VNA measurements of antennas with connector - Page 1
Forum section: electronics / rf-microwave

author: tszaboo
author_rank: Super Contributor
timestamp: June 16, 2025, 08:45:18 am
content: So here is the two topic I would like to discuss: Antennas that come with a connector.Specifically, antennas that have a coax cable already attached to them and end with an SMA male. I tried doing measurements of them.I 1-port calibrate the VNA with the typical SMA male calibration kit right on the VNA connector. Then connect the antenna. The coax cable is supposed to be "just 50 Ohm" it will drive the Smith chart around in circles. The cable length has tolerance, the cable is typically some RG with varying quality.How do you measure an antenna with a cable already attached? Is it necessary to take these apart and use your own cable? What do you do when this isn't practical?My other question would be about whip antennas. How do you measure them? Do you have some sort of ground plane with a test connector for the DUT Antenna? Can you share a practical setup for measuring these?

author: tszaboo
author_rank: Super Contributor
timestamp: June 16, 2025, 08:48:47 am
content: Here is a simulation of a non-matched load, with a cable in front of it, just to give an idea on what would be seen on the VNA

author: rteodor
author_rank: Frequent Contributor
timestamp: June 16, 2025, 09:44:17 am
content: Quote from: tszaboo on June 16, 2025, 08:45:18 amDo a calibration with a similar cable of the same length, maybe ? Or use 'port extension'. Or both if length does not match.I do not know, I'm interested on the subject too.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 16, 2025, 11:44:38 am
content: Assuming you are measuring VSWR, why do you feel it matters? Then I would also ask why you are wanting to use VNA to make this measurement rather than a VSWR meter?

author: tszaboo
author_rank: Super Contributor
timestamp: June 16, 2025, 11:58:54 am
content: Quote from: joeqsmith on June 16, 2025, 11:44:38 amBecause I have a VNA and not a VSWR meter.But I think I understand what you are saying. Set the VNA to only display VSWR and stop worrying about the Smith chart. I guess fair enough.Why am I interested in the measurements? Typically it's a pass-fail test for antennas before connecting it to a system. If you have a product with an SMA connector and you need to define an antenna to be used with it, you might need to measure it. For example a 4G modem will say that you need 50% efficiency at certain bands, and the antenna datasheet might not have the info in it. Or the installation environment for sure will have an effect on it.

author: xrunner
author_rank: Super Contributor
timestamp: June 16, 2025, 12:19:29 pm
content: Quote from: tszaboo on June 16, 2025, 08:45:18 amWell the system you are measuring is as you know not just the antenna but includes the cable and a connector that came installed. The VNA simply tells you the response of this system. This system may include a low quality cable and a low quality connector. Ideally you want to include the cable and connector that came with the the antenna in the calibration, or if it's suspect quality to remove that variable from the system under test. If you leave it on then it's showing you valuable information about the "system" you are attaching to the transmitter and if you do not approve of the response then you might want to separate these two components and test each one separately.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 16, 2025, 03:05:45 pm
content: If it were a tower mounted outside in the elements, there have been a fair number of hams who have reported damaging their VNAs. If its running below 500MHz, a simple in-line VSWR meter may be a better choice. Below 50MHz, you may be able to toss something together.One advantage of the VNA is you could potentially look at the radiation pattern. After all, that's really what you want the antenna to do. In the GHz ranges you're talking about, the far field would be manageable.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 16, 2025, 03:35:53 pm
content: Simple VSWR meter used for my spark gap transmitter running in the 40ish meter band.https://www.eevblog.com/forum/rf-microwave/fly-swatter-ham-radio-transmitter/msg5650363/#msg5650363

author: pdenisowski
author_rank: Super Contributor
timestamp: June 16, 2025, 03:57:32 pm
content: Quote from: tszaboo on June 16, 2025, 08:45:18 amAre you trying to characterize _only_ the antenna? If the antenna is manufactured with a ("permanently") attached coaxial feeder, then it probably makes more sense to measure it as a unit (antenna + cable).As I think a few of the other posters have already mentioned, you could use de-embedding / port extension if your VNA supports this (hobbyist / entry level VNAs typically do not)Quote from: tszaboo on June 16, 2025, 08:45:18 amI'm assuming you mean a whip on a handheld / portable radio and not a whip mounted on a vehicle. Ideally, you would want a "ground plane" that's similar is size, shape, and composition to the body of the radio the whip will be connected to. If you have an empty chassis (or can sacrifice a radio to make one), a simple bulkhead connector can be used (VNA attached to the "inside" side and the antenna attached to the "outside" side.

author: rteodor
author_rank: Frequent Contributor
timestamp: June 16, 2025, 04:17:48 pm
content: Quote from: pdenisowski on June 16, 2025, 03:57:32 pmI'm sure I wrote this in some other post: on those tiny VNA's (Lite, F2, etc) port extension is under DISPLAY >>> SCALE >>> E-DELAY.I guess 'E-DELAY' fits better on the small screen menu.Later edit: E-DELAY is not quite the same as port extension. Reply #22 below.

