What does a VNA tell us about this antenna ? - Page 1
Forum section: electronics / rf-microwave

author: tautech
author_rank: Super Contributor
timestamp: September 07, 2019, 10:42:31 am
content: In this thread I want to demonstrate the use of SVA1015X for analysis of a number of antennas I have hoarded over the years.In another thread I built and tuned a 315 MHz J-Pole antenna to get 500m range with a key-fob remote and in doing so learned getting maximum antenna emissivity at the fundamental frequency was paramount to getting good results.https://www.eevblog.com/forum/rf-microwave/antenna-project-log/Recent demonstrations of SVA1015X to customers doing various wireless projects have shown the 'off the shelf' commercial antennas they have selected couldn't be considered as optimal for their intended frequencies and their wireless range expectations are therefore generally not met.As antennas have the useful property of reciprocity we can sweep them with a signal and find their transmission properties which are also very close to their receptive properties. We can get basic information with a spectrum analyser and a reflection bridge but not the degree of info that can be acquired from a VNA.2.4 GHz SMA WiFi antenna analysisWhile I can't fully sweep a 2.4 GHz antenna with a 1.5 GHz VNA let's look at a full 1.5 GHz sweep and see what info arises.The subject:It should be noted that this antenna connected directly to the S11 port has its performance impacted by adjustments of the VNA due to proximity effects and it need be connected more remotely to reduce them for accurate repeatable measurements.Emissivity, aka Log MagWhere we look for the largest dip in the trace that represents the greatest absorption of energy from the feedline/transmitter and the frequencies this might occur at.Interestingly here it's at ~1/2 of 2.4 GHz that this antenna is designed for.Well could this antenna be useful at 1.28 GHz ?We need to check the SWR to look for efficiencies:Results of 1.6 and 1.2:1 indicate it might useful however how will they match the optimal 50 Ohm impedance we always seek ?A Smith chart will give us the info of impedance and inductive or capacitive match/mismatch.Overlay all these results using a trace for each measurement and we can see the full picture.We need remember all these results are with a full 100 Khz-1.5 GHz sweep which is not what we would normally do.Instead we would narrow the sweep considerably and set the center frequency to the frequency of interest.Next up, some antennas under 1 GHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 07, 2019, 11:10:55 am
content: But S11, SWR, Z/Zo and the need for tuning, are all equivalent (give or take the first two being shown in scalar form)...The real question is: is it absorptive or transmissive at those frequencies? It could very well be a better antenna say in that 400-800MHz range, despite the poor SWR, if it's more transmissy than lossy. (In that case you'd apply a matching or tuning network to use it there, at 50 ohms.)Most antennas with reasonable construction and no intentionally introduced losses, do in fact couple nicely, so it's probably that the markers show the best. But in general, you never knowTim

author: hagster
author_rank: Frequent Contributor
timestamp: September 07, 2019, 12:41:30 pm
content: As commented, the Return Loss and SWR are the same, but on a different scale.You can often get an idea of how lossy an antenna is likely to be by inspection. If the antenna is basically just metal and air, then there really isn't much mechanism available to create loss.You also need to check that the power is radiated from the antenna and not the cable. Normally you can get a good idea by moving your hand over the cable and watching for large changes on the plot. A good antenna should not have currents flowing on the outside of the coax and hence should not be sensitive to changes in cable length of proximity of other objects. For small low efficiency antennas it can be a real challenge to separate the cable effects from the performance of the antenna.

author: tautech
author_rank: Super Contributor
timestamp: September 09, 2019, 09:48:28 am
content: Next subject:Magnetic base whip antenna ~360 mm long.What is it as there's no markings and just a ~3m long coax feedline BNC terminated ? ? ?Full 1.5 GHz Log Mag span to find frequencies of resonance.Marker 3 indicates 876 MHz best Log MagLet's narrow the Span to 500 MHz centered on 876 MHzAnd a SWR check at the same settingsSmith chart with just 100 MHz span on 876 MHzFinally Smith, SWR and Log Mag with markers all on 876 MHz and a 100 MHz span.It should be noted to get selected marker measurements displayed it is necessary to select the correct trace from the trace menu. Once selected the diamond marker indicator is filled with color and not a hollow shape.Antenna labelled and stored.Next up a little Yagi...........

