Microstripline SWR pickup: question regarding working mechanism - Page 1
Forum section: electronics / rf-microwave

author: cprobertson1
author_rank: Frequent Contributor
timestamp: January 04, 2017, 10:59:38 am
content: Firstly, Happy New Year!TL;DR -- skip to the actual question -->I've been dabbling in antenna analysis recently, and decided to make a (moderately) simple antenna analyser that measures Standing Wave Ratio (SWR), Impedance and Reactance of the antenna (or any other transmission line, I suppose) under test.My original plan was to use to a simpleresistive SWR bridgewith selectable 50/75precision resistors1with three measurement points to give SWR and impedance, and simply ignoring reactance to begin with2- but while carrying out research I've happened to come across an articlecriticising standard SWR bridgesas simply being a measure of impedance, not SWR - and suggested the use ofmicrostripline SWR pickupsas a viable alternative.--> Actual QuestionI was just wondering howthisis actually working to pick up the reflected wave? It appears to be a pair of peak detectors situated3in opposite directions across the central "test region" of the transmission line (in this case a microstripline). What I'm not able to appreciate is how this circuit is obtaining directionality to measure the forward/reflected voltages? Am I missing something obvious or having a stupid moment?I thought for a little while that signal is being capacitively coupled across from the central stripline to the two detector striplines: but if this is the case then I can't see how both aren't just detecting the largest voltage present (as opposed to the largest forward voltage on one side and the largest reflected voltage on the other).The provided explanation (as far as I can make out) is that for any given node, the two detectors will be coupling the signal across equally as I suspected, but because of their positions will be at different places within the node: but when I try to model this I end up with the detection becoming incredibly frequency-dependent (or alternatively dependent on the length of the stripline sections depending on which way you want to look at it)This brings us back round to the original backstory of developing a simple antenna analyser: will an SWR bridge suffice on it's own (since I also want to explicitly measure impedance)? Should it be used in conjunction4or even in series with the above stripline meter for better SWR accuracy?5This post was definitely longer than it could have been! Thanks for bearing with me!1or a group of closely matched resistors if precision resistors are too expensive2and later developing a new board with the reactance-measurement circuitry integrated into the measurement circuits, or alternatively an add-on board that can be switched in and out with a relay at the expense of EMC3"situated" is such a nice word: it rolls straight off the tongue; has a pleasing sound; and is actually useful in describing where something is while sounding a little bit smarter than the norm, but not so smart that you come across as smarmy.4as in switched into and out of circuit with relays5although, having said that, I can't imagine a situation where there would be a high SWR but no impedance mismatch - excepting maybe a ground loop? I might just be having a mind-blank moment and forgetting something simple though!

author: CJay
author_rank: Super Contributor
timestamp: January 04, 2017, 12:29:41 pm
content: I'd be interested to see how MFJ and the other manufacturers of antenna analysers do it, I know at least one uses a resistive bridge.I think the stripline approach is the one used by almost (if not every) SWR meter I've seen, it's simple to produce for HF to VHF and probably beyond.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 04, 2017, 12:36:28 pm
content: How?Simple! Find the definition for forward and reflected power, and SWR, and apply them to the circuit (you can model the stripline circuit as a wideband transformer structure, often used in the SW band). You will find that incident power is represented on one port of the directional coupler structure, and reflected power on the other.Tim

author: cprobertson1
author_rank: Frequent Contributor
timestamp: January 04, 2017, 04:04:06 pm
content: Quote from: T3sl4co1l on January 04, 2017, 12:36:28 pmAHA! I think I understand it nowYou put me on the right track for modeling it!I ended up doing further research based on the model (trying to determine the effect of the stripline length in particular) when I came acrossthis blogand the Monimatch SWR meter which is what the core of the aforementioned SWR pickup from the other article (the one with the terrible/horrific/cringe-inducing colour template on it).While doing further research I happened across the Monimatch, and in particularthis blogQuote from: CJay on January 04, 2017, 12:29:41 pmI am not actually entirely sure! One one hand the stripline approach is very simple - but on the other hand it doesn't measure impedance which is one of the characteristics you're wanting out of an antenna analyser.I'd imagine the dedicated SWR meters use stripline detectors due to their simplicity and more complex analysers use either a combination of stripline and an impedance bridge - or some other topology that allows them to measure each characteristic separately.According to another article I read,this circuitis also fairly common amongst analysers - but I can't comment, as I've not had a look inside any of them!I can, however, say that I'm visiting the Glasgow/WOSARS radio club tonight and I might be able to have a gander inside their MFJ analyser and get back to you on thatNo promises though, all depends on if it's in use/being borrowed/etc etc!

