S11 of dipole - Page 1
Forum section: electronics / rf-microwave

author: SparkFly
author_rank: Regular Contributor
timestamp: December 14, 2020, 10:39:38 am
content: Hi, I'm looking to measure the s11 of a dipole antenna, but I can only measure each element independently. Does this mean I should expect to see a resonance dip at half the intended dipole frequency? Ie, for a 500MHz dipole, the s11 of a single element should be 250 MHz?

author: radiolistener
author_rank: Super Contributor
timestamp: December 14, 2020, 12:02:54 pm
content: Quote from: SparkFly on December 14, 2020, 10:39:38 amNo, you cannot measure each element of antenna independently. Because antenna works as a complex of all elements.S11 is a reflection coefficient, it depends on impedance difference between feeder impedance Z0 (coax cable impedance) and antenna impedance Za:S11 = (Za - Z0) / (Za + Z0)So, S11 measurement for antenna doesn't make sense, because it depends on the feeder and matching circuit impedance.You're needs to measure antenna impedance Z = R + jX. And then see how to match it with your feeder.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 14, 2020, 01:06:21 pm
content: Quote from: SparkFly on December 14, 2020, 10:39:38 amhow do you measure each element indepently? you'll screw the property of the antenna as a whole. as radiolistener typed.Quote from: SparkFly on December 14, 2020, 10:39:38 amno, you should see a dip at exactly the frequency where source and the antenna matched, ie 50 ohm impedance. when the power is mostly absorbed by the antenna.Quote from: radiolistener on December 14, 2020, 12:02:54 pmsorry maybe i misinterpret you. all your statement make sense except this.

author: SparkFly
author_rank: Regular Contributor
timestamp: December 14, 2020, 01:48:16 pm
content: Thanks for the replies, I see my errors now. I am trying to optimise the lengths of dipole elements for a turnstile antenna made of two dipoles and a phasing block. I want to transmit on 436MHz. I've included the diagram below woth my setup. Can I use my vna to tune the lengths of each element, as in the picture below.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: December 14, 2020, 01:51:01 pm
content: Quote from: Mechatrommer on December 14, 2020, 01:06:21 pmditto; the S11 / return loss in dB can easily be converted to the SWR; sure, it's nice to have the complex impedance, but it's not absolutely necessary. before the cheap VNAs flooded the market, we were mostly living just with SWR when measuring our antennas

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 14, 2020, 02:11:56 pm
content: Quote from: HB9EVI on December 14, 2020, 01:51:01 pmcomplex impedance is required to match feeder at particular frequency of interest. VNA can easily shows this, unlike VSWR meter. from S11, we can derive the return loss, complex impedance, and VSWR. VNA is a superset of a VSWR meter.

author: radiolistener
author_rank: Super Contributor
timestamp: December 14, 2020, 02:15:14 pm
content: Quote from: Mechatrommer on December 14, 2020, 01:06:21 pmwhen you measure S11 you're doing it for some base impedance in the VNA. Usually this is 50 Ohm.But:1) antenna impedance usually is NOT equals to 50+j0 Ohm.2) if you measure S11 of antenna through a piece of cable, it cannot help you to tune your antenna properly because antenna impedance will be transformed by piece of cableThis is why measurement of S11 is useless. You're needs to measure impedance Z (R and X component) at the antenna feeding point. In such way you can tune your antenna into resonance to get X = 0. Also you can get actual antenna impedance and will be able to do proper antenna match with feeder.

author: radiolistener
author_rank: Super Contributor
timestamp: December 14, 2020, 02:22:36 pm
content: Quote from: HB9EVI on December 14, 2020, 01:51:01 pmIf you use S11/SWR, you're tuning complete system (antenna+feeder) simultaneously. But this is not a good way, because if you use feeder as impedance match circuit, you will get high loss in the feeder. And you never know if your antenna is tuned properly or not. You may loss a lot of power in the feeder and even don't know about that, because you see good S11/SWR on the feeder input, but it doesn't mean that antenna is tuned for better efficiency.In some cases you can get good S11/SWR on the feeder input with broken antenna and high power loss. Because good SWR doesn't mean that your antenna is tuned properly and works efficiently. It just means that your feeder get all power from transmitter and nothing else. You don't know what happens with this power. It can be dissipated to heat your coax cable or it can be radiated with antenna or... You don't know that from SWR value on the feeder input.The better way is to tune antenna properly first and then properly match antenna with the feeder.

author: SparkFly
author_rank: Regular Contributor
timestamp: December 14, 2020, 02:30:08 pm
content: Thanks again for the information, becomibg clearer to me. In this case, how do I tune the individual elements?