author: borjam
author_rank: Unknown
timestamp: September 09, 2019, 10:09:40 am
content: For an antenna you need to check a couple of things:- First, coupling. When checking S11, as a rule of thumb better than -8 dB (less than 2.5 VSWR) will be good.- Second a resistor would give perfect matching but it would be useless. So you would need to check how well it radiates, which is expensive.The best thing you can do is to buy antennas from reputable sellers who provide a datasheet with the correct measurements.Antenna looks can be deceiving, sometimes there is a bit of trickery hidden inside a plain looking one that helps it work at more than one frequency.And unreputable antennas... I remember a batch of super cheap Chinese antennas. I tested 30 of them with a VNA and all of them were different.

author: tautech
author_rank: Super Contributor
timestamp: September 09, 2019, 10:23:14 am
content: Quote from: borjam on September 09, 2019, 10:09:40 amEven for mW ?Nah, when you have a VNA why not get them way down under 1.1:1 ?With the one hendrog and I conjured up we got it down to 1.0137:1 !QuoteYep all that and that's one of the reasons I started this thread especially after sweeping a customers '925' MHz antenna only to find it worked best at 870 MHz ! No wonder they had range issues !The little Yagi up next is labelled and I can find some info on it so it will be interesting to see how well it meets its spec. After that I have a couple more at least to write up here and one is a monster sized whip so that will need a fine day, something that's rare here ATM.

author: borjam
author_rank: Unknown
timestamp: September 09, 2019, 10:42:57 am
content: Quote from: tautech on September 09, 2019, 10:23:14 amYes, even with mW level powers. Otherwise efficiency would be too low and at least according to manufacturers you can damage a chip if the mismatch is too large. And I forgot,even if you don't break anything a large mismatch will likely cause distortion. So the problems will mountAlways read the datasheetsRegarding such awesome VSWR it will depend on the application. On HF you can get an outstanding match for a bandwidth of about 100 KHz. But if you are using it for WiFi (mininum channel size is 20 MHz) or cellular (several multi MHz bands on different frequencies) it will be difficult.QuoteYou would be amazedI had an issue at work with that. Some think that as long as it has a connector and it looks like an antenna it will workQuoteAlso, remember that antenna position matters, especially if you have stuff within the near field.

author: OwO
author_rank: Super Contributor
timestamp: September 09, 2019, 10:48:08 am
content: For transmit I would say any VSWR < 1:3 is "good enough" because that's -6dB return loss which means 75% of the power is radiated. Another way to think of it is in terms of "mismatch loss", which at 1:3 VSWR is -1.25dB. It might be unintuitive to most but 1-2dB is completely negligible in a typical wireless system. I've done propagation experiments on various ISM bands and you can easily get 10-20dB variations in received signal just walking around.For receive it's usually even less important because the ambient noise is going to be at least around 10dB above thermal noise floor so any antenna with less than 10dB mismatch loss (VSWR 1:37!) is likely not noticeable. That is the reason most broadcast receivers get away with very crappy antennas that are completely untuned. The exception to this is the higher bands (think >5GHz) or satellite receivers where you can have a noise floor below thermal noise (because the antenna is pointing at the sky).

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: September 09, 2019, 11:35:21 am
content: What S11 is good enough also really depends what your antenna is connected to and what it's application is. If you are dealing with some kind of radar, a poor S11 could result in your receiver being saturated more than you want due to the transmitted pulse.And mismatch can hamper the performance of the amplifier - the amplifier's matching network will be designed for maximum power (usually) and a poor S11 can impact that significantly.I have always frowned at people assuming good S11 = good antenna, because of two reasons: S11 can be changed with a matching network, and just because it is not reflected back doesn't mean it is going anywhere (losses).Yeah, at the few GHz frequencies you might be able to ignore any dielectric/conductor losses in an antenna, but that stops once you get past a few GHz or want high accuracy of your antenna (eg for imaging or sensing)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 09, 2019, 03:19:18 pm
content: Heh, the S11 notch is probably the antenna's tuning, but the SWR around the notch isn't bad either -- suspect cable loss, is that just ordinary shitty RG-58 or 174?So we see that good SWR may not necessarily give good antenna performance; obviously, a transmission test would be required to prove that.Tim

