NanoVNA input protection  - Page 1
Forum section: electronics / rf-microwave

author: joeqsmith
author_rank: Super Contributor
timestamp: August 13, 2023, 01:35:25 pm
content: I had forgotten that the administrators for group.io supporting the V2Plus4 censor the posts. There were a few comments about a video I had made on protecting the VNA from transients which I would like to address.The group is locked to non-members, but you can view the comments from their website:https://nanorfe.com/forum/ESD-safer-circuit.htmlFor now, this is only a placeholder. Of course feel free to flame the video.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 13, 2023, 01:48:18 pm
content: For some context, I was reminded of the professional ham who demonstrates the proper use of the NanoVNA. Video is linked at 9:00 in. Watch carefully. The best part is the comments.https://youtu.be/tCHAa-sjgcQ?t=535

author: joeqsmith
author_rank: Super Contributor
timestamp: August 13, 2023, 02:33:18 pm
content: And while I do not disagree that it is wise to discharge anything you plan to connect your VNA to, you may want to consider something other than a drill bit like this ham demonstrates.I recently made a video showing proper RF connector repair after watching another ham "pulling dents" of an RF connector with drill blanks.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 13, 2023, 04:38:38 pm
content: QuoteI doubt there is anyone who would suggest that any GDT would protect a VNA in of itself. Of course, this not what the video shows.QuoteI had mentioed a third where the ham connects the VNA directly to their PA.QuoteGood practices was covered during the video. In addition I talk about the use of ESD matts and wrist straps.QuoteSorry you lost me. I don't believe I have ever suggested a resistor would act as a clamp.QuoteAs I demonstrated, at the frequencies I was working with (<300MHz), the capacitance of the parts chosen had minimal effect on the measurements.QuoteI am not too surprised. As I stated in that video, following good practices should help avoid most of the common failures.QuoteAs I talked about in the video, there are two attenuators. One between the GDT and TVS. The other between the TVS and port 1. Obviously you are giving up dynamic range. I demonstrated the effects of adding various attention had on the SWR measurement. But again, I have heard from hams suggesting they need to measure their SWR out three places beyond the decimal. As I talk about in the video, any design is a give and take. The circuit proposed wasn't suggested as an end all to protection but rather just going through the motions on what to consider if a ham were to attempt to design their own. Yes, I know hams no longer enhance the art. Well maybe the 1%ers are still out there..The attenuators are there only to give the TVS something to work against. They are not their to act as a fuse as you suggest. Coax makes a good capacitor. Once the GDT fires, the stress on the first stage attenuator is gone. You could try creeping up on the GDT with a DC voltage and damage the attenuator. In that case, it may very well act as a fuse but it is not the intent. Again, what was presented it's not a one size fits all and based on what some of the hams have posted, I doubt you could add enough protection to save them in all cases.QuoteThey vary like any device, which is why there are so many and data sheets. One place you may find them is multi-meters where they commonly sit behind a surge rated resistor and PTC which limits the current they see.QuotePin diodes are also common. Again, it all depends. As I mentioned during the video, I have seen neon bulbs being used in some equipment.QuoteThe LiteVNA has clamps built-in. For my home equipment, I don't add any protection. Normally I follow good practices. I did manage to damage an amplifier a few months back when I decided to not wear a strap. I touched a horn (antenna) which was gounded via several feet of coax back to the amp.QuoteI had a DC blocking cap in that design I show and went over it. I've had hams suggest that a blocking cap was enough to protect their equipment. The idead that the capacitor will present a low impedance to high frequencies and that transients, like those from an ESD event have fairly high frequency content goes over their heads.QuoteOh, I can assure you we are giving up performance. Again, there is a reason I don't add any proction when working with my equipment. Most of my equipment is 20 years old or more and still the expense of repairs could easily be in the $1k and up. So, wrist straps and other practices are a must.QuoteThere have been several posts from hams who have damaged theirs which is what prompted the video in the first place. Don't assume hams have any engineering background. I suspect had I ran those grill starter ESD transients directly into that VNA, it would have been game over. I saw no reason to demonstate that. The pulse from that grill starter is fairly weak compared with the IEC standards. I could have ran that same test using this generator without any concern. Higher energy transients like you may see with a charged section of coax may have been more interesting to demonstrate but the video was getting long.QuoteI don't see the VNA as an antenna analyzer like many of the hams market them and can certainly believe a purpose built antenna analyzer would out perform it for measuring SWR. I can even believe a basic SWR meter may do a better job (certainly more robust and easier for the hams to setup and learn to use).As far as getting the data into the PC, for me that's the least of my problems. I started writing software for VNAs when I bought my first one a few decades past (and not to measure antennas). Porting over my software to support these various low cost VNA has not presented any real problems. Once the LiteVNA was released, I pretty much stopped using any of the others. Dislord has done a wonderful job with the firmware and the hardware, for the $120, is a tough combo to beat.I'm do not have a CB or ham license and no interest in them as a hobby. I noticed you built some sort of drag car. I would have far more interest in that. I've been in the 7's myself in the 1/4.***Bad formatting

author: xrunner
author_rank: Super Contributor
timestamp: August 13, 2023, 06:33:28 pm
content: Something bad went down I take it.Let me watch the video later I can't do it now ...

author: joeqsmith
author_rank: Super Contributor
timestamp: August 13, 2023, 07:10:39 pm
content: Not really. Just the typical ham culture. With Tom suggesting the video was "factual useless nonsense", I thought I would offer them an open dialog where they could ask the author directly. I would have kept the discussion on the group.io, but I suspect OWO has their own agenda as my last two posts were nicked. I couldn't see a point in wasting anytime using that group but am certainly willing to still discuss it on a more open platform. Sadly, they even censored my post on where to direct them.

