Maximum power when sweeping an antenna with an VNA? - Page 1
Forum section: electronics / testgear

author: bayjelly
author_rank: Regular Contributor
timestamp: June 20, 2022, 01:36:52 am
content: When sweeping an antenna with a VNA, is there a maximum output power that is deemed "safe", i.e. that won't cause problems for other users of the swept band? Is -10dBm, which seems to be the default for my VNA, still safe then?

author: tautech
author_rank: Super Contributor
timestamp: June 20, 2022, 02:07:00 am
content: Never bothered about this too much as your stimulus is a sinewave whereas most coms are digital.IIRC everything I did in this thread was at default stimulus levels:https://www.eevblog.com/forum/rf-microwave/antenna-project-log/

author: xrunner
author_rank: Super Contributor
timestamp: June 20, 2022, 02:15:40 am
content: Quote from: bayjelly on June 20, 2022, 01:36:52 amNot possible to answer in a simple way. It depends on what "band" you are sweeping. What devices use the band, what services use the band ...It probably wouldn't be much of an issue since sweeping would only momentarily introduce rf into another system, but no one can answer unless more is known.

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2022, 04:55:41 am
content: You could look up the emission limits for Part 15 devices, but in practice the output from a VNA is not powerful enough to matter.

author: knudch
author_rank: Regular Contributor
timestamp: June 20, 2022, 05:09:11 am
content: Quote from: David Hess on June 20, 2022, 04:55:41 amI am no so sure about that.....0dBm is 107 dBuV and that is a lot if you measure emissionsBut antenna type and position, distance to "victim"...factors to take into accountbeside those factors already mentioned

author: mag_therm
author_rank: Frequent Contributor
timestamp: June 20, 2022, 04:24:37 pm
content: USA Extra Class Amateur Exam question E4A07:"Which of the following is an advantage of using an antenna analyser compared to an swr bridge to measure antenna swr?"Correct answer: "Antenna analysers do not need an external RF source"So by inference, the vna is regarded as a better alternative to a transmitter power test.Further explanation is in the ARRL License Manual.The Nooelec vna here emits about -13dBm, I have not found any labels or manual notes about compliance.

author: pdenisowski
author_rank: Super Contributor
timestamp: June 21, 2022, 06:38:57 am
content: I worked professionally in interference hunting all across the US and internationally for over a decade. A CW (i.e. continuous) signal at -10 dBm would definitely get my attention in cellular bands, but not so much at HF. That said, most interferers have to be “on the air” for significant periods of time in order to cause problems: unless you’re sweeping your antenna multiple times per day, every day, the chance that you’ll create noticeable / “harmful” interference is pretty low. I worked well over a hundred interference cases in the field and in only one case (on a tradeshow floor!) was the source an RF test and measurement instrument.I should add that in the cellular bands I did sometimes see very narrow, very fast “sweepers” which crossed 10s of MHz in a few seconds and which only appeared a single time. I always suspected (but could never prove, since they were “there and gone” ) that these were someone sweeping an antenna or running a PIM test.

author: jeremy
author_rank: Super Contributor
timestamp: June 21, 2022, 07:12:46 am
content: Quote from: pdenisowski on June 21, 2022, 06:38:57 amSorry for going off-topic, but I would be interested to read more about how interference hunting works on a professional level. I once spoke to someone who also worked in this area who did a lot of overseas work. He said the US embassy was a repeat offender in his country for mystery signals, but they were too afraid to do anything about it

author: tszaboo
author_rank: Super Contributor
timestamp: June 21, 2022, 08:18:04 am
content: Anecdotal evidence, take it as it is: I had Bluetooth headset on, working fine, while continuously sweeping a 2.4GHz antenna.

author: iMo
author_rank: Super Contributor
timestamp: June 21, 2022, 08:39:59 am
content: With 100mW into a shortwave antenna (I guess it is -10dBm) you can do a transatlantic QSOPS: the ham radio people claim the world record is 1.6billion miles per Watt (KL7YU and W7BVV in 10m band)..

author: Performa01
author_rank: Super Contributor
timestamp: June 21, 2022, 09:07:31 am
content: Quote from: imo on June 21, 2022, 08:39:59 am-10 dBm is inevitably 0.1 mW.100 mW would be +20 dBm.

author: iMo
author_rank: Super Contributor
timestamp: June 21, 2022, 09:14:45 am
content: Quote from: Performa01 on June 21, 2022, 09:07:31 amOk, thanks, still with 0.1mW you can make a transatlantic QSO then..