author: borjam
author_rank: Unknown
timestamp: September 09, 2019, 03:29:30 pm
content: Quote from: T3sl4co1l on September 09, 2019, 03:19:18 pmIndeed, new product idea for Siglent: The PAC1000T (Portable Anechoic Chamber 1000 Tati Anniversary Edition)Outside view:Inside view:(Operator not included)

author: TheSteve
author_rank: Unknown
timestamp: September 09, 2019, 04:49:05 pm
content: Tautech you missed out the on the best part - now that you found a resonant frequency do a little math on it and determine how the antenna works and see if it all makes sense.Likely the lower section of that antenna before the coil will be roughly 1/4 wavelength long at your resonant frequency. The coil itself is a matching network(many people think it is just there to look cool). The upper section should also work out to be mathematically related to the resonant frequency. From there you will get a picture of the gain it will ideally have and the ideal radiation pattern.

author: coppercone2
author_rank: Super Contributor
timestamp: September 10, 2019, 04:20:06 am
content: is that like a russian hippy van?

author: tautech
author_rank: Super Contributor
timestamp: September 10, 2019, 08:05:48 am
content: Quote from: TheSteve on September 09, 2019, 04:49:05 pmYeah, maths is my weak point.Anyways, from base to start of coil = 90mmCoil = 3.5t over 20mm @ 15mm dia.Coil to tip (top section) 200mm

author: TheSteve
author_rank: Unknown
timestamp: September 10, 2019, 09:13:55 am
content: Quote from: tautech on September 10, 2019, 08:05:48 amLet's see - speed of light in meters per second divided by the frequency in Hz. 300000000/876000000 = 0.342 meters - divide by 4 for a quarter wave and we get 85.6 mm for the lower section.The upper section is likely a 5/8 wave which would be 214 mm.The numbers line up reasonably well.That would make the antenna a collinear, the 5/8 over 1/4 design typically gives around 3.2 dB of gain.To work well and keep the RF radiating in a nice pattern(vs wanting to go up) the antenna should have a ground plane under it - such as the roof of a car. This would likely alter the resonant frequency a little as well but it should work fine for any use between 800 and 900 MHz.

author: borjam
author_rank: Unknown
timestamp: September 10, 2019, 09:49:16 am
content: But it can get much more complicated at high frequencies, where its shape, the actual material used for the antenna, its thickness, the radome if present, etc, can have a significant impact.Some apparently simple antennas, for example, can work on several LTE/UMTS/GSM/WiFi bands.

author: radiolistener
author_rank: Super Contributor
timestamp: September 10, 2019, 09:52:58 am
content: any piece of wire can work as antenna. The problem here is to put RF energy into this piece of wireNot all RF energy which is put into a wire will be radiated as EM, just a small part. Some part will be spent on heating loss. And the rest of energy will be returned back to the transmitter, where it can be added with a new transmitting energy and the sum may damage your transmitterIn order to avoid that, it's better to use piece of wire with resonant length for desired frequency. When wire has resonant frequency it will not return energy back, it will accumulate it. So, RF energy will oscillating within wire and don't come back to transmitter.Each oscillation in the wire will leads to heat loss and radiation loss. Since we using resonant length, the energy will be accumulated in the wire and amplitude will be increased for each oscillation. Since amplitude is increased, heat loss and radiation loss also will be increased. At some point incoming energy will be balanced with energy loss (heat and radiation).The oscillation cycle count needed in order to radiate applied energy is named as antenna Q factor. Usual half wavelength dipole has Q factor about 12. Higher Q factor leads to higher amplitude in the wire, so it will have higher thermal and radiation loss. But at the same time higher Q factor leads to smaller antenna bandwidth.With vector analyzer you can analyze if your antenna with matching network and with feeding cable returns energy back to transmitter or not. But you cannot say if this energy is spent on heat loss or it is spent on EM radiation.

author: tautech
author_rank: Super Contributor
timestamp: September 10, 2019, 11:20:15 am
content: Quote from: radiolistener on September 10, 2019, 09:52:58 amYep, this:Inside a tin shed !~250mm of stainless wire dropped into a BNC socket worked sporadically for us over 500m for years until I made a J-Pole that solved all our problems.It need be mentioned good design and proven measurement can vastly improve reception quality as that's often all you have to work with when transmission antennas are set in stone by the manufacturers of say a short range keyfob device.