author: pdenisowski
author_rank: Super Contributor
timestamp: June 16, 2025, 06:41:47 pm
content: Quote from: rteodor on June 16, 2025, 04:17:48 pmThanks! Good to know!

author: tszaboo
author_rank: Super Contributor
timestamp: June 16, 2025, 09:24:49 pm
content: Quote from: pdenisowski on June 16, 2025, 03:57:32 pmThat makes sense. My simple mind tells me that coax attenuation and antenna radiation will be impossible to distinguish. But probably that's not something I need to worry about for short cables.De-embedding is possible, however from what I've seen, for me it was just a bunch of guesswork trying to punch in some numbers. At larger frequency even a few ps length turns the Smith chart upside down. From what I can see, without disassembling the antenna, it's not always obvious what's the length of the coax, because it continues inside the antenna body.By whip antenna I mean something like this:This is an antenna for LTE for example. Your suggestion to try to measure in-situ is good. But imagine that you want to use such an antenna with an evaluation board. The board might come with other style antenna. So how would I know if the antenna even close to being good at the frequency? Not to mention, manufacturers when selling these boards are notorious for just packaging something close to resemble an antenna with it, without marking or any type of part number or description.So basically is there a way to measure such an antenna by itself. Like is there a best industry practice?Quote from: joeqsmith on June 16, 2025, 03:05:45 pmI typically work with LTE, GNSS, 802.15.4 and other higher frequency signals, not HAM.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 17, 2025, 01:02:53 am
content: I would imagine that there are some good papers available on antenna testing starting with a chamber similar to what I have shown. IMO, cheaper just to buy name brand parts. I saw a video a ham made who boiled his antenna tests by taking a radio remote and just seeing how well different ones performed when attached to the same radio. Hard to see any fault with that technique.I've ran a few experiments using various antennas to look at their radiation patterns, gains and directivity. Really a poor mans setup and I doubt very useful from getting any absolute data. Showing my early setup with a homemade Vivaldi and custom software. Single axis only. It's advanced a bit since then.Various antennas for the fun of it.https://www.eevblog.com/forum/rf-microwave/antennas-for-experimentation/Testing plastic horns with 2-axis rotorTrimming a simple dipole using the NanoVNA

author: rsjsouza
author_rank: Super Contributor
timestamp: June 17, 2025, 01:37:59 am
content: I usually do matching and parameter definition using the antenna as an entire system, as pdenisowski mentioned.The S11 VNA is connected to the point of the feed (if trying to characterize the antenna) or at the feed to the match network (if I am doing antenna matching).I compensate (extensions) or calibrate out any cables or connectors that do not "belong" to the final system.Also, if using cables, make sure they are still in ok shape. If your VNA supports S21/S12, use the magnitude and test the cable attenuation on the frequency range of interest (up to 2nd or 3rd harmonic if you are interested in suppressing these during matching) and make sure you wiggle the cable to check if there are any problems with it. Believe me, this will prevent you from going crazy.Smith charts are great for getting a feel for the necessary changes for the matching circuit. This can also be used with the magnitude and VSWR to double-triple check your matching results.

author: tszaboo
author_rank: Super Contributor
timestamp: June 17, 2025, 07:32:35 am
content: Quote from: rsjsouza on June 17, 2025, 01:37:59 amI've matched several PCB mounted antennas on products that I have designed. The workflow for that is different though, you have your enclosure, you can setup your environment the way it's going to be used. And you can drill, cut up or change anything on your own PCB, as many as needed, to make sure this test works.For a ready made antenna in an unknown environment, this is not the case.Quote from: joeqsmith on June 17, 2025, 01:02:53 amWe've been doing range testing at work for some radio interfaces. It's a system validation test, and for those RF interfaces where you need to walk a few hundred meters, it easy to do. But you cannot do that for 4G for example. The radio tower has a range of tens of kilometers, with overlapping coverage. And your local 4G tower will only work on a few bands out of the dozen or so a modem will support. We had DUTs sent to overseas just to see if they would connect there. It's not just the antenna, there is also roaming and network set up to take care, but you see the issue. I wouldn't even be allowed to transmit on some of the bands of 4G which are used in the US.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 17, 2025, 10:00:39 am
content: Quote from: tszaboo on June 17, 2025, 07:32:35 amI just assumed that you have some procedure where you operate the antenna using a low power source (<0dBm) inside the chamber and look at the pattern, gain, directivity... that way. For outside testing, I would guess you would do something similar. Some fixed location from tower and constant radio. Add attenuators until the signal drops. Swap antennas and try again. Not great but I doubt you get 10s of kilometers of range with a 100dB attenuator in front of the antenna.Again, I want to be clear, I have no background in antenna design or testing.