author: radiolistener
author_rank: Super Contributor
timestamp: December 14, 2020, 02:33:42 pm
content: Quote from: SparkFly on December 14, 2020, 02:30:08 pmIt depends on antenna, but first step - you're needs to tune your antenna without feeder (without coax cable). Just at feeding point.Antenna is at resonance when X component of it's impedance is 0. You're needs to tune antenna size to get X=0. Or you can use impedance match circuit to match reactive impedance with your feeder. If you use impedance match circuit it has some power loss. So, it's better to tune antenna to keep RF energy within antenna body, in such way you will minimize power loss.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 14, 2020, 02:48:23 pm
content: Quote from: radiolistener on December 14, 2020, 02:15:14 pmcorrect...Quote from: radiolistener on December 14, 2020, 02:15:14 pmcorrect...Quote from: radiolistener on December 14, 2020, 02:15:14 pmwe are not measuring at VNA's input plane, ie measuring the whole cable and antenna....Quote from: radiolistener on December 14, 2020, 02:15:14 pmwe are measuring at feeding point end plane, ie at the antenna input... after calibration on the cable's end (feeding point). hence we are measuring antenna's complex impedance alone (of course + tiny bit of "rig" or adapter to make it insertable to VNA cable) we can later tune the matching circuit for the antenna to match the actual feeding line impedance be it 50, 75 ohm or whatever.with your statement... then i wonder how you figure out the antenna's complex impedance without using a VNA? (i'm not HAM, so sorry for the noob question)

author: SparkFly
author_rank: Regular Contributor
timestamp: December 14, 2020, 03:02:17 pm
content: Sorry by tune in this instance I mean cut the conductors ofnthe turnstile antenna to the optimum length. I know each element should be wavelength/4, but ive left them a little long to be better clipped using real measurements. However, I'm becoming confused now as to what I should be measuring to do this.

author: radiolistener
author_rank: Super Contributor
timestamp: December 14, 2020, 03:07:46 pm
content: Quote from: Mechatrommer on December 14, 2020, 02:48:23 pmWhen you calibrate your VNA through cable, your calibration load don't use outer side of coax cable braid.Your way can work (with calibration include cable), but only if antenna is balanced and don't use outer side of coax braid as a part of antenna.You never know if your antenna is well balanced. So, it's better to measure antenna without cable.Or at least you're needs to use RF chokes on the cable.

author: radiolistener
author_rank: Super Contributor
timestamp: December 14, 2020, 03:15:28 pm
content: Quote from: SparkFly on December 14, 2020, 03:02:17 pmYou're needs to measure X component of impedance. Cut your antenna body piece by piece to get X = 0 at desired frequency.But try to cut it in such way to keep symmetry. Try to avoid asymmetry, it will help to reduce common mode currents on the feeder.When your antenna has X = 0, or close to 0, now look at R value. You're needs to match R value with coax cable impedance.For example when your antenna has X=0 and R = 100, then you're needs to connect your 50 Ohm feeder with antenna through 1:2 balun. Because 100 / 50 = 2If your antenna has R close to 75 at resonance frequency, there is a sense to use 75 Ohm coax cable.

author: SparkFly
author_rank: Regular Contributor
timestamp: December 14, 2020, 03:30:43 pm
content: I think I see. However, I'm limited as I need to use a phasing block as per the picture below, and cannot change the baluns. Is it the case that I cannot tune each of the 4 elements individually unless I know exactly what is inside this phasing block?

author: radiolistener
author_rank: Super Contributor
timestamp: December 14, 2020, 03:32:56 pm
content: Quote from: Mechatrommer on December 14, 2020, 02:48:23 pmwithout VNA it's very hard to tune your antenna for best efficiency. If you use SWR meter on the feeder input, it just show you if there is reflected energy from the feeder to transmitter, nothing else. With SWR meter you never know if your transmitter antenna works or not.For example, you can put 50 Ohm dummy load instead antenna at the end of feeder. You will see SWR=1, this is very good. But there is no antenna and no transmission at all

author: radiolistener
author_rank: Super Contributor
timestamp: December 14, 2020, 03:43:57 pm
content: Quote from: SparkFly on December 14, 2020, 03:30:43 pmit's hard to tune antenna array. You can tune individual antenna for specific frequency. But then you're needs to place them in such way against each other to get proper phase between them. Or tune your phasing block. It will needs to take into account cable length, because cable works as delay line. So, you will need very precise cable length between each antenna and phasing block.If you want to use two perpendicular dipoles, I think you can tune them by connect VNA first for two horizontal elements and then two vertical elements. But I'm not sure, because I never tried to tune antenna arrayLet me know if such method works, I'm wonderProbably there is a sense to put dummy load on unused dipole pair while tuning.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: December 14, 2020, 04:16:51 pm
content: Quote from: radiolistener on December 14, 2020, 02:22:36 pmcertainly; this is why I always use to measure as close as possible to the feedpoint of the antenna, still with VNAs nowadaysbut it's never wrong to that remember again