author: tautech
author_rank: Super Contributor
timestamp: September 10, 2019, 11:21:49 am
content: Next subject:YagiHG909Y-NFDatasheet attached.Coupled to SVA1015X with Siglent 6 GHz rated 700mm N-N cable.Full sweep Log MagThis is looking better so lets narrow the span to 100 MHz centered on 930 MHz.Log MagSWRSmith chartPhaseAll 4And for giggles we'll look at a DTF of the feedline and antenna with default DTF settings.......set marker to peak and out with the tape measure and just 50mm difference in results.

author: borjam
author_rank: Unknown
timestamp: September 10, 2019, 11:24:32 am
content: I will post later the S11 graph of an antenna with a crazy high Q: a tuned loop antenna for HF.It has an extremely small bandwidth, like 4 KHz or so.

author: borjam
author_rank: Unknown
timestamp: September 10, 2019, 06:44:35 pm
content: So this is itIt's a MFJ1788 loop antenna. It's in a bad placement (balcony) so the SWR is higher than it should be.Tuned to 7080 KHz offering a VSWR of 1.6:1, climbs to 2.5:1 with an offset of just 3.3 KHz.By the way this is an example showing the VNA function working below 10 MHz.Now, a really interesting feature would be a "continuous valley" monitoring and linked markers for the desired performance.For example, let's see I am adjusting this antenna. While I adjust the tuning controls the resonance frequency will change. Unfortunately the VNA function doesn't adjust the "valley" peak marker when it changes. So I must keep pressing peak->valley while I tune the antenna if I want the marker to be updated. It should "follow" the valley.Linked markers: It would be great to specify something like: I'd like markers set to a certain RL or VSWR value. Again, updated if the antenna behavior changes because I am making adjustments.These two features would be really awesome because thanks to the web server you can be monitoring the VNA while you are adjusting the antenna, even if you are high on a tower.This would be a good use case for a ham operator.

author: TheSteve
author_rank: Unknown
timestamp: September 10, 2019, 07:11:10 pm
content: Does it not have an auto-tracking feature when a max or minimum peak has been selected?

author: borjam
author_rank: Unknown
timestamp: September 10, 2019, 10:19:20 pm
content: Quote from: TheSteve on September 10, 2019, 07:11:10 pmDo you mean the VNA?If you press the peak buttona and select peak or valley it will place a marker on the maximum or minium value. But it won't follow the maximum or minimum value if you change something in the DUT (like for example tuning the antenna).

author: TheSteve
author_rank: Unknown
timestamp: September 10, 2019, 10:31:42 pm
content: Yes, that is what I was referring to. It is a feature that should be requested.

author: tautech
author_rank: Super Contributor
timestamp: September 11, 2019, 12:52:35 am
content: Quote from: TheSteve on September 10, 2019, 10:31:42 pmDone !Thanks borjam and Steve.Useful indeed and a feature that certainly would've helped when I did final tuning of the J-Pole I made to resolve the proximity effects once it was installed.

author: hendorog
author_rank: Super Contributor
timestamp: September 11, 2019, 01:07:26 am
content: Quote from: borjam on September 10, 2019, 06:44:35 pmI'm not sure what you mean when you say Linked Markers set to a certain RL/SWR? Do you mean something like a reference line, which is drawn across the screen at a particular level?

author: tautech
author_rank: Super Contributor
timestamp: September 11, 2019, 05:50:36 am
content: Quote from: tautech on September 09, 2019, 10:23:14 amIn preparation for this 3m long antenna let's check this ~5m N type M-F RG-213/U cable I have.DTF set to 8m range and markers on the join to a 700mm cable and the RG-213 end afterwards.RG-213/U cable specs:https://www.rfparts.com/rg213-u.html

author: borjam
author_rank: Unknown
timestamp: September 11, 2019, 06:35:43 am
content: Quote from: hendorog on September 11, 2019, 01:07:26 amLet's imagine I am tweaking an antenna. I am interested maybe not only on the resonance bandwidth, but also on the usable bandwidth. Let's see I want VSWR < 2:1. If I could set markers based on that value instead of frequency, it could display a table of markers with their frequency and the frequency difference between them.In the case of my loop antenna it would create two markers and the delta frequency display would be really useful. In the past I had to adjust it by changing the shape of an internal coupling loop that affected both minimum VSWR and bandwidth a bit.It could be a bit more useful now that you mention a reference line. It could show a table of "usable frequencies" by looking at the VSWR/RL line crossing the reference line.And the real plus: imagine you are tweaking your antenna and your VNA is in the shack. You can access it from your phone via web, it could show a simple "tuning screen" with the information I am mentioning here.

