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RF, Microwave, Ham Radio / Re: How to measure VSWR of tiny antennas - how to eliminate effect of VNA cables?
« Last post by E Kafeman on Today at 01:50:37 am »If it is a real small DUT with integrated antenna such as a BT earphone and antenna relies on a ground as part of antenna, monopoles, F-antennas, which mirrors itself in PCB ground => adding a measurement cable will change measured antenna impedance as cable braids will extend PCB ground size.
Correct impedance matching is then not possible if not some action are taken.
Even if PCB ground length is long relative actual wavelength will there be hot places along ground where a coaxial cable leaving PCB more heavily will affect measured impedance. Depending on situation can cable effect be reduced in a number of ways.
Sleeves and ferrite tubes to "cut off" cable braid works relative well at 1-3 GHz even if ferrite tubes material not are totally cutting or absorbing the braid current. Select ferrite material intended for as high frequencies as possible.
Never let test cable leave PCB ground at a ground corner or near antenna location.Especially if PCB ground is lossy from RF view or very short, such as split in small isles and traces, is it hard to avoid that coax cable braid becomes a vital part of antenna, which will result in a very different and probably poor impedance matching when cable is removed.
Sometimes must several ways be tested how to attach test cable without affect too much how RF current behaves in PCB ground. Hard to explain but I use a finger to search for places along ground that reacts more heavily at Smith chart, to find places to avoid letting cable leave PCB.
Do below show how a cable is attached at a medium small PCB using just two small ferrite tubes around measurement cable. It is not ideal high impedance blockers or absorbers but it is just a marginal antenna affect by test cable remaining. https://youtu.be/RyMFun_KhAc?si=PkSXL6TAsHgfyyEe
As the PCB in above video will be used close to a human head must also that be taken in account in how ground behaves and how body nearness can move around ground hot spots. If above video had been a customer project had I probably done more detailed testing in different environments to find best impedance matching. Now was it a fictive job just to show principles how to do a 10 minutes quick impedance matching using AnTune software.
>Many of the antenna designers I have met have used "Carbon-Based Foam Absorbers"
None of my antenna design colleges can use such material to avoid that cable braid affects antenna impedance as its absorption effect is very low for short cable lengths, close to PCB.
>ferrites that are narrowband in comparison
High freq. ferrites can very hardly be said to be narrow band but do mostly peak at 1-1.5 GHz in absorption efficiency and at 5 GHz are effect a bit too low to result in avoiding test cable very measurable acts as an low loss ground extension.
So fare have I been lucky with antenna design and matching at 5-6 GHz as in such cases have PCB ground been low loss and several lambdas in size which reduces problem that braid adds to ground size and hot spots can easier be avoided.
A sleeve as alternative will always work. It is more narrow band but mostly wide enough to cover bands of interest at 5-6 GHz.
Correct impedance matching is then not possible if not some action are taken.
Even if PCB ground length is long relative actual wavelength will there be hot places along ground where a coaxial cable leaving PCB more heavily will affect measured impedance. Depending on situation can cable effect be reduced in a number of ways.
Sleeves and ferrite tubes to "cut off" cable braid works relative well at 1-3 GHz even if ferrite tubes material not are totally cutting or absorbing the braid current. Select ferrite material intended for as high frequencies as possible.
Never let test cable leave PCB ground at a ground corner or near antenna location.Especially if PCB ground is lossy from RF view or very short, such as split in small isles and traces, is it hard to avoid that coax cable braid becomes a vital part of antenna, which will result in a very different and probably poor impedance matching when cable is removed.
Sometimes must several ways be tested how to attach test cable without affect too much how RF current behaves in PCB ground. Hard to explain but I use a finger to search for places along ground that reacts more heavily at Smith chart, to find places to avoid letting cable leave PCB.
Do below show how a cable is attached at a medium small PCB using just two small ferrite tubes around measurement cable. It is not ideal high impedance blockers or absorbers but it is just a marginal antenna affect by test cable remaining. https://youtu.be/RyMFun_KhAc?si=PkSXL6TAsHgfyyEe
As the PCB in above video will be used close to a human head must also that be taken in account in how ground behaves and how body nearness can move around ground hot spots. If above video had been a customer project had I probably done more detailed testing in different environments to find best impedance matching. Now was it a fictive job just to show principles how to do a 10 minutes quick impedance matching using AnTune software.
>Many of the antenna designers I have met have used "Carbon-Based Foam Absorbers"
None of my antenna design colleges can use such material to avoid that cable braid affects antenna impedance as its absorption effect is very low for short cable lengths, close to PCB.
>ferrites that are narrowband in comparison
High freq. ferrites can very hardly be said to be narrow band but do mostly peak at 1-1.5 GHz in absorption efficiency and at 5 GHz are effect a bit too low to result in avoiding test cable very measurable acts as an low loss ground extension.
So fare have I been lucky with antenna design and matching at 5-6 GHz as in such cases have PCB ground been low loss and several lambdas in size which reduces problem that braid adds to ground size and hot spots can easier be avoided.
A sleeve as alternative will always work. It is more narrow band but mostly wide enough to cover bands of interest at 5-6 GHz.