author: tszaboo
author_rank: Super Contributor
timestamp: June 17, 2025, 01:28:25 pm
content: Quote from: joeqsmith on June 17, 2025, 10:00:39 amSure, I could do that, but then again, it would be better to quickly evaluate an antenna without leaving the office.The attenuator based method has also the issue that it changes the impedance. Reflected power from the antenna passes the attenuator twice. I'm not sure how much effect this makes on the output stage of the RF amplifier, maybe none. It also doesn't provide absolute results, only relative.I see someone using Danish butter cookies box to do the testing:Funny enough it's something I did myself for GPS antennas.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 17, 2025, 04:16:25 pm
content: Quote from: tszaboo on June 17, 2025, 01:28:25 pmIf you are looking for absolute results, I would expect chamber, calibrated antennas, amps, receivers...... similar to what I have shown in the past. For just playing at home, getting some sort of relative measurements, what I have shown at home is fine for me. It's actually been fairly reproducible. Mainly this because the frequency I have been testing it, I can get into the far field and stay away from junk in the office. A tape measure gets me close.Measuring for example the VSWR doesn't tell you much. Say the antenna mfg. stuffs a 50ohm resistor into the connector end of the antenna with nothing else. You measure the VSWR and it's very good over your entire band. Best antenna ever. Till you measure the forward gain...That home made 2 axis rotor and my office is no substitute for an actually test physicality but it also didn't cost anything.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 17, 2025, 04:54:55 pm
content: Member Marsupilami was helping out running simulations of another member's antenna. We ended up running a little experiment where they modeled one of my test horns and we compared their simulation against my measured results. For a homemade setup, I'm surprised it worked as well as it did. Looks like I had the higher directivity Narda directional coupler when I collected the data. But the frequency converter was not in the new clam shell case with improved routing.https://www.eevblog.com/forum/rf-microwave/cantenna-implementation-just-how-tricky-is-it/

author: Bud
author_rank: Super Contributor
timestamp: June 17, 2025, 06:23:59 pm
content: Quote from: rteodor on June 16, 2025, 04:17:48 pmPort extensions and delay are two different things.

author: rteodor
author_rank: Frequent Contributor
timestamp: June 18, 2025, 01:56:20 pm
content: Quote from: Bud on June 17, 2025, 06:23:59 pmPlease share what you know.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 18, 2025, 04:07:06 pm
content: Quote from: Bud on June 17, 2025, 06:23:59 pmhttps://docs.keysight.com/kkbopen/port-extension-vs-electrical-delay-in-network-analyzers-588262547.htmlTime stamped on terminologyhttps://youtu.be/bEPUePy_buM?t=164Full video

author: joeqsmith
author_rank: Super Contributor
timestamp: June 18, 2025, 04:22:38 pm
content: A decent article about port extension:https://www.mwrf.com/technologies/test-measurement/article/21842522/eliminate-fixture-effects-on-device-measurementsSolver64 refers to the delay as port extension. It assumes no losses or discontinuities (which in my case is basically true). There is no provision to add, say a model for a test fixture. I've thought about it but have yet to have a need for it.

author: rteodor
author_rank: Frequent Contributor
timestamp: June 18, 2025, 05:56:56 pm
content: Quote from: joeqsmith on June 18, 2025, 04:22:38 pmSo there is e-delay, port extension and de-embedding. I was wrong but at least I had a good lesson on what is what.Thank you.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 18, 2025, 06:24:18 pm
content: Quote from: rteodor on June 18, 2025, 05:56:56 pmI would not say anyone was wrong but rather it depends on the context. If someone like our ham friend in the video was talking about port extension with the NanoVNA, I will know what they are referring to.The following shows the menu settings to define port extension for a more modern, professional VNA:https://helpfiles.keysight.com/csg/N52xxB/S3_Cals/Port_Extensions.htmWe have some old HP 8753 VNAs. The reference plane and port extensions on these are very primitive. Just a simple delay like the low cost VNAs or my software.On my first VNA (predates 8753), HP had what they referred to as electrical length compensation. They used a combination of different length cables and a trimmer (pot) to adjust it.

author: pdenisowski
author_rank: Super Contributor
timestamp: June 19, 2025, 03:18:56 pm
content: Quote from: rteodor on June 18, 2025, 01:56:20 pmI made a video that covers these topics

author: rteodor
author_rank: Frequent Contributor
timestamp: June 25, 2025, 12:13:39 pm
content: Quote from: joeqsmithBeen watching W2AEW excellent videos from some years ago. Funny thing, it is exactly his video no. 317 that made me confuse things:@2:44Quote from: w2aewThe mwrf.com article nails it this way: Its one solution with 3 refinement levels:1.electrical delay: just asumes an ideal transmission line only, i.e. no loss, just propagation delay2.port extension: assumes time delay and constant loss on a single and uniform line3.de-embedding: use realistic models for each section to be extended, i.e. takes into account each 'sub-extension' with its delay and frequency dependent lossIn the last case things get complicated. Its '2x Thru de-embedding' method from pdenisowski's video that for me at least, makes most practical sense.