author: hendorog
author_rank: Super Contributor
timestamp: September 11, 2019, 07:05:21 am
content: Quote from: borjam on September 11, 2019, 06:35:43 amAhh right I get it. Actually I think I have seen something similar now you mention it.

author: tautech
author_rank: Super Contributor
timestamp: September 11, 2019, 08:31:01 am
content: Quote from: hendorog on September 11, 2019, 07:05:21 amYou can already do this of sorts.For example in Log Mag (other modes too) adjust one trace in Amplitude to the Ref level you desire and place marker/s upon it where you desire. Then in the Trace menu enable a second trace and use that against the level set with the first trace.Here we have trace 1 set to some Log Mag level and trace 2 the selected/active trace with markers un-coupled so to be able to move them independent of trace 1 markers.Not all all hard to do, have a play borjam.

author: borjam
author_rank: Unknown
timestamp: September 11, 2019, 09:10:01 am
content: Quote from: tautech on September 11, 2019, 08:31:01 amI'll check later at home. Anyway, a remote controlled VNA you can operate or at least watch from the discomfort of your tower using your phone can be a sort of a killer application for hamsNot "sort of", but actually doing it

author: tautech
author_rank: Super Contributor
timestamp: September 11, 2019, 09:17:25 am
content: Quote from: borjam on September 11, 2019, 09:10:01 amWell didn't I just demonstrate now you can set some arbitrary level with another trace then use another to do the tune ?WRT SVA1015X remote control, behind the scenes one using the SVA's inbuilt webrowser is being prepared between NZ and Oregon as we speak !Hopefully there'll be some evidence to study later.......

author: borjam
author_rank: Unknown
timestamp: September 11, 2019, 09:23:23 am
content: Quote from: tautech on September 11, 2019, 09:17:25 amActually it would be possible to make a phone app to do something like this.I'll play a bit more this evening and get back to you. Anyway, not criticizing, just making a suggestion.

author: tautech
author_rank: Super Contributor
timestamp: September 11, 2019, 09:25:10 am
content: Quote from: borjam on September 11, 2019, 09:23:23 amThings already exist but I'll let those doing it explain later.

author: borjam
author_rank: Unknown
timestamp: September 11, 2019, 09:27:20 am
content: I might have another interesting suggestion for automatic port extension correction, but I need to run some more tests.(And given that I am far from a VNA guru, more a little grasshopper, the idea needs validation from more experienced folks here)

author: tautech
author_rank: Super Contributor
timestamp: September 11, 2019, 08:58:29 pm
content: Quote from: tautech on September 11, 2019, 12:52:35 amWord back from the factory is Peak and Valley marker tracking is already on the 'to do' list.No indication of when it might be added to firmware.....

author: hendorog
author_rank: Super Contributor
timestamp: September 12, 2019, 08:27:50 am
content: I have had the opportunity to play with an SVA which is sitting in Canada - from here in NZ. This was to see how usable the live web interface is over a WAN.Latency was actually really good considering the distance, the ping is about 170ms. I was expecting it would be much worse than that.The SVA was quite usable from a PC over VNC or the web interface which uses VNC internally.The best result was using the RealVNC Google Chrome add-in with marker values updating up to 5 times per second.The web interface, and any other VNC client I tried, including the RealVNC native client, was a bit slower. and any attempt to change settings like colour depth to improve the speed also made it slower.I installed a VNC app on my phone. It wasn't as fast, and it had that age old problem of controlling a mouse cursor with a touch screen.However it would have been perfect used readonly for monitoring a trace and/or marker values when tuning an antenna. Pinch to zoom the screen and the phone shows the full screen image landscape.All in all it was surprisingly good over such a long distance.

