return loss from the calibration load. The only inference you can draw about the return loss of the load in this is that it is below the noise floor of the instrument.As an illustration, I have attached a chart showing the results of three different 3 point calibrations. I used two random "50 ohm" terminations from my junk box and the calibration load supplied with the nanoVNA. After each of the three calibrations I recorded the indicated return loss for all three loads in succession.As you can see from the chart, the reading for any of the the three loads when used as the calibration load is the noise floor i.e. "<-70 dB". What can also be seen from the chart is the symmetry of the readings with different calibration loads. For example, when I calibrate with Load 1, Load 2 indicates -53.6 dB. When I calibrate with Load 2 then Load 1 indicates a similar -54.3 dB.The point is that, by definition, it is impossible to get an accurate measurement of the return loss of a calibration load on the instrument that was calibrated by that load. The instrument has been set by the calibration to assume that load is perfect and will return only its noise floor level.The nanoVNA cannot and should not be expected to match the performance of a $15,000 lab instrument. But it does a remarkable job for its cost. In my "lab" I do not care about the difference in return loss between -40.5 and -41.0 dB. What I care about is at what frequency the return loss is best and, at that frequency, whether or not the load is reactive or not. I also care about having an instrument that can show a filter profile and allow me to optimize it for the performance I want. I care about what the input to my linear amplifier looks like for return loss and impedance. For these things I find the performance of the nanoVNA to be more than adequate and the value to be tremendous!
"Hand capacitance"

 

Hello team,

While I was measuring an handheld VU antenna, I noticed that the nanovna was suffering of "Hand capacitance" influence.

My unit is an original one, shielded. What can I do to improve it? Have you noticed the same? What about calibration, do you think your hand will influence it?

Cheers!

LL
Re: NanoVNA Under The Covers

 

On Mon, Aug 5, 2019 at 06:41 AM, Jeff Anderson wrote:


must be less than 50.032 ohms
Just a slight correction -- actually, I should have said that the load's resistive "deviation from 50 ohms" must be less than 0.032 ohms.

Sorry about that!

- Jeff
Re: NanoVNA Under The Covers

 

Warren, excellent illustrative post demonstrating that if a VNA measures a load to have a return loss of 70 dB, it isn't necessarily so.

Over the years I've purchased a number of 50 ohm loads (generic as well as calibration loads) at swap meets, and upon returning home I'll usually do a quick check of their resistance (at DC) with a 4-wire ohm-meter (e.g. HP 3468A). The measured resistance can deviate significantly from the ideal of 50 ohms.

For a 50 ohm load to have a return loss of greater than 70 dB, assuming it has no reactive component, its resistive component must be less than 50.032 ohms. Anyone can do a quick check of their own calibration load if they have a 4-wire ohmmeter. This won't be a guarantee of return loss at high frequencies (where reactance effects become more prevalent), but, if you have a selection of a few loads designed for calibration (i.e. designed to minimize reactive effects), I believe this would allow you to find the one which ought to have the best "actual" return loss.

- Jeff, k6jca
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