author: CJay
author_rank: Super Contributor
timestamp: January 04, 2017, 05:03:58 pm
content: Quote from: cprobertson1 on January 04, 2017, 04:04:06 pmStripline is dirt cheap, hence the use in most SWR meters but I am lead to believe by some happy coincidence, that it's also a very good solution as well.I breadboarded the detector section from the cheap Chinese antenna analysers you find on eBay, they're a rip off of an open source design kit, and it worked fairly well, a couple of precision op-amps and a meter gave me usable measurments but then I happened on the same blog casting doubt on the usefulness of the resistive bridge which made sense, yes, the impedance of an antenna is important but as that site says, it's not necessarily resonant.I have a feeling the MFJ uses a sort of automated GDO arrangement, schematics are on the web so I'll print a set out next monday.W2AEW however has a video on the resistive bridge method for 'tuning' an HF antenna:But also one on a directional coupler:

author: rfeecs
author_rank: Frequent Contributor
timestamp: January 04, 2017, 07:01:31 pm
content: Quote from: cprobertson1 on January 04, 2017, 10:59:38 amYou were right in the first place. Measuring impedance will give you SWR. VK5AJL.com is wrong. Most broadband network analyzers use a bridge. That gives you the best directivity. A microstrip coupler typically has terrible directivity.You have a one port measurement. So you only can know the voltage and current at that port. Given the system impedance Z0 (typically 50 ohms), you calculate SWR. That's all you can do. Whether you use a bridge or directional coupler, you are ending up doing the same thing.Quote from: cprobertson1 on January 04, 2017, 10:59:38 amLook up microstrip directional coupler:http://kilyos.ee.bilkent.edu.tr/~microwave/programs/magnetic/dcoupler/theory.htm(Maybe this article is too heavy on math and light on intuitive understanding. There are tons of other articles out there.) These are coupled transmission lines. Note that it depends on incident and reflected waves canceling out at various points, and it is typically designed to be a quarter wavelength long at the frequency of interest. Consequently not very broad band.

author: CJay
author_rank: Super Contributor
timestamp: January 04, 2017, 07:36:15 pm
content: Quote from: rfeecs on January 04, 2017, 07:01:31 pmHis site made sense to me but I'm always prepared to try and learn more.

author: rfeecs
author_rank: Frequent Contributor
timestamp: January 04, 2017, 10:43:08 pm
content: Quote from: CJay on January 04, 2017, 07:36:15 pmThis page is loaded with errors:http://vk5ajl.com/projects/SWRBridge.phpThe problems start with his definition of SWR:QuoteA few references on the definition of SWR (or VSWR):https://www.microwaves101.com/encyclopedias/452-vswrhttp://www.antenna-theory.com/definitions/vswr.phphttps://en.wikipedia.org/wiki/Standing_wave_ratiohttps://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdfThe last one, "Understanding SWR by Example" from ARRL looks like it is from the point of view of a HAM.

author: langwadt
author_rank: Super Contributor
timestamp: January 04, 2017, 11:32:56 pm
content: what frequencies are you aiming for ?

author: cprobertson1
author_rank: Frequent Contributor
timestamp: January 05, 2017, 10:46:58 am
content: Quote from: langwadt on January 04, 2017, 11:32:56 pmAs it's an antenna analyser, I'm aiming for everything from 160M up to 70cm: so that's 1.8MHz right up to 470MHz - which is a pretty broadbanded range!The for the v1 prototype I'm aiming to use a vcxo1covering 2.5 kHz to 200 MHz as the main oscillator - I'm currently investigating oscillators covering an even larger range, or alternatively having a separate circuit for the HF, VHF and UHF bands, each with a dedicated oscillator and bridge. The v1 prototype will give me an idea of the broadband performance of whatever method I choose: if acceptable I'll expand the oscillator range, if not, I'll make a separate analyser for each region2.Quote from: rfeecs on January 04, 2017, 10:43:08 pmSome very useful links thereI came across a few of the links in my travels before - but a good collection there!I did think that the vk5ajl.com swr page was a little error-proneI was taking everything there with a pinch of salt: particularly on the details!Quote from: rfeecs on January 04, 2017, 07:01:31 pmI'm almost certainly going to use a bridge for my analyser project: I might make a couple of cheap-o SWR meters using stripline, just to see how it works out - but for the analyser I don't think they'll meet my needs (based on a cursory glance on various internet articles and your own input, I suspect the bandwidth may be a problem)I take it if it's not 1/4 wavelength then it's just less efficient in it's pickup and will need more amplification before using the output? Ooft.....! Just occured to me that a quarter-wavelength of 160 meters is 40 meters :O... I'm not going to lie, I kinda want to make a 40-meter long piece of test gear and then try to sell it on ebay... or better still, send it to Dave for a two-minute-teardownANYWAY - my original goal was to try and figure out how the microstripline directional coupler was achieving it's directionality xD Mission accomplished! Thanks folks!I'm sure I'll post up the v1 Antenna Analyser project in the projects section as well - I really need to get in the habit of pictorially documenting my projects more often3: I even considered a live-stream on youtube at one point, but decided I don't have the gift of the gab required to make "soldering" entertaining for anybody save a very, very niche audience (solderaphiles... which is odd because my spellchecker recognises that word: but google doesn't... weird.)1Im currently planning on using theSI5351A-B04486-GT- with some supporting circuitry to transform, clean up and amplify the waveform. Maybe not the best choice, but it's cheap, moderately easy to implement, and covers almost all the frequencies I'd need it for (the UHF region is obviously lacking, but I can always add that later by changing the oscillator module in the v2 model2which has the added bonus of giving me three times as many boxes-with-dials-and-knobs on my benchwho doesn't want gauges-and-dials-and-knob-boxesTMon their bench!?3I'm surprisingly fastidious in documentation for my projects: schematics, notes, research, operating procedures all stored under a unique serial number and stored in a database/filing cabinet. Actually photographing my work though? I think I done it once or twice to show off something? But I haven't really bothered recently!