author: pdenisowski
author_rank: Super Contributor
timestamp: June 21, 2022, 12:16:43 pm
content: Quote from: jeremy on June 21, 2022, 07:12:46 amThis series might be a good introductionhttps://www.rohde-schwarz.com/us/knowledge-center/webinars/ihunter-webinars_250575.htmlAnd here is an intro whitepaperhttps://www.rohde-schwarz.com/us/solutions/test-and-measurement/mobile-network-testing/interference-hunting/white-paper-an-introduction-to-interference-hunting_251161.htmlI’m the author of all this content, so if you have any questions, please let me know

author: bayjelly
author_rank: Regular Contributor
timestamp: June 23, 2022, 07:38:49 pm
content: Thanks a lot pdenisowski!Seems it's overall still good to be cautious. As an extreme, I do not think the 0dBm that my VNA's tracking generator could be set to are negligible, as some folks here suggested, but maybe I'm overly cautious?I also didn't ask for specific bands or situations because I was interested in levels that weregenerallyconsidered safe. It's easier to know what to do in, say, a HAM radio band, than if the question is just "I wonder what frequencies this funky antenna I built resonates at across the entire range of my VNA".Worse, there's also the situation where you might end up sweeping something thatshouldn'tbe an antenna, but ends up being one.

author: TimFox
author_rank: Super Contributor
timestamp: June 23, 2022, 10:17:35 pm
content: FCC Part 15 regs are rather complex, and usually refer to radiated field strength (uV/m) rather than power.https://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet63/oet63rev.pdfNote that the complexity is due to different limits depending on the frequency bands.

author: tszaboo
author_rank: Super Contributor
timestamp: June 23, 2022, 10:22:27 pm
content: Quote from: TimFox on June 23, 2022, 10:17:35 pmYes, because of antenna gain. Or an internal antenna in an enclosure will behave differently, and you are only measuring the emitted power with a wide bandwidth antenna at a fixed distance. There are some practical way to measure the power output of the DUT, like replacing the antenna with a coax, but that doesn't tell the whole story.

author: mag_therm
author_rank: Frequent Contributor
timestamp: June 23, 2022, 11:08:41 pm
content: For approximate conversion of E [V/m] at distance D , to antenna power P , seePage 29 of the FCC Part 15 doc referenced by TimFox in post #14

author: tautech
author_rank: Super Contributor
timestamp: June 24, 2022, 01:50:48 am
content: Quote from: bayjelly on June 23, 2022, 07:38:49 pmI may be being picky but when using a VNA the terminology TG is left behind where it belongs with SA's.Yes, the modern SA/VNA use the SA mode TG port however when it's in VNA mode you should refer to it as Port 1.This makes more sense with proper 2 and 4 port VNA's where every port is capable of providing a test stimulus.QuoteReally, you're overthinking this.With the plain and ordinary VNA single port sweeps of several antennas either just for learning about them or the process of tuning them, the radio a few yards away, our WiFi network and cellphones don't take a damn's worth of notice about what you're doing at typical stimulus levels you might use for this work.Might you be exceeding local ERIP levels, possibly but as it's not normally at a fixed frequency but instead swept, any sensitive local device might only be impacted on for milliseconds at most.Still, it's an interesting subject however if VNA's caused that much unintended mayhem in the RF spectrum they would've been banned from common use decades ago.

author: pdenisowski
author_rank: Super Contributor
timestamp: June 25, 2022, 04:41:55 am
content: Regarding units in interference hunting: most everyone uses units of dBm (although usually without specifying a bandwidth). The only people who use V/m are the cable TV folks, primarily because the FCC specifies permissible egress limits this way.When LTE was first deployed at 700 MHz, there were a LOT of cases of cable egress causing interference in the new cellular bands, and part of the problem in getting the two sides (cellular and cable ) to work together is that they had these different ways of “measuring” interference.In the end, what really matters (especially to FCC) is whether it isharmfulinterference. If you can show that the interference is harming your ability to use the spectrum licensed to you, then it’s an issue regardless of the absolute signal levels. I wrote several white papers on this topic and spoke at several SCTE events to make this point.Returning to the VNA antenna sweep question: I can’t imagine this ever meeting the definition of “harmful” interference. I’ve seen RF interference from almost every imaginable source, but with one notable exception (on the floor of the CTIA trade show one year), I have never seen or heard of interfence being caused by test equipment.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 25, 2022, 02:06:41 pm
content: I have some screw in Chinese imported 120V LED lights that conduct enough wide band spatter onto the house wiring that when you turn them on, anything below about 200MHz no longer works. All the way down to the AM radio bands. I would imagine the house wiring makes a decent antenna and our government has yet to stop the imports. I think your going to be fine...

author: rfclown
author_rank: Frequent Contributor
timestamp: June 27, 2022, 02:20:28 am
content: I would venture to say that most of us who have been measuring antennas for some time (myself for 40 or so years) have not bothered to consider this. I measure, and I'm done. I don't usually think about the default VNA power. The only time I stop to consider interference is when I find that someone else is transmitting on the same frequency and is screwing up my measurement.