author: tautech
author_rank: Super Contributor
timestamp: September 12, 2019, 10:53:34 am
content: Just had a quick play with the SVA1015X that hendorog connected to earlier.Right now in NZ it's peak online time so the results with an iPad and the VNC app were a little disappointing due to latency of more than 1s which is not too bad at peak time considering it's in Oregon !Still I made a couple of small adjustments to the setup and a marker just to prove to myself it works.I installed the VNC Viewer from the Apple app store yet to get it working you need open a free account at realvnc.com Then it's just the matter of verifying yourself to them from emails and within the app and you're good to go.The SVA1015X we've been able to connect too has been graciously made available to us on an open port by an EEVblog member.While there I grabbed an iPad screenshot as proof of connection.

author: tautech
author_rank: Super Contributor
timestamp: September 12, 2019, 11:00:51 am
content: Quote from: tautech on September 11, 2019, 08:58:29 pmPlaying around with settings as a result of suggestions from borjam I've asked for another feature from the SVA team:Limit lines.SVA and SSA have them in SA mode so a Limit UI button is already on the front panel so I've asked if limits can be added into VNA mode.We've got to keep the engineers busy don't we ?

author: hendorog
author_rank: Super Contributor
timestamp: September 12, 2019, 11:05:54 am
content: Quote from: tautech on September 12, 2019, 10:53:34 amTry it again now, it was on 751 points and taking a while to sweep. I changed it to 201 and its updating much faster.I noticed it was on 201 points when I tested it.QuoteI was able to skip the RealVNC registration thing on the PC install. Not sure if that is the case with the iPad version though.

author: borjam
author_rank: Unknown
timestamp: September 12, 2019, 11:06:38 am
content: Quote from: tautech on September 12, 2019, 11:00:51 amAs Groucho Marx would say, MORE WOOD!!Yet another suggestion. It's confusing when you have more than one trace on screen but only one of the scales (the one for the currently selected graph).What about adding scales with color matching the trace color? Or maybe at least balancing them left and right of the graph.

author: tautech
author_rank: Super Contributor
timestamp: September 12, 2019, 11:15:00 am
content: Quote from: borjam on September 12, 2019, 11:06:38 amKeep em coming.QuoteAgreed but with the flag on the active trace visible on the left you do get used to it.QuoteDunno how best to implement that and we best all have a bit of a chat and throw some ideas around.Me, well it's getting late here and the brain fog is coming down.hendorog, we can chat about it tomorrow by mail.

author: tautech
author_rank: Super Contributor
timestamp: September 12, 2019, 11:47:04 am
content: Next the 3m monster:3x 450mm stainless radialsSmall branding just above the radials identifies a NZ brand from which I've found a matching datasheet.Datasheet attached below.As it's big it will much lower frequency than those tested before therefore we limit the sweep to just 500 MHz.Look at all those dips, we better look closer and find their frequencies.OK so as I know were this antenna came from and have the repeater too I know the bands it operated in as 136 - 174 MHz so let's look closer at the best Log Mag dip and its SWR.Here I've disconnected and set a second trace to a Ref level representing an acceptable SWR then switched back to trace 1 and reconnected.Not ideal I know as RF connectors have a finite life and an important reason why I've asked Siglent to implement user Limits into VNA modes.Finally a quick check with a Smith chart that raises some questions on the impedance match of this antenna.I will add its location for testing was not optimum with several near field structures that would impact on its performance.

author: tautech
author_rank: Super Contributor
timestamp: October 10, 2019, 08:58:43 am
content: Now with a SVA1032X let's revisit the first subject I looked at, this 2.4 GHz WiFi antenna:With the SVA1015X only 1.5 GHz sweep this was the Log Mag result:Now with a 3.2 GHz sweep let's see how some of the markers placed align with the previous screenshot.Markers 1 and 5 placed ~2.3 - 2.5 GHzDiscovered is another Wifi whip so that's the next subject with this new SVA1032X.

author: tautech
author_rank: Super Contributor
timestamp: October 15, 2019, 06:32:36 am
content: This time the Wifi antenna I mentioned above. It's a little longer than the white one tested previously and here I've added 100x averaging so to negate the operator proximity effects each time I adjust something on this SVA1032X.Full 3.2 GHz sweep.