author: rfeecs
author_rank: Frequent Contributor
timestamp: January 05, 2017, 06:37:41 pm
content: Quote from: cprobertson1 on January 05, 2017, 10:46:58 amYes, the coupling is maximum at the quarter wave frequency, and rolls off at lower frequencies. A quick simulation of his design shows the quarter wave frequency is about 800MHz, with coupling about 20dB. Directivity is less than 10dB.The coupling goes from 20dB at 800MHz to 22dB at 400MHz, 41dB at 50MHz, 73dB at 1MHz.FYI, the schematic for the MJF-259C is here, under downloads:http://www.mfjenterprises.com/Product.php?productid=MFJ-259CYou can see on page 4 of the schematic that it uses a bridge.

author: w2aew
author_rank: Super Contributor
timestamp: January 05, 2017, 11:16:21 pm
content: I also did one on a homebrew directional coupler:Quote from: CJay on January 04, 2017, 05:03:58 pm

author: cprobertson1
author_rank: Frequent Contributor
timestamp: January 06, 2017, 11:28:43 am
content: Quote from: rfeecs on January 05, 2017, 06:37:41 pmNice! I didn't realise the schematics were on their own webpage ... in retrospect I probably should have checked...@w2aew + CJay- I've still to look at the linked videos; they're going to be my evening viewing tonight methinks!Thanks again everybody

author: CJay
author_rank: Super Contributor
timestamp: January 06, 2017, 12:51:42 pm
content: Also take a look at the SARK100 which is (I Think) an open hardware/source analyser that uses a resistive bridge, the firmware gave me a few pointers on how to measure but I didn't get further than building the bridge ugly style and a multimeter to measure outputs from the diodes before it got put away.It was repeatable and useable to the maximum frequency I could accurately generate at the time (~50MHz) and I suspect it would have been usable much higher with a little more care taken in layout and selection of SMT resistors and diodes.

author: cprobertson1
author_rank: Frequent Contributor
timestamp: January 10, 2017, 09:24:42 am
content: Quote from: CJay on January 06, 2017, 12:51:42 pmYup!Open source(though superseded by the SARK-110... which costs more than an MJF; but hey-ho, it *does* have a nice screen)So, bearing in mind this is my firstrealRF project (excusing a pretty terrible 5mW 433MHz transmitter / receiver I made with LC tanks and amplifiers - it done what I needed to do, which was to get a signal through a window using two suction-cups while not using very much power (in retrospect, I should have used an IR diode and receiver - but I wanted an excuse to use my frequency counter)ANYWAY - So, bearing in mind this is my firstrealRF project - what sort of bottlenecks are limiting the frequency of this bridge?I take it in terms of PCB layout, parasitic noise from the oscillator, and impedance matching of the tracks are the big concern?How about the choice of resistors and diodes? Resistors, I take it should be as small as feasible while keeping inductance and capacitance as low as possible? How about the diodes - what sort of characteristics am I looking out for when designing this?(Primary goal is HF: secondary is VHF; UHF would be a bonus, though the v2 board won't have an oscillator that can run fast enough for!)There's probably a lot of resources out there for that side of things actually: application notes for VHF/UHF components would probably be a good place to start!

author: CJay
author_rank: Super Contributor
timestamp: January 10, 2017, 10:26:37 am
content: Well, again letting MFJ do the work for you, HSMS2820 would appear to be suitable, into the low GHz ranges even.I knocked up a simple diode detector using one diode in a dual HSMS2820 device to detect output from a cell phone VCO before I had my SA so they'd be at least good enough for UHF.SMD thin film resistors seem to be usable, again up to low GHz and *way* beyond,http://www.vishay.com/docs/60107/freqresp.pdfYou may need to add some method of preventing static discharge from damaging the input (look at SARK but bear in mind that's HF only), especially if you're going to use it on antennae that are mounted outdoors.Capacitors will be more of a challenge to select as the range of types can be bewildering but Vishay*, again, offer easily available SMD parts that seem to be used by people building UHF 'stuff'.(*I chose Vishay as their SMD resistors and capacitors are easily available from RS in the UK, as are some of the HSMS2820 variants)I don't think you'll need any esoteric PCB material and there are plenty of impedance calculators out there on the web which will give you a head start.If you're a Designspark user there's one built in.'Proper' RF design with groundplane, screening etc. would be a good idea for the detector and oscillator and I'd make it modular but you're not dealing with spectrum analyser -137dBm levels of signal here so I think that while it's important to make sure your oscillator doesn't grossly affect the bridge, it won't be *that* important that it'll stop it working, plus you can probably calibrate out offsets as you're not looking for absolute measurements, just relative ones.