SWR meter for signals below 1mW? - Page 1
Forum section: electronics / rf-microwave

author: xwarp
author_rank: Frequent Contributor
timestamp: February 11, 2023, 08:42:31 am
content: I'm diving into unknown territory here and RF isn't my strong suit.What are the options for looking at FM signals below 1 milliwatt?Any help would be appreciated.

author: AFTORF
author_rank: Contributor
timestamp: February 11, 2023, 09:32:53 am
content: You have some options :- spectrum analyzer with tracking generator and swr bridge or directional coupler- spectrum analyzer with external generator and swr bridge or directional coupler- Vna if you need only SWR without FM demodulationYou could swap spectrum analyzer with selective receiver

author: radiolistener
author_rank: Super Contributor
timestamp: February 11, 2023, 12:02:47 pm
content: you can check the VSWR with a VNA it uses signal level below 1 mW for measurements.If you want to know actual power efficiency with a specific transmitter, you can measure it's output impedance and by knowing antenna impedance measured with VNA you can calculate actual VSWR.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 11, 2023, 05:53:57 pm
content: Quote from: xwarp on February 11, 2023, 08:42:31 amI assume you are looking for an inline meter. Are you wanting to buy or build something? How far below 1mW? What frequency range?The more details you provide, the better the answers.

author: xwarp
author_rank: Frequent Contributor
timestamp: February 12, 2023, 09:39:11 am
content: Quote from: joeqsmith on February 11, 2023, 05:53:57 pmAn inline meter would be perfect.approx specs would be 15 to 45 Mhz about -40 dB

author: radiolistener
author_rank: Super Contributor
timestamp: February 12, 2023, 01:23:33 pm
content: you can also use directional coupler

author: joeqsmith
author_rank: Super Contributor
timestamp: February 12, 2023, 04:41:16 pm
content: Quote from: xwarp on February 12, 2023, 09:39:11 amYou didn't state if you wanted to buy or make something. Cost? Impedance? Assuming you have time searching for something that meets your needs, which ones and why did you rule them out?It would be difficult to offer much help with such vague requirements.I'm sure you have spent a few minutes using Google before you posted. This one came up in my search just now:https://datasheet.datasheetarchive.com/originals/crawler/preciserf.com/f06acc509553a7d45bb598d60e260927.pdfIf you wanted to attempt to make something, several people have posted about using the low cost Analog Devices log detectors. Combined with a coupler, micro, software and or course some way to calibrate it, seems like you could measure down to -40. I cobbled an in-line meter together just for the fun of it. I was looking for something in the several Watts but the principle is the same. In my final setup, I used two of the AD8307 boards. Spent most of the time making decent couplers that allowed me to just use a simple point slope calculation. Datasheet and link final video is below. Even with the couplers I show, you can see it can measure down to -40 but there's a lot of error. To work at 1mW, I would need to design a new coupler.https://www.analog.com/en/products/ad8307.html#product-overviewhttps://www.eevblog.com/forum/rf-microwave/cb-and-ham-radio-techs-love-their-bird-wattmeters/msg4338097/#msg4338097

author: xwarp
author_rank: Frequent Contributor
timestamp: February 12, 2023, 07:24:56 pm
content: Quote from: joeqsmith on February 12, 2023, 04:41:16 pmThank you, your replies are very much appreciated. I'm a little scatter brained trying to get a grasp on what I am trying to accomplish. I've never had any formal training with RF and everything I've learned so far has been hands on and reading as much as I can understand at the moment.The impedance is 50 ohms. The cost, well I figure below 50 dollars would be good.I have been searching and as stated, not being familiar with RF scattering parameters and the math that goes with it has been a real challenge for me. At 52, remembering formulas and how different values can be manipulated for the unknowns is hard for me to grasp and retain.It sounds like that I can take the output impedance/VSWR of the amplifier, then take the input impedance/VSWR of the next section, do the math for the value I am looking for without actually having to build anything. I just don't know the words to exactly explain that value enough to google the formula that would give me the answer.Hope that makes sense.On a side note, I've got a HP 8505A NA with the S-Parameter test set helping me learn...LOL, but I love that old HP gear.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 12, 2023, 07:58:10 pm
content: I recently constructed a slotted line for a video I made and used it to measure frequency by measuring the distance between the waves using a mechanical scale. It could also be used to measure VSWR by measuring the voltage levels along the transmission line. It's always good to have some understanding of the basics.Your 8505A would have been a big improvement over my first VNA (8754A). This is pretty much what I used to learn the basics on how a VNA works and how to use one. Unlike the 8505A, mine requires everything be hand calculated which is what forced me to learn some of the math and how I ended up writing software for the new low cost pocket VNAs a few decades later.Your $50 limit may be too low, even if you make something. I think those Analog Devices boards were about $10 ea. By the time you add the cost of the micro, display, coupler... I think you will exceed it. Then you have the problem of trying to get it to give you meaningful data. I'm not aware of anything that you could purchase, ready to use, that would fit your criteria.https://www.amazon.com/Logarithmic-Keenso-Testing-Detector-0-1-600M/dp/B07RKHD86B

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 14, 2023, 12:21:54 pm
content: How about one of these?I have a similar one and if you like can check it down to 1 mw.https://www.ebay.com/itm/314291162168?hash=item492d368c38:g:A1EAAOSwTuJjqS9q&amdata=enc%3AAQAHAAAA4CO9HFSy4aADLzITtkpKbEAz%2FbPwtYfKJpEWAQqC4FtINRUcyBc%2F2%2FqKZ%2BQxZei%2F9ywi%2FiRVz9q8J%2BSH0az1K%2F8H5MWqcRercU0ZVpULiNZjz6wMJBhybgUmZlzD05IA3Zdkevh5X5OFVZTaZt%2F5UV%2FEC0ZgaQHuYfSwQEvsuDkr9SUTXvzcQq45qzLH6hAumP3JEaY7umJ8vOsX2b217Woy6MA0tjxM1HWtYwjIPNY0w%2BGub5xLI4FxTFk1RDp1PkRIpP12t81UwzRhJ2j1mmfoujH1GXiFC9CS5DtAD9wY%7Ctkp%3ABk9SR56h4f3JYQ

author: joeqsmith
author_rank: Super Contributor
timestamp: February 15, 2023, 12:41:46 am
content: Quote from: Wallace Gasiewicz on February 14, 2023, 12:21:54 pmAs I understand from reading their posts, they need to measure below 1mW, down to 100nW,want the meter to be in-lineand are willing to pay $50 for a complete turn key solution.***additional requirements.