author: xrunner
author_rank: Super Contributor
timestamp: August 14, 2023, 12:09:01 pm
content: Yea protect your test equipment. But another simple thing, (not just us here but all possible users), wev'e all got used to these cheap devices coming with no documentation whatsoever. Sure it may reside on some website, but is the URL even printed on the device? Again, sure you can Google the thing by name and find it (probably).But what happened to the days where you got at least a couple pages of documentation. The inexperienced user just grabs it and starts trying to make sense of what it can do. It has connections - it's for RF right? Let's connect RF to them and see what happens on the screen! Why not provide at least ONE simple piece of paper for a list of simple warnings and the most basic instructions? Can the MAX RF power be printed on the case right by the connections?I know, I know - they wouldn't read that either.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 14, 2023, 12:57:29 pm
content: Looks like OWO has filtered the last set of posts and indeed, they had censored my last post as well. Tough to have any discussions when the third party censors most of the posts. Looks like a few more chimed in.QuoteThat's been my experience. Inexpensive doesn't suggest ease of use.Doing a search on my friend Tom, my guess is he has reached the highest levels of knowledge and just didn't want to waste his time watching something that he already has a deep understanding of. Then he felt the need to show his expertise to his peers. Looks like he has a YT channel, website and a small business. It makes sense that he feels he needs to present himself as an expert. I'm sure I would have enjoyed the dialog.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 26, 2023, 12:07:47 am
content: QuoteI'm not sure why the poster would consider my clamp a tank or what these RLC time constants are that refer to. As I responded, the NanoVNA is limited to 300MHz. I am also not sure why they feel the circuit would effect the DUT's resonance frequency. This is fairly low frequency and while I did demonstrate making a few measurements with the clamp inline, I am guessing the OP may not have watched that part or didn't understand what was being shown.Shown looking at S11 from the antenna side of the clamp with the backside terminated to 50ohms. Also shown measuring S21. The clamp uses both a 3 and 7dB attenuator and should measure roughly 10dB total. At 300MHz, it's within a half dB. Normally, you would cal with the clamp in place.I invited the OP to join up and better explain what they are getting at. Maybe there is a problem I am not understanding.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: November 26, 2023, 03:51:06 pm
content: There is a reason for not including much input protection on antenna analysers and VNAs, it fouls up the measurements, otherwise they would all have “back to back” diodes on their ports.You just have to use care, one of my technicians managed to blow an expensive commercial antenna analyser testing a VHF dipole on a bench. We guessed it was due to static.SJ

author: joeqsmith
author_rank: Super Contributor
timestamp: November 26, 2023, 06:05:45 pm
content: Quote from: Solder_Junkie on November 26, 2023, 03:51:06 pmCare to provide any details as to why you feel this way? Just stating it doesn't make it true and how products are designed has nothing to do with it.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 26, 2023, 07:17:07 pm
content: For fun, I took a few measurements using a homemade antenna I used in a previous thread.https://www.eevblog.com/forum/rf-microwave/antennas-for-experimentation/msg4539512/#msg4539512I had designed that clamp assuming it would be used with the original NanoVNA for up to 300MHz. This antenna was designed for around 950MHz and I will be using the LiteVNA to collect the data. The LiteVNA has a TVS on each port. This particular one can get some useful data to about 8GHz. Good enough for this demonstration which should push the limits of that poorly constructed clamp.trans6_slimjim: Showing the setup. Antenna wedged into a block of wood and then set on top of the PC with a section of coax. The clamp was installed between the coax going back to the VNA and the coax feeding the antenna. Calibration was performed at the end of the clamp when installed, or at the end of the coax attached to the VNA. Both calibrations will use the standards supplied with the LiteVNA with my sorted load, and will use the ideal model.trans5_slimjim: Just a quick sanity check looking at the return loss of the antenna with and without the clamp, no calibration. Direct measurement appears to be around 966MHz. So I set the sweep range from 900MHz to 1GHz, 1601 points, 2kHz IFBW and calibrated the system with and without the clamp.trans7_slimjim: Measuring the VSWR with and without the clamp after calibration. The antenna isn't really a standard, and again we are working outside the range of what I had intended that clamp to be used. Then if you want to tighten things down further, I would ditch my home office, use real standards that have been characterized.... Still for what it is, seems like little change and good enough.

author: p.larner
author_rank: Frequent Contributor
timestamp: November 30, 2023, 02:01:54 pm
content: a few years ago t was stormy,i decided to unplug my coax from my atu,i did that and had a metal windowframe,i held the coax pl259 centerpin about 3mm from the frame only to see a spark jump the gap,i left it about 10 seconds and it did it againe,now i always ground the antenna before testing,shame i always seem to forget to discharge caps before testing them a blown up a couple of component testers ,i have an olds hunts leakage tester,this seems to charge them to 300volts ish,i always forget to discharge them!.

author: Helix70
author_rank: Unknown
timestamp: December 05, 2023, 10:41:13 am
content: I tried to fly the flag....Josh AVK4JNA

author: joeqsmith
author_rank: Super Contributor
timestamp: December 05, 2023, 03:37:53 pm
content: Guessing your flag poll snapped.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 19, 2023, 02:05:05 am
content: "Are you a HAM operator?"*****They put some effort into this post."I think you are putting major headaches on new guys that now think you need some kind of major rediculous extras like you are using,and I am sure selling, not a bad advert to scare guys into buying some of your stuff huh, ..."