author: RCinFLA
author_rank: Contributor
timestamp: October 25, 2019, 05:54:33 pm
content: Saying that anything below a given VSWR number is good can be misleading, other then lower VSWR is generally better.A given VSWR number is a circle of a given radius around the center of Smith chart. The number given in the mismatch loss equation is based on 10*Log10( 1-r^2) where 'r' is the circle radius ('rho'). This is only power transfer mismatch loss and does not take into account possible detuning effects on resonance of a transmitter output tank circuit or receiver input tank circuit with its optimum noise termination resistance.A given VSWR can produce any impedance along the circle, from resistive, to inductive, to capacitive. The given impedance can have a greater detrimental effect on the tank circuit tuning that results in more degradation in transmitter RF output power or receiver sensitivity then yielded by the mismatch loss equation alone.It can also cause instabilities in the transmitter PA or receiver LNA. A 'load pull' test checks the stability when a given rho magnitude is rotated around all 360 degrees while checking for stability. Putting a 100 ohm load on a 50 ohm line stretcher and moving length over half wave length range checks load pull for all impedances along 2:1 VSWR circle.

author: E Kafeman
author_rank: Regular Contributor
timestamp: October 27, 2019, 05:44:56 am
content: What does a VNA tell us about this antenna ?It is a bit misleading label for this tread.Antenna mounted directly at VNA is a very poor mounting place if it not is by purpose that VNA should be a part of the antenna measurement. Actual measurements curves shows dips at 400 and 1200 MHz. Antenna in it self is too short to provide resonances at these frequencies.What is measured is to a degree the antenna and its nearfield such as the VNA. It is neither an antenna designed to be placed parallel and close to a big metallic surface.Measurement setup is less optimal and VNA probably not correct calibrated.Normally when measuring this type of antenna do we try to avoid that measurement cable is included in actual antenna measurement.A free space with nothing that resonated near antenna and ferrite tubes around measurement cable is much the standard setup.Connecting antenna directly at VNA and VNA is a bit hard to hide from actual measurement.There is also some kind of problem with instrument calibration. For actual setup and type of antenna and frequency range, it is not even close to realistic impedance in last Smith Chart above. Guess that correct port forwarding not is performed.I repeated a similar measurement below as a comparison. Measured a D-Link router antenna.Connecting cable have several ferrite tubes and nearfield is not a part of the measurement. Note that Smith Chart is stable and not rotates a lot of turns. No low frequency anomalies are seen.Similar result for most of my collection of old router antennas.It is an interesting tool and price but very hard to judge what it is able to measure. Is it a network analyzer or single port analyzer (can it measure S12)?Is it maybe a problem with its calibration? I have searched in the data sheet that could be found but could not find such information.

author: tautech
author_rank: Super Contributor
timestamp: October 27, 2019, 08:33:24 am
content: Quote from: E Kafeman on October 27, 2019, 05:44:56 amPlease suggest a more suitable topic name.Maybe one that has the VNA model # ?QuoteYes all this is a problem for both the SMA WiFi whips I tested. On the last one I engaged averaging to somewhat negate operator proximity affects.QuoteWhat cable length in your experience might be suitable to mostly exclude nearfield influences ?Just a few hundred mm or a meter or so ?QuoteCorrect, no port extensions were used for the WiFi antennas so the N-SMA and SMA M-F adapters will impact some on results.QuoteThanks for the lesson on how to do it !QuoteNo, it's a single port and through to the spectrum analyser input so only S11 and S21. Let's not forget the SVA models are spectrum analyser and VNA combo units.QuoteMember hendorog and I have checked the Cal against a few SOL's that he has and the factory port Cal is pretty darn close.Of the 2 units I have used in the antenna tests in this thread (SVA1015X and later SVA1032X) the SVA1032X is a spanking new unit that I already have a pre-public FW version that I have yet to install. I don't have the release notes for it but AFAIK it was to address some issues that Sharihar (TSP) found when he did a recent review.https://www.youtube.com/watch?v=ToVJTKCyIU8&feature=youtu.beRight how both my demo units are with hendorog for deeper comparisons against his 6 GHz HP and SH VNA's.

author: borjam
author_rank: Unknown
timestamp: November 06, 2019, 11:06:04 am
content: Quote from: tautech on October 27, 2019, 08:33:24 amThe near field is determined by the wavelength. You can consider it a wavelength and, if you want to be safer, also include the transition zone. That would be two wavelengths.https://en.wikipedia.org/wiki/Near_and_far_fieldIn order to check some WiFi/cellular antennas I used the actual cabinet of the equipment that would mount them, putting the miniVNA Tiny inside and using an adapter to